<html><head><meta http-equiv="Content-Type" content="text/html charset=utf-8"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class=""><div><blockquote type="cite" class=""><div class=""><div style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class=""><div class=""><div class=""><blockquote type="cite" class=""><div class=""><div style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class=""><br class=""><div class=""><blockquote type="cite" class=""><div class="">On 1 May 2019, at 5:42 PM, Oakley Sound via Synth-diy <<a href="mailto:synth-diy@synth-diy.org" class="">synth-diy@synth-diy.org</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><span style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; orphans: auto; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; widows: auto; word-spacing: 0px; -webkit-text-stroke-width: 0px; float: none; display: inline !important;" class="">The actual VCO core is based around the SCR Q103. It's a linear VCO with the main timing cap C102. I believe the 555 is being used as a 'semi-fixed' monostable to produce an error CV, courtesy of IC101 pins 1, 2 & 3, that partly compensates for high frequency tracking droop and also allows some control over the overall pitch of the VCO core.</span><br style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; orphans: auto; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; widows: auto; word-spacing: 0px; -webkit-text-stroke-width: 0px;" class=""></div></blockquote><div class=""><br class=""></div><div class="">Many thanks for this Tony. I’m not familiar with SCR-based oscillators. I had a look around the net, and found a couple that indeed use a 555, but nothing really like this one. This is more complicated. I poked around with the scope to try to get an idea of what’s happening, I’ve put the new scope shots up here along with the VCO schematic...</div><div class=""><a href="https://mezzoauto.blogspot.com/2019/05/roland-sh-2000.html" class="">https://mezzoauto.blogspot.com/2019/05/roland-sh-2000.html</a></div><div class=""><br class=""></div><div class="">You can see there is a big change to the mark-space ratio between these frequencies at pin 3 of the 555. This pulse output is then integrated by the first stage of IC 101, it would seem (see pic)… but I wonder what is happening at the second stage where the negative key CV is introduced?</div><div class="">Any further enlightenment from anyone will be greatly appreciated!</div><div class=""><br class=""></div><div class=""><br class=""></div></div></div></div></blockquote></div></div></div></div></blockquote><blockquote type="cite" class=""><div class="" style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;"><div class="">The keyboard is 3 octaves not 4. </div><div class="">F0 is approx 1.5 kHz and F3 is approx 11kHz.</div></div></blockquote></div><br class=""><div class=""><br class=""></div><div class="">I found some good stuff in the archives about SCR based VCOs.</div><div class="">I didn’t realise the VCO in the 303 is one such thing - the SCR there is made from two transistors (thanks Richie!)</div><div class="">Neil mentions a Jim Williams App note AN299 that features a VCO with a similar idea.</div><div class="">Juergen Haible was fascinated by them. He wondered why they were considered inferior, when they were put to such effective use in the early Korg synths. This led him on to develop his “JH-720” which had the most temperature stable VCO of all his synths “by far”. (Funnily enough, Juergen DIDN’T like stair-stepped waveforms, so maybe he wouldn’t have liked the SH-2000. I think they sound great in this synth, but that’s probably because of the ladder filter.)</div><div class=""><br class=""></div><div class="">So I think after reading all this, what is happening in the SH-2000 at the top half of IC101 is that the pitch CV directly affects the rate the timing cap charges, hence the frequency. There’s a lot of other stuff going on that I don’t get, but that was the bit that was annoying me (cos I’m a control freak about control voltages.) </div><div class=""><br class=""></div><div class="">I’ve pasted some interesting posts below.</div><div class=""><br class=""></div><div class=""><br class=""></div><div class=""><br class=""></div><div class=""><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">From 1997:</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier; min-height: 19px;" class=""><span style="font-kerning: none" class=""></span><br class=""></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Times;" class=""><span style="font-kerning: none" class=""><b class="">Synthaholic </b></span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Times;" class=""><span style="font-kerning: none" class=""><i class="">Sun Aug 10 19:23:58 CEST 1997</i></span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">After reading about the thyratron VCO, I thought: "Why not an SCR</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">vco?"</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier; min-height: 19px;" class=""><span style="font-kerning: none" class=""></span><br class=""></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">I have quite a pile of them of various ratings, and I wondered if</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">anyone has tried this? Assuming a saw waveform, the question (for me)</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">is will an SCR discharge the cap fast enough to make a VCO with decent</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">range?</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">- Scott Gravenhorst (Synthaholic)</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier; min-height: 19px;" class=""><span style="font-kerning: none" class=""></span><br class=""></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier; min-height: 19px;" class=""><span style="font-kerning: none" class=""></span><br class=""></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Times;" class=""><span style="font-kerning: none" class=""><b class="">Haible Juergen</b> </span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Times;" class=""><span style="font-kerning: none" class=""><i class="">Mon Aug 11 14:02:24 CEST 1997</i></span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier; min-height: 19px;" class=""><span style="font-kerning: none" class=""></span><br class=""></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">Most Korg VCOs actually use SCR circuits !</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier; min-height: 19px;" class=""><span style="font-kerning: none" class=""></span><br class=""></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">(MS-20, Polysix, Trident, to name just a few.)</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier; min-height: 19px;" class=""><span style="font-kerning: none" class=""></span><br class=""></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">Only that they don't use an SCR *component* - they</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">build their own SCR circuit from two bipolar transistors.</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">But it works exactly the same way: It starts to conduct</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">when a certain gk voltage is reached, and it stops when</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">the current thru the device reaches zero (i.e. when the</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">capacitor is discharged).</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">The first Moog VCOs used unijunction transistors - not</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">the same, but similar in operation.</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">An actual SCR component I have only seen in Roland </span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">System 700 docs. Not in the VCO's, but in the envelopes,</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">if memory serves.</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier; min-height: 19px;" class=""><span style="font-kerning: none" class=""></span><br class=""></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">JH.</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier; min-height: 19px;" class=""><span style="font-kerning: none" class=""></span><br class=""></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">PS: The Korg circuit looks very elegant, as you don't need</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">any voltage comparator (it's buit in), nor hysterestic switch</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">or monoflop (the end condition is "current == 0").</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">Does this circuit also have some drawbacks ?</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">Tolerances of trigger voltage ?</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">Temperature dependance ?</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">Any idea why it wasn't used by everybody?</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier; min-height: 19px;" class=""><span style="font-kerning: none" class=""></span><br class=""></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Times;" class=""><span style="font-kerning: none" class=""><b class="">Eric Barbour</b> </span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Times;" class=""><span style="font-kerning: none" class=""><i class="">Mon Aug 11 19:18:06 CEST 1997</i></span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">UJTs and four-layer devices (SCRs, triacs, etc.) have all</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">been used for building VCOs. Their major problem is one of</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">temperature drift--it's serious. Especially in the case of the</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">UJT, which is often referred to as a "junk part" by analog</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">design engineers. (If you can find a copy of ANALOG DESIGN </span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">ENGINEERING: ART, SCIENCE AND PERSONALITIES by Jim Williams,</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">EDN Electronic Series, 1994, you will find a section in there</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">by Bob Pease about the agony of making a linear V/F converter</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">(VCO) with a UJT.)</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class=""> The gas thyratron is vaguely similar to a four-layer device</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">in electrical operation; it does not conduct until the control</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">electrode reaches the threshold, whereupon conduction occurs.</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">Shutting the device off requires shutting off or reversing the </span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">voltage across it. In a relaxation oscillator, this is done by</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">the capacitor which sets the oscillator rate.</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class=""> Where the difference comes in is in the utter un-transistor-like</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">behavior of the thyratron. It is dependent on a hot cathode and</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">gas ionization, not on avalanching of a 4-layer semiconductor structure.</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">The 4-layer device varies greatly with temperature, while the</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">thyratron shows almost no variation. Both devices are not very</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">linear V/F converters, unless many schemes are undertaken to make</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">them linear. Still, there is one advantage of the tube over its</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">analogous semiconductor device.</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class=""> As for the sound and behavior, well, Bob Edgar got a chance to</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">try my modular tube synthesizer this past Sunday. Ask him if it's</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">real. Any comments Bob?</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier; min-height: 19px;" class=""><span style="font-kerning: none" class=""></span><br class=""></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">-- </span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">Eric Barbour</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">Svetlana Electron Devices</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">Portola Valley CA USA</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier; min-height: 19px;" class=""><span style="font-kerning: none" class=""></span><br class=""></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Times;" class=""><span style="font-kerning: none" class=""><b class="">Don Tillman</b> </span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Times;" class=""><span style="font-kerning: none" class=""><i class="">Mon Aug 11 19:40:01 CEST 1997</i></span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier; min-height: 19px;" class=""><span style="font-kerning: none" class=""></span><br class=""></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class=""> Date: Mon, 11 Aug 1997 10:18:06 -0700</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class=""> From: Eric Barbour </span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier; min-height: 19px;" class=""><span style="font-kerning: none" class=""></span><br class=""></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class=""> UJTs and four-layer devices (SCRs, triacs, etc.) have all</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class=""> been used for building VCOs. </span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier; min-height: 19px;" class=""><span style="font-kerning: none" class=""></span><br class=""></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class=""> [Gas thyratron stuff...]</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier; min-height: 19px;" class=""><span style="font-kerning: none" class=""></span><br class=""></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">And neon lamps too!</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier; min-height: 19px;" class=""><span style="font-kerning: none" class=""></span><br class=""></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class=""> -- Don</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier; min-height: 19px;" class=""><span style="font-kerning: none" class=""></span><br class=""></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Times;" class=""><span style="font-kerning: none" class=""><b class="">Haible Juergen</b></span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Times;" class=""><span style="font-kerning: none" class=""><i class="">Mon Aug 11 19:40:51 CEST 1997</i></span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier; min-height: 19px;" class=""><span style="font-kerning: none" class=""></span><br class=""></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class=""><span class="Apple-tab-span" style="white-space:pre"> </span>>Their major problem is one of</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class=""><span class="Apple-tab-span" style="white-space:pre"> </span>>temperature drift--it's serious. Especially in the case of the</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class=""><span class="Apple-tab-span" style="white-space:pre"> </span>>UJT, which is often referred to as a "junk part" by analog</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class=""><span class="Apple-tab-span" style="white-space:pre"> </span>>design engineers.</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier; min-height: 19px;" class=""><span style="font-kerning: none" class=""></span><br class=""></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">Ok, the UJT may be crap. But the thyristor structure</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">(composed of a discrete npn and pnp) is widely</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">used in Korg synths and apparetly doesn't cause too </span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">much problems (?).</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier; min-height: 19px;" class=""><span style="font-kerning: none" class=""></span><br class=""></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class=""><span class="Apple-tab-span" style="white-space:pre"> </span>>Both devices are not very</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class=""><span class="Apple-tab-span" style="white-space:pre"> </span>>linear V/F converters, unless many schemes are undertaken to</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">make</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class=""><span class="Apple-tab-span" style="white-space:pre"> </span>>them linear.</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier; min-height: 19px;" class=""><span style="font-kerning: none" class=""></span><br class=""></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">I don't understand this. Nonlinear V/F, or better I/F, would mean</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">that the trigger point would depend on frequency. Just curious ...</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier; min-height: 19px;" class=""><span style="font-kerning: none" class=""></span><br class=""></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">JH.</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier; min-height: 19px;" class=""><span style="font-kerning: none" class=""></span><br class=""></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Times;" class=""><span style="font-kerning: none" class=""><b class="">Tim Cockram</b> </span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Times;" class=""><span style="font-kerning: none" class=""><i class="">Mon Aug 11 22:51:59 CEST 1997</i></span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier; min-height: 19px;" class=""><span style="font-kerning: none" class=""></span><br class=""></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">The system 700 lfo uses a SCR too.</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier; min-height: 19px;" class=""><span style="font-kerning: none" class=""></span><br class=""></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier; min-height: 19px;" class=""><span style="font-kerning: none" class=""></span><br class=""></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">-------------------------------------</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">Tim Cockram</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier; min-height: 19px;" class=""><span style="font-kerning: none" class=""></span><br class=""></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier; min-height: 19px;" class=""><span style="font-kerning: none" class=""></span><br class=""></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Times;" class=""><span style="font-kerning: none" class=""><b class="">SCR VCOs (was: AW: 303 clone filter and VCO not reproducable</b></span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Times;" class=""><span style="font-kerning: none" class=""><b class="">Haible Juergen</b> </span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Times;" class=""><span style="font-kerning: none" class=""><i class="">Thu Nov 5 14:37:37 CET 1998</i></span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier; min-height: 19px;" class=""><span style="font-kerning: none" class=""></span><br class=""></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class=""><span class="Apple-tab-span" style="white-space:pre"> </span>>And I think that the SCR method is no good idea for a vco at all</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class=""><span class="Apple-tab-span" style="white-space:pre"> </span>>(not stable).</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier; min-height: 19px;" class=""><span style="font-kerning: none" class=""></span><br class=""></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">I tend to agree, because some simulations I've done say</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">the same thing. But I'm still wondering about all these</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">Korg synths that use them - they are not exactly known</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">as not stable. Rather the contrary. It's still a mystery to me.</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier; min-height: 19px;" class=""><span style="font-kerning: none" class=""></span><br class=""></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">JH.</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier; min-height: 19px;" class=""><span style="font-kerning: none" class=""></span><br class=""></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Times;" class=""><span style="font-kerning: none" class=""><b class="">Martin Czech</b></span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Times;" class=""><span style="font-kerning: none" class=""><i class="">Thu Nov 5 14:53:27 CET 1998</i></span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier; min-height: 19px;" class=""><span style="font-kerning: none" class=""><i class=""></i></span><br class=""></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier; min-height: 19px;" class=""><span style="font-kerning: none" class=""></span><br class=""></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">Not to boost with it, but I deal with this SCR stuff every day now.</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">For ESD-Protection devices. I've just measured about 40 of such structures</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">@ 27C and @120C Tj. From this I can see that at higher temperature</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">the leakage gets very much worse for an SCR (implicit current gain,</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">this gain is quite high for currents in the leakage range). Also the</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">threshold voltage for the snap-back (this is over head triggering, not</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">gated trigger) varries widely with temperature. geometry and parameters.</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">And even worse: aging. I mean these devices see a lot of current, and</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">this causes parameter shift.</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier; min-height: 19px;" class=""><span style="font-kerning: none" class=""></span><br class=""></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">Excactly the kind of stuff I don't want to have in my osc. Principally</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">faulty. Ok, Korg may have used them, but what is the meaning of "they</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">are not exactly known as not stable" ? If we compare it with tempco</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">effects it may look not so important, because tempco is put into some exp</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">function, whereas trigger level and leakage are more "linear" effect,</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">but I don't want to call them "second order effects".</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">At low frequencys the leakage should be noticeable. </span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier; min-height: 19px;" class=""><span style="font-kerning: none" class=""></span><br class=""></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">m.c.</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier; min-height: 19px;" class=""><span style="font-kerning: none" class=""></span><br class=""></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Times;" class=""><span style="font-kerning: none" class=""><b class="">Haible Juergen</b> </span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Times;" class=""><span style="font-kerning: none" class=""><i class="">Thu Nov 5 16:14:36 CET 1998</i></span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier; min-height: 19px;" class=""><span style="font-kerning: none" class=""></span><br class=""></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier; min-height: 19px;" class=""><span style="font-kerning: none" class=""></span><br class=""></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">What I meant is "what tricks did they use to make them useful</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">*despite* all these effects you mentioned". Their circuit is not</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">just a naked npn and pnp, but some passive circuitry around</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">that. </span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">It's clear that the 3rd transistor is a temperature compensation</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">of sort, but how good is it ?</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">They *could* compensate for constant leakage (even if its quite</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">high) in linear VCOs (where you need an offset trimmer anyway),</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">but not if ageing plays a role ...</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">I mean, I know that SCRs have some lousy specs, but I'm after</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">the design tricks to make them usable for such a delicate </span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">application as VCOs nevertheless. It's done, and not too bad,</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">and my aim is to understand why and how it works. </span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">(Sorry that I have been a little unprecise in my first mail.)</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier; min-height: 19px;" class=""><span style="font-kerning: none" class=""></span><br class=""></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">JH.</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier; min-height: 19px;" class=""><span style="font-kerning: none" class=""></span><br class=""></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier; min-height: 19px;" class=""><br class=""><span style="font-kerning: none" class=""></span></div><div style="margin: 0px; font-size: 15px; line-height: normal; font-family: Courier; min-height: 19px;" class=""><span style="font-kerning: none" class=""></span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Times;" class=""><span style="font-kerning: none" class=""><b class="">My new VCO</b></span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Times;" class=""><span style="font-kerning: none" class=""><b class="">jh</b> </span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Times;" class=""><span style="font-kerning: none" class=""><i class="">Wed Jun 2 01:12:54 CEST 1999</i></span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier; min-height: 19px;" class=""><span style="font-kerning: none" class=""></span><br class=""></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">Hi,</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier; min-height: 19px;" class=""><span style="font-kerning: none" class=""></span><br class=""></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">I have attatched the schematics of my new VCO. </span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">It's "Poly Modular Compatible" (if anybody is still interested),</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">i.e. V/Hz keyboard input (from a MUX'ed expo converter etc.),</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">and it has a tempco-less exponential current-in summing node for</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">exponential Modulation / Detuning etc.</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier; min-height: 19px;" class=""><span style="font-kerning: none" class=""></span><br class=""></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">But I've actually built this VCO for different reasons, namely</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">a little performance synth inspired by the old Korg Monosynths.</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier; min-height: 19px;" class=""><span style="font-kerning: none" class=""></span><br class=""></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">I've been bringing up the topic of thyristor-based VCOs on the</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">list some time ago, and I remember there were quite some</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">critical comments. But I wondered why Korg used them over</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">such a long span of time, in so many different products from their</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">earliest stuff up to the Trident. I thought they cannot be *that* bad</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">then. </span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">Though they were also used in true wide range VCOs (MS-20),</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">I guess that they really have their biggest advantage in the</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">upper frequency range: Very short discharge time. There is no</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">delay from a high impedance buffer, no comparator and monoflop,</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">just the two complementary transistors that immediately discharge</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">the capacitor when the voltage across them gets over the threshold.</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">So it's probably the best idea to use them in a configuration</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">like the 700 (Minikorg), 800DV etc.: Run them in the highest</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">footage (2'), and use a digital divider to generate the lower</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">octaves. This is no crappy staircase stuff, mind you. A SAW</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">wave is reconstructed for every footage by adding just the right</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">amount of the top octave SAW to the staircase. Apart from</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">*very* short glitches (inaudible ?) you get very nice waveforms</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">for all octaves. A bunch of cheap 1% resistors do the job. (Sorry</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">Jorgen B. - no E12 values; even with E24 I still needed some </span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">combinations.)</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">With the VCO running at rather high frequencies, a (possible)</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">leakage of the thyristor (is there any ? Martin ?) would not be</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">harmful either.</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">Remains the infamous Interlock problem. Everybody who played</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">an MS-20 knows how bad this can be. (Maybe it's part of the</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">MS-20's "thin" sound ?). In order to avoid the use of the unregulated</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">voltage for the buffer FET (MS-20) or a negative reference voltage</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">for the exponential converter (Polysix), I chose to generate two</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">positive auxiliary voltages, +5V and +10V, anyway. Now I</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">carefully designed that part to get a precise DC voltage, and very</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">strong AC decoupling at the same time: A slow dual opamp is used</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">for the reference voltages, with a tantal across the outputs to provide</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">the HF current spikes as good as possible. (the cap is large enough</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">to form a dominant pole with the opams' output resistance in order</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">to preserve stability.) In case this decoupling was not enough, I</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">have RC-decoupled *both*, the GND and +15V supplies of the</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">dual opamp. Only the resistor divider for the reference voltage is</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">connected directly to the rails to provide exact DC values. </span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">I can proudly report that this prevents any interlock. I've built two</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">of these circuits in close proximity on a single Eurocard (16x10cm),</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">and I have very pleasant fluctuations of slow beat rates in all</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">octaves, but no unwanted sync. Needless to say that a pair of these</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">VCOs has a built in "Linear Detune" feature without extra tricks:</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">You can set the constant term of the beat rate with the Offset trimmers,</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">and the frequency proportional term with the Tune trimmers (or with</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">a tiny DC voltage at the Expo Mod input).</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier; min-height: 19px;" class=""><span style="font-kerning: none" class=""></span><br class=""></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">No waveform converters in the drawing - of course you need these</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">in addition, depending on the desired VCO waveforms.</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier; min-height: 19px;" class=""><span style="font-kerning: none" class=""></span><br class=""></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">(BTW, if you don't like V/Hz, just use a tempco resistor and a better</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">expo converter.)</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier; min-height: 19px;" class=""><span style="font-kerning: none" class=""></span><br class=""></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">More notes about the circuit, especially the components, in the</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">drawing.</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier; min-height: 19px;" class=""><span style="font-kerning: none" class=""></span><br class=""></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">Hope you'll like it,</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier; min-height: 19px;" class=""><span style="font-kerning: none" class=""></span><br class=""></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">JH.</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier; min-height: 19px;" class=""><span style="font-kerning: none" class=""></span><br class=""></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Times;" class=""><span style="font-kerning: none" class=""><b class="">Ingo Debus</b> </span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Times;" class=""><span style="font-kerning: none" class=""><i class="">Mon Jun 7 16:30:30 CEST 1999</i></span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier; min-height: 19px;" class=""><span style="font-kerning: none" class=""></span><br class=""></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">Hi Juergen,</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier; min-height: 19px;" class=""><span style="font-kerning: none" class=""></span><br class=""></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">this reminds me of my very first "serious" DIY music project, the</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">Tuenker "Sound Organ". Oscillator was with SCR too, no VCO though. The</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">four sawtooth footages were generated the same way with resistor</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">networks. No ICs, two frequency dividers on one PCB.</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier; min-height: 19px;" class=""><span style="font-kerning: none" class=""></span><br class=""></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">Anyway, I have some questions (sorry if some of these have been covered</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">before):</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier; min-height: 19px;" class=""><span style="font-kerning: none" class=""></span><br class=""></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">What is the usable frequency range of the VCO itself (without frequency</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">divider), using the V/Hz input? Are the frequency dividers just to</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">select the footage or do their outputs have to be switched by the</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">keyboard, when a larger (88 keys) keyboard is used?</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier; min-height: 19px;" class=""><span style="font-kerning: none" class=""></span><br class=""></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier; min-height: 19px;" class=""><span style="font-kerning: none" class=""></span><br class=""></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">I think it's very useful when the footage outputs could be mixed too.</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">Especially if there were tri/sine converters for each footage, a organ</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">drawbar thing could be done.</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier; min-height: 19px;" class=""><span style="font-kerning: none" class=""></span><br class=""></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">However, if only one footage needs to be available at a time there's no</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">need for a complete resistor network for each footage. To get the next</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">lower footage just add the next lower square wave with double the</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">amplitude of the previous one. The Reset input of the frequency divider</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">IC could be used to disable/enable the squares. With a GAL as frequency</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">divider, the footage could be selected with static digital signals;</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">useful if several VCOs are ganged for a polyphonic system.</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier; min-height: 19px;" class=""><span style="font-kerning: none" class=""></span><br class=""></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">Of course the amplitude of the saw is twice as high then (could be</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">compensated with another switch scaling down the output level).</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">Instead of the resistor network, a multiplying DAC IC could be used. Not</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">necessarily cheaper, but maybe space saving. The top saw had still to be</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">added with a discrete resistor.</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier; min-height: 19px;" class=""><span style="font-kerning: none" class=""></span><br class=""></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">Even if all footages are available the same time, with an output buffer</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">for each footage, only two voltages need to be added for each footage:</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">the saw of the next higher footage and the square.</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier; min-height: 19px;" class=""><span style="font-kerning: none" class=""></span><br class=""></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier; min-height: 19px;" class=""><span style="font-kerning: none" class=""></span><br class=""></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">Another question: Is the + input of the TL071 really connected to two</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">caps only?</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier; min-height: 19px;" class=""><span style="font-kerning: none" class=""></span><br class=""></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">Ingo</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier; min-height: 19px;" class=""><span style="font-kerning: none" class=""></span><br class=""></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Times;" class=""><span style="font-kerning: none" class=""><b class="">Haible Juergen</b> </span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Times;" class=""><span style="font-kerning: none" class=""><i class="">Mon Jun 7 19:56:03 CEST 1999</i></span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier; min-height: 19px;" class=""><span style="font-kerning: none" class=""></span><br class=""></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class=""><span class="Apple-tab-span" style="white-space:pre"> </span>>this reminds me of my very first "serious" DIY music project, the</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class=""><span class="Apple-tab-span" style="white-space:pre"> </span>>Tuenker "Sound Organ". Oscillator was with SCR too, no VCO though.</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">The</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class=""><span class="Apple-tab-span" style="white-space:pre"> </span>>four sawtooth footages were generated the same way with resistor</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class=""><span class="Apple-tab-span" style="white-space:pre"> </span>>networks. No ICs, two frequency dividers on one PCB.</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier; min-height: 19px;" class=""><span style="font-kerning: none" class=""></span><br class=""></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">Real saw wave outputs or just staircase waveforms ? (This makes a big</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">difference)</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier; min-height: 19px;" class=""><span style="font-kerning: none" class=""></span><br class=""></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class=""><span class="Apple-tab-span" style="white-space:pre"> </span>>What is the usable frequency range of the VCO itself (without</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">frequency</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class=""><span class="Apple-tab-span" style="white-space:pre"> </span>>divider), using the V/Hz input? Are the frequency dividers just to</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class=""><span class="Apple-tab-span" style="white-space:pre"> </span>>select the footage or do their outputs have to be switched by the</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class=""><span class="Apple-tab-span" style="white-space:pre"> </span>>keyboard, when a larger (88 keys) keyboard is used?</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier; min-height: 19px;" class=""><span style="font-kerning: none" class=""></span><br class=""></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">It runs down to 0Hz. The closer you come to the offset voltage of opamps,</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">the larger will the relative error be, however. That's why Korg 700 etc</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">used the freq. dividers, and MS-20 etc switched different resistors for</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">fottage selection (to change the current, but keep the voltage the same !)</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier; min-height: 19px;" class=""><span style="font-kerning: none" class=""></span><br class=""></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">I've tested the circuit over approx 6 Octaves. No problem here. You have</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">to adjust the offset voltage of the opamps, of course. It's easy: zero</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">input voltage -> adjust for zero frequency (in practice, adjust for one</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">click every few seconds for the 2' footage)</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier; min-height: 19px;" class=""><span style="font-kerning: none" class=""></span><br class=""></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">LF411 will be better than TL071 in terms of drift.</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier; min-height: 19px;" class=""><span style="font-kerning: none" class=""></span><br class=""></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier; min-height: 19px;" class=""><span style="font-kerning: none" class=""></span><br class=""></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class=""><span class="Apple-tab-span" style="white-space:pre"> </span>[good divider ideas snoípped]</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier; min-height: 19px;" class=""><span style="font-kerning: none" class=""></span><br class=""></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class=""><span class="Apple-tab-span" style="white-space:pre"> </span>>Another question: Is the + input of the TL071 really connected to</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">two</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class=""><span class="Apple-tab-span" style="white-space:pre"> </span>>caps only?</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier; min-height: 19px;" class=""><span style="font-kerning: none" class=""></span><br class=""></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">No. </span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">The GND connection here.</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier; min-height: 19px;" class=""><span style="font-kerning: none" class=""></span><br class=""></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">JH.</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier; min-height: 19px;" class=""><span style="font-kerning: none" class=""></span><br class=""></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Times;" class=""><span style="font-kerning: none" class=""><b class="">Understanding thyristors (?) (was:AW: My new VCO)</b></span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Times;" class=""><span style="font-kerning: none" class=""><b class="">Haible Juergen</b> </span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Times;" class=""><span style="font-kerning: none" class=""><i class="">Wed Jun 2 17:53:56 CEST 1999</i></span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier; min-height: 19px;" class=""><span style="font-kerning: none" class=""></span><br class=""></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class=""><span class="Apple-tab-span" style="white-space:pre"> </span>> is this chinese or what? :)</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier; min-height: 19px;" class=""><span style="font-kerning: none" class=""></span><br class=""></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">Japanese, actually. Designed by the clever people of Korg. (;->)</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier; min-height: 19px;" class=""><span style="font-kerning: none" class=""></span><br class=""></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">But seriously, I'm not sure if I have understood the circuit myself.</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier; min-height: 19px;" class=""><span style="font-kerning: none" class=""></span><br class=""></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">I'm not that familiar with thyristors at all.</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">What I *think* to know is this:</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">If you connect a npn and pnp to a typical scr circuit (base to collector,</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">and vice versa), it would normally not conduct, even if there is a voltage</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">applied between the two emitters (what you'd call anode and cathode</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">on a scr). Only if you have a trigger pulse of the right polarity on either</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">base (what you'd call gate then), one transistor would conduct, and </span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">make the other one conduct as well, even if the gate pulse is not there</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">anymore. The circuit would lock up in a conductive state. Only when the</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">current thru the scr would get close to zero because of external reasons</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">(e. g. the external voltage breaking down because a capacitor is</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">discharged).</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">This would end the locked state, and even when the external voltage is</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">growing again, another gate pulse is needed to start the action again.</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier; min-height: 19px;" class=""><span style="font-kerning: none" class=""></span><br class=""></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">Now there would be another way to trigger the scr - I think it's called</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">"über Kopf zünden" in German (is that "overhead trigger" (?) ): When</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">an external voltage would exceed the BC reverse breakdown voltage of one</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">of the transistors, the current would start to flow even without a gate</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">pulse.</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">(Please note that this is more a *question* than an explanation - I really</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">don't know for sure.)</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier; min-height: 19px;" class=""><span style="font-kerning: none" class=""></span><br class=""></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">Anyway, if you look at the VCO circuit, neither of these mechanisms would</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">happen, would it ? No external gate pulses, and no excess voltage.</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">So I suspect that the leakage currents of the transistors would be *used*</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">for the desired operation. If you remove Q3, wouldn't the tinyest current</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">from the collector of Q2 be amplified in Q1, and again amplified in Q2,</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">and the circuit would lock up at once ? </span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">This would be prevented by Q3. I'd say there is a *current* divider built</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">from Q1 and Q3, the current distribution being determined by the voltages</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">on the emitters (E) of Q1 and Q3. As long as E(Q1) is more positive</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">than E(Q3), the vast majority of current would flow thru Q3 and prevent</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">the positive feedback described above. Only when E(Q1) - the capacitor</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">voltage ! - comes down near +5V, the current amplification circle would</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">begin.</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">Now the cap becomes discharged, E(Q1) rising again. But as we now have</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">*high* currents to distribute between Q1 and Q3, the resistor R6 would</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">prevent</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">the most of the current to flow thru Q3, and the Q1/Q2 circle is not broken.</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">You'll get a voltage pulse across R6, however, which is used to trigger</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">the CMOS divider.</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier; min-height: 19px;" class=""><span style="font-kerning: none" class=""></span><br class=""></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">Does this make sense ? And is it different from "normal" thyristor action ?</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">(What prevents the leakage currents in a normal tyristor from forming</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">a positive feedback without external gate voltage, btw. ?)</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">And what is the purpose of the diode ?</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier; min-height: 19px;" class=""><span style="font-kerning: none" class=""></span><br class=""></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">I don't know if this shed some light. If not, I hope it will "trigger"</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">someone</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">else to put it right or explain it further. (I admit when I did my first</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">experiments</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">with these circuits, I was looking for a reverse breakdown to trigger it</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">(;->) )</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier; min-height: 19px;" class=""><span style="font-kerning: none" class=""></span><br class=""></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">JH.</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier; min-height: 19px;" class=""><span style="font-kerning: none" class=""></span><br class=""></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Times;" class=""><span style="font-kerning: none" class=""><b class="">Rene Schmitz</b> </span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Times;" class=""><span style="font-kerning: none" class=""><i class="">Thu Jun 3 01:47:11 CEST 1999</i></span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier; min-height: 19px;" class=""><span style="font-kerning: none" class=""></span><br class=""></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">How I think it is triggered: the "Gate" of the thyristor, the point where</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">D1 and Q1s base meet, is held at 5volts (well almost there is a drop at Q3</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">and R6), the current sink sinks towards 0V. Initially the cap is</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">discharged, both plates at 10V. When the cap reaches (5volts - Ube) of Q1</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">it starts to conduct. (A thyristor can be triggered from the anode too.)</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">Then the current thru Q1 turns on Q2 which in turn acts to raise the gate</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">potential, further increasing conduction in Q1 and so on... The two</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">transistors are very soon fully saturated, and now the cap can get</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">discharged very fast.</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">When the cap is discharged only small currents can flow and the thyristor</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">resets. I *guess* D1 is there to prevent the thyristor to keep conducting</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">on the current that the expo convertor sinks, by altering the turn off</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">current.</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier; min-height: 19px;" class=""><span style="font-kerning: none" class=""></span><br class=""></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">Bye</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class=""> René</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier; min-height: 19px;" class=""><span style="font-kerning: none" class=""></span><br class=""></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">Forgot to mention, that the leakage of Q2 cannot forward bias the BE</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">junction of Q1, since the emitter of Q1 is at a higher potential than the</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">base. With that diode back biased the current is forced to flow out of the</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">gate, or if the gate is open it cannot flow! </span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier; min-height: 19px;" class=""><span style="font-kerning: none" class=""></span><br class=""></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier; min-height: 19px;" class=""><span style="font-kerning: none" class=""></span><br class=""></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Times;" class=""><span style="font-kerning: none" class=""><b class="">VCOs (time to change the subject line (;->) )</b></span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Times;" class=""><span style="font-kerning: none" class=""><b class="">jh.</b> </span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Times;" class=""><span style="font-kerning: none" class=""><i class="">Mon Oct 2 12:35:50 CEST 2000</i></span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier; min-height: 19px;" class=""><span style="font-kerning: none" class=""></span><br class=""></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">><i class=""> Me either. A few years ago I asked a question here about what kind of</i></span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">><i class=""> temperature dependencies the Korg-style SCRs might have and I still</i></span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">><i class=""> haven't gotten or reasoned out the answer. I'd start by guessing that</i></span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">><i class=""> any temperature dependencies must cancel out in the various devices,</i></span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">><i class=""> otherwise it wouldn't be a practical VCO</i></span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier; min-height: 19px;" class=""><span style="font-kerning: none" class=""></span><br class=""></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">In practice, of all the synths I've built, the JH-720 with its MS-20 style</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">thyristor core is the most temperature stable by far. Switching it on</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">it is spot on tune to a few cents (compared to divide-down string</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">ensembles and organs at least). As you normally need 2 BJTs to emulate</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">a SCR, the 3rd BJT would be for temperature compensation of the</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">threshold.</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier; min-height: 19px;" class=""><span style="font-kerning: none" class=""></span><br class=""></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">Temperature stable as it is, the general "stability" may be inferior to</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">other</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">designs. It needs some "headroom" to start oscillating (That is, the fist</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">saw cycle after power on is larger then the normal ones), the series</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">resistor between the cascode FET and the composite SCR is for some reason</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">important, and using different BJTs makes a difference too.</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier; min-height: 19px;" class=""><span style="font-kerning: none" class=""></span><br class=""></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">I could spice it, I guess.</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier; min-height: 19px;" class=""><span style="font-kerning: none" class=""></span><br class=""></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">I did, and reporting PSpice resuluts with the usual grain of salt, I can say</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">the</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">reset times are really impressive. I don't have the numbers here, but from</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">memory: it was not as fast as Ian's latest saw VCO with 319 comparators,</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">but in a similar order of magnitude (and therfore better than many other</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">designs). Just think of it, in the JH-720 (and in the Mini/Maxi-Korgs) it</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">always runs in the highest octave (2'), with *no* means of HFT compensation.</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier; min-height: 19px;" class=""><span style="font-kerning: none" class=""></span><br class=""></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">Speaking of HFT compensation: I'm aware that it's not as crucial in linear</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">VCOs as in exponential ones. We don't have the Rbb error, but we still</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">have the reset time error, and (depending on the implementation) we also</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">have a latency time before a comparator or SCR or hysteretic switch</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">triggers.</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">Now how can we measure that latency time ??</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier; min-height: 19px;" class=""><span style="font-kerning: none" class=""></span><br class=""></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier;" class=""><span style="font-kerning: none" class="">JH.</span></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; font-size: 15px; line-height: normal; font-family: Courier; min-height: 19px;" class=""><span style="font-kerning: none" class=""></span><br class=""></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; line-height: normal; font-family: Courier; min-height: 19px;" class=""><br class=""></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; line-height: normal; font-family: Courier; min-height: 19px;" class=""><div style="margin: 0px 0px 0px 112.1px; text-indent: 2px; font-size: 14px; line-height: normal; font-family: Helvetica; min-height: 20px;" class=""><br class=""></div><div style="margin: 0px 0px 0px 112.1px; text-indent: 2px; font-size: 14px; line-height: normal; font-family: Helvetica;" class=""><span class="Apple-tab-span" style="white-space:pre"> </span><b class="">From: </b><span class="Apple-tab-span" style="white-space:pre"> </span> <a href="mailto:rburnett@richieburnett.co.uk" class="">rburnett@richieburnett.co.uk</a></div><p style="margin: 0px 0px 1px 112.1px; text-indent: 2px; font-size: 14px; line-height: normal; font-family: Helvetica;" class=""><span class="Apple-tab-span" style="white-space:pre"> </span><b class="">Subject: </b><span class="Apple-tab-span" style="white-space:pre"> </span><b class="">[sdiy] Roland 303 VCO questions??</b></p><p style="margin: 0px 0px 1px 112.1px; text-indent: 2px; font-size: 14px; line-height: normal; font-family: Helvetica;" class=""><span class="Apple-tab-span" style="white-space:pre"> </span><b class="">Date: </b><span class="Apple-tab-span" style="white-space:pre"> </span>15 August 2011 6:37:09 PM</p><div style="margin: 0px 0px 0px 112.1px; text-indent: 2px; font-size: 14px; line-height: normal; font-family: Helvetica;" class=""><span class="Apple-tab-span" style="white-space:pre"> </span><b class="">To: </b><span class="Apple-tab-span" style="white-space:pre"> </span> <a href="mailto:synth-diy@dropmix.xs4all.nl" class="">synth-diy@dropmix.xs4all.nl</a></div><div style="margin: 0px 0px 0px 112.1px; text-indent: 2px; font-size: 14px; line-height: normal; font-family: Helvetica;" class=""><span class="Apple-tab-span" style="white-space:pre"> </span><b class="">Reply-To: </b><span class="Apple-tab-span" style="white-space:pre"> </span> <a href="mailto:rburnett@richieburnett.co.uk" class="">rburnett@richieburnett.co.uk</a></div><div style="margin: 0px 0px 0px 112.1px; text-indent: 2px; font-size: 14px; line-height: normal; font-family: Helvetica; min-height: 20px;" class=""><br class=""></div><div style="margin: 0px 0px 0px 112.1px; text-indent: 2px; font-size: 14px; line-height: normal; font-family: Helvetica; color: rgb(150, 33, 148);" class="">i have been looking at this schematic all day and I would really</div><div style="margin: 0px 0px 0px 112.1px; text-indent: 2px; font-size: 14px; line-height: normal; font-family: Helvetica; color: rgb(150, 33, 148);" class="">like to understand how it works.</div><div style="margin: 0px 0px 0px 112.1px; text-indent: 2px; font-size: 14px; line-height: normal; font-family: Helvetica; color: rgb(150, 33, 148);" class="">anyone want to take a shot at it?</div><div style="margin: 0px 0px 0px 112.1px; text-indent: 2px; font-size: 14px; line-height: normal; font-family: Helvetica; color: rgb(150, 33, 148);" class="">the schematic I am working from is HERE:</div><div style="margin: 0px 0px 0px 112.1px; text-indent: 2px; font-size: 14px; line-height: normal; font-family: Helvetica; color: rgb(150, 33, 148); min-height: 20px;" class=""><br class=""></div><div style="margin: 0px 0px 0px 112.1px; text-indent: 2px; font-size: 14px; line-height: normal; font-family: Helvetica; color: rgb(150, 33, 148);" class=""><a href="http://www.ladyada.net/media/x0xb0x/mainboard_beta.png" class="">http://www.ladyada.net/media/x0xb0x/mainboard_beta.png</a></div><div style="margin: 0px 0px 0px 112.1px; text-indent: 2px; font-size: 14px; line-height: normal; font-family: Helvetica; color: rgb(150, 33, 148); min-height: 20px;" class=""><br class=""></div><div style="margin: 0px 0px 0px 112.1px; text-indent: 2px; font-size: 14px; line-height: normal; font-family: Helvetica; color: rgb(150, 33, 148);" class="">I am interested in parts in the VCO, basically IC11B to a little</div><div style="margin: 0px 0px 0px 112.1px; text-indent: 2px; font-size: 14px; line-height: normal; font-family: Helvetica; color: rgb(150, 33, 148);" class="">bit past Q24</div><div style="margin: 0px 0px 0px 112.1px; text-indent: 2px; font-size: 14px; line-height: normal; font-family: Helvetica; min-height: 20px;" class=""><br class=""></div><div style="margin: 0px 0px 0px 112.1px; text-indent: 2px; font-size: 14px; line-height: normal; font-family: Helvetica;" class="">IC11B is the VCO CV buffer from the R-2R ladder DAC. It also forms part of the</div><div style="margin: 0px 0px 0px 112.1px; text-indent: 2px; font-size: 14px; line-height: normal; font-family: Helvetica;" class="">portamento circuit with analogue switches IC12C/D and capacitor C35 to limit CV</div><div style="margin: 0px 0px 0px 112.1px; text-indent: 2px; font-size: 14px; line-height: normal; font-family: Helvetica;" class="">slew rate during programmed slides.</div><div style="margin: 0px 0px 0px 112.1px; text-indent: 2px; font-size: 14px; line-height: normal; font-family: Helvetica; min-height: 20px;" class=""><br class=""></div><div style="margin: 0px 0px 0px 112.1px; text-indent: 2px; font-size: 14px; line-height: normal; font-family: Helvetica;" class="">Matched transistor pair Q26 and it's servo amplifier IC11A convert the linear</div><div style="margin: 0px 0px 0px 112.1px; text-indent: 2px; font-size: 14px; line-height: normal; font-family: Helvetica;" class="">pitch CV from IC11B into an exponential current sink. This achieves the</div><div style="margin: 0px 0px 0px 112.1px; text-indent: 2px; font-size: 14px; line-height: normal; font-family: Helvetica;" class="">approximate 1v/octave law for the oscillator. The expo converter continuously</div><div style="margin: 0px 0px 0px 112.1px; text-indent: 2px; font-size: 14px; line-height: normal; font-family: Helvetica;" class="">drains current from capacitor C33 in order to make it's voltage ramp downwards</div><div style="margin: 0px 0px 0px 112.1px; text-indent: 2px; font-size: 14px; line-height: normal; font-family: Helvetica;" class="">linearly. Q24, 25 and 27 form a thyristor. This triggers when the voltage on</div><div style="margin: 0px 0px 0px 112.1px; text-indent: 2px; font-size: 14px; line-height: normal; font-family: Helvetica;" class="">C33 falls below a preset level determine by the 5.333V applied to Q24 emitter</div><div style="margin: 0px 0px 0px 112.1px; text-indent: 2px; font-size: 14px; line-height: normal; font-family: Helvetica;" class="">via R101. When this thyristor implementation fires Q27 and Q25 both turn on,</div><div style="margin: 0px 0px 0px 112.1px; text-indent: 2px; font-size: 14px; line-height: normal; font-family: Helvetica;" class="">with each providing the base current to keep the other turned on. This results</div><div style="margin: 0px 0px 0px 112.1px; text-indent: 2px; font-size: 14px; line-height: normal; font-family: Helvetica;" class="">in C33 being rapidly recharged up to approximately 12V via Q27 and Q25. They</div><div style="margin: 0px 0px 0px 112.1px; text-indent: 2px; font-size: 14px; line-height: normal; font-family: Helvetica;" class="">stay in the conductive state like a thyristor would stay latched until the</div><div style="margin: 0px 0px 0px 112.1px; text-indent: 2px; font-size: 14px; line-height: normal; font-family: Helvetica;" class="">charging current into C33 falls below their holding current (typically a mA or</div><div style="margin: 0px 0px 0px 112.1px; text-indent: 2px; font-size: 14px; line-height: normal; font-family: Helvetica;" class="">so.) Then both transistors drop out of conduction and the expo current sink</div><div style="margin: 0px 0px 0px 112.1px; text-indent: 2px; font-size: 14px; line-height: normal; font-family: Helvetica;" class="">causes the capacitor voltage to ramp downwards again at a rate determined by the</div><div style="margin: 0px 0px 0px 112.1px; text-indent: 2px; font-size: 14px; line-height: normal; font-family: Helvetica;" class="">pitch CV.</div><div style="margin: 0px 0px 0px 112.1px; text-indent: 2px; font-size: 14px; line-height: normal; font-family: Helvetica; min-height: 20px;" class=""><br class=""></div><div style="margin: 0px 0px 0px 112.1px; text-indent: 2px; font-size: 14px; line-height: normal; font-family: Helvetica;" class="">So in summary, you really have an exponential current sink centred around</div><div style="margin: 0px 0px 0px 112.1px; text-indent: 2px; font-size: 14px; line-height: normal; font-family: Helvetica;" class="">Q26/IC11A, and a crude voltage comparator/reset circuit based around Q24, Q25,</div><div style="margin: 0px 0px 0px 112.1px; text-indent: 2px; font-size: 14px; line-height: normal; font-family: Helvetica;" class="">and Q27. This gives you a raw downwards ramping sawtooth with fixed amplitude</div><div style="margin: 0px 0px 0px 112.1px; text-indent: 2px; font-size: 14px; line-height: normal; font-family: Helvetica;" class="">and exponentially conforming frequency.</div><div style="margin: 0px 0px 0px 112.1px; text-indent: 2px; font-size: 14px; line-height: normal; font-family: Helvetica; min-height: 20px;" class=""><br class=""></div><div style="margin: 0px 0px 0px 112.1px; text-indent: 2px; font-size: 14px; line-height: normal; font-family: Helvetica;" class="">Q28 acts as a high input-impedance unity-gain buffer to buffer the voltage on</div><div style="margin: 0px 0px 0px 112.1px; text-indent: 2px; font-size: 14px; line-height: normal; font-family: Helvetica;" class="">the integrating capacitor before passing the sawtooth output on to the VCF or</div><div style="margin: 0px 0px 0px 112.1px; text-indent: 2px; font-size: 14px; line-height: normal; font-family: Helvetica;" class="">square wave shaper. Q8 and it's associated circuitry form an over-driven</div><div style="margin: 0px 0px 0px 112.1px; text-indent: 2px; font-size: 14px; line-height: normal; font-family: Helvetica;" class="">common-emitter amplifier and act to shape the sawtooth waveform into something</div><div style="margin: 0px 0px 0px 112.1px; text-indent: 2px; font-size: 14px; line-height: normal; font-family: Helvetica;" class="">that is occasionally vaguely square in shape at some pitches! Then it's onwards</div><div style="margin: 0px 0px 0px 112.1px; text-indent: 2px; font-size: 14px; line-height: normal; font-family: Helvetica;" class="">to the VCF...</div><div style="margin: 0px 0px 0px 112.1px; text-indent: 2px; font-size: 14px; line-height: normal; font-family: Helvetica; min-height: 20px;" class=""><br class=""></div><div style="margin: 0px 0px 0px 112.1px; text-indent: 2px; font-size: 14px; line-height: normal; font-family: Helvetica;" class="">I hope this helps,</div><div style="margin: 0px 0px 0px 112.1px; text-indent: 2px; font-size: 14px; line-height: normal; font-family: Helvetica; min-height: 20px;" class=""><br class=""></div><div style="margin: 0px 0px 0px 112.1px; text-indent: 2px; font-size: 14px; line-height: normal; font-family: Helvetica;" class="">-Richie,</div></div><div style="margin: 0px 0px 0px 83.7px; text-indent: -0.6px; line-height: normal; font-family: Courier; min-height: 19px;" class=""><span style="font-kerning: none" class=""></span><br class=""></div><div style="margin: 0px 0px 0px 83.7px; 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