<html><head><meta http-equiv="Content-Type" content="text/html; charset=us-ascii"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; line-break: after-white-space;" class="">Hah, wow. I have never seen the datasheet "voltage-controlled resistor" used! I admit I've never even gave it much thought. When I was new to all of this, and asked the community "hey, how do I make an OTA work like a voltage controlled resistor" the response was "well you can't, figure out how to need a VCA instead." wild. It works really well in the this context. <div class=""><div class=""><br class=""></div><div class="">Cheers, </div><div class="">Chris</div><div class=""><br class=""><div><blockquote type="cite" class=""><div class="">On Apr 30, 2020, at 3:20 PM, Kevin Walsh <<a href="mailto:kadub@outlook.com" class="">kadub@outlook.com</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><div style="font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration: none; font-family: Calibri, Helvetica, sans-serif; font-size: 12pt;" class="">Hi Colin,</div><div style="font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration: none; font-family: Calibri, Helvetica, sans-serif; font-size: 12pt;" class=""><br class=""></div><div style="font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration: none; font-family: Calibri, Helvetica, sans-serif; font-size: 12pt;" class="">Very nice.</div><div style="font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration: none; font-family: Calibri, Helvetica, sans-serif; font-size: 12pt;" class="">This looks like it's just what I have been looking for.</div><div style="font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration: none; font-family: Calibri, Helvetica, sans-serif; font-size: 12pt;" class="">Thank you very much.</div><div style="caret-color: rgb(0, 0, 0); font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration: none;" class=""><div style="font-family: Calibri, Helvetica, sans-serif; font-size: 12pt;" class="">I'll try it out this evening :D </div><div id="Signature" class=""><div class=""><br class=""></div><div class=""><div class="">Best Regards,</div><div class=""><br class=""></div><div class="">Kevin Walsh.</div><div class=""><br class=""></div><div class=""><br class=""></div><div class="">-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------</div><div class=""><br class=""></div></div></div></div><div style="caret-color: rgb(0, 0, 0); font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration: none;" class=""><div id="appendonsend" class=""></div><div style="font-family: Calibri, Helvetica, sans-serif; font-size: 12pt;" class=""><br class=""></div><hr tabindex="-1" style="display: inline-block; width: 643.859375px;" class=""><div id="divRplyFwdMsg" dir="ltr" class=""><font face="Calibri, sans-serif" style="font-size: 11pt;" class=""><b class="">From:</b><span class="Apple-converted-space"> </span>ColinMuirDorward <<a href="mailto:colindorward@gmail.com" class="">colindorward@gmail.com</a>><br class=""><b class="">Sent:</b><span class="Apple-converted-space"> </span>30 April 2020 18:07<br class=""><b class="">To:</b><span class="Apple-converted-space"> </span>Kevin Walsh <<a href="mailto:kadub@outlook.com" class="">kadub@outlook.com</a>><br class=""><b class="">Cc:</b><span class="Apple-converted-space"> </span>SDIY <<a href="mailto:synth-diy@synth-diy.org" class="">synth-diy@synth-diy.org</a>><br class=""><b class="">Subject:</b><span class="Apple-converted-space"> </span>Re: [sdiy] OTA to replace variable resistor</font><div class=""> </div></div><div class=""><div dir="ltr" class=""><div class="">Hi Kevin, not sure if this is helpful but you could check the befaco kickall module which has VC time on a simple decay (release?) envelope.<span class="Apple-converted-space"> </span><br class=""></div><div class="">See schematics<span class="Apple-converted-space"> </span><a href="https://befaco.org/docs/Kickall/Kickall_V1C_Schematic.pdf" class="">here</a>.</div><div class="">Colin<br class=""></div></div><br class=""><div class="x_gmail_quote"><div dir="ltr" class="x_gmail_attr">On Thu, Apr 30, 2020 at 4:01 PM Kevin Walsh <<a href="mailto:kadub@outlook.com" class="">kadub@outlook.com</a>> wrote:<br class=""></div><blockquote class="x_gmail_quote" style="margin: 0px 0px 0px 0.8ex; border-left-width: 1px; border-left-style: solid; border-left-color: rgb(204, 204, 204); padding-left: 1ex;"><div dir="ltr" class=""><div style="font-family: Calibri, Helvetica, sans-serif; font-size: 12pt;" class="">Hi all,</div><div style="font-family: Calibri, Helvetica, sans-serif; font-size: 12pt;" class=""><br class=""></div><div style="font-family: Calibri, Helvetica, sans-serif; font-size: 12pt;" class="">Thanks for the advice.</div><div style="font-family: Calibri, Helvetica, sans-serif; font-size: 12pt;" class=""><br class=""></div><div style="font-family: Calibri, Helvetica, sans-serif; font-size: 12pt;" class="">I have made a VC envelope generator using the as3310 but I think that's a bit fancy for a drum simulator. I like the open and closed logic of the original Thomas Henry circuit so I will keep all of that for this module too. I just want to be able to modulate the decay times. Nothing too complicated...</div><div style="font-family: Calibri, Helvetica, sans-serif; font-size: 12pt;" class=""><span style="font-family: Calibri, Helvetica, sans-serif; font-size: 12pt;" class=""><br class=""></span></div><div style="font-family: Calibri, Helvetica, sans-serif; font-size: 12pt;" class=""><span style="font-family: Calibri, Helvetica, sans-serif; font-size: 12pt;" class="">Ben, thanks. That was helpful information.</span></div><div style="font-family: Calibri, Helvetica, sans-serif; font-size: 12pt;" class=""><span style="font-family: Calibri, Helvetica, sans-serif; font-size: 12pt;" class="">I need to breadboard this yet but I think I have mostly figured it out now.</span></div><div style="font-family: Calibri, Helvetica, sans-serif; font-size: 12pt;" class=""><span style="font-family: Calibri, Helvetica, sans-serif; font-size: 12pt;" class=""><span style="font-family: "Times New Roman"; font-size: 12px; display: inline;" class=""><a href="http://tinyurl.com/y7pem69d" title="http://tinyurl.com/y7pem69d" target="_blank" class="">Simplified Falstad Simulation</a></span> - (simplified because my browser crashes when I try adding OTA's to the full circuit...) doh!</span><br class=""></div><div style="font-family: Calibri, Helvetica, sans-serif; font-size: 12pt;" class=""><span style="font-family: Calibri, Helvetica, sans-serif; font-size: 12pt;" class="">As you mentioned, the discharge rate is linear so I may need to keep looking for a better solution. I guess I'll give it a listen and decide.</span></div><div style="font-family: Calibri, Helvetica, sans-serif; font-size: 12pt;" class="">There's always the vactrol option...</div><div style="font-family: Calibri, Helvetica, sans-serif; font-size: 12pt;" class="">Maybe someone might enjoy this simulation of the trigger logic circuit so I'll link it in anyway but it's otherwise irrelevant :p </div><div style="font-family: Calibri, Helvetica, sans-serif; font-size: 12pt;" class=""><span style="font-family: "Times New Roman"; font-size: 12px; display: inline;" class=""><a href="http://tinyurl.com/ycvp5gv8" id="x_gmail-m_-3652080007391272438LPlnk378842" target="_blank" class="">http://tinyurl.com/ycvp5gv8</a></span><br class=""></div><div style="font-family: Calibri, Helvetica, sans-serif; font-size: 12pt;" class=""><br class=""></div><div style="font-family: Calibri, Helvetica, sans-serif; font-size: 12pt;" class="">Thanks again,</div><div class=""><div id="x_gmail-m_-3652080007391272438Signature" class=""><div class=""><div class=""><br class=""></div><div class="">Kevin Walsh.</div><div class=""><br class=""></div><div class=""><br class=""></div><div class="">-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------</div><div class=""><br class=""></div></div></div></div><div class=""><div id="x_gmail-m_-3652080007391272438appendonsend" class=""></div><div style="font-family: Calibri, Helvetica, sans-serif; font-size: 12pt;" class=""><br class=""></div><hr style="display: inline-block; width: 631.8125px;" class=""><div id="x_gmail-m_-3652080007391272438divRplyFwdMsg" dir="ltr" class=""><font face="Calibri, sans-serif" style="font-size: 11pt;" class=""><b class="">From:</b><span class="Apple-converted-space"> </span>Ben Bradley <<a href="mailto:ben.pi.bradley@gmail.com" target="_blank" class="">ben.pi.bradley@gmail.com</a>><br class=""><b class="">Sent:</b><span class="Apple-converted-space"> </span>28 April 2020 06:05<br class=""><b class="">To:</b><span class="Apple-converted-space"> </span>Kevin Walsh <<a href="mailto:Kadub@outlook.com" target="_blank" class="">Kadub@outlook.com</a>><br class=""><b class="">Subject:</b><span class="Apple-converted-space"> </span>Re: [sdiy] OTA to replace variable resistor</font><div class=""> </div></div><div class=""><font size="2" class=""><span style="font-size: 11pt;" class=""><div class="">I'm looking over your schematic, and it looks like a floating VC<br class="">resistor would be overkill, as each one requires two OTAs. Some<br class="">simpler circuitry with one OTA per voltage-controlled function, should<br class="">work.<br class=""><br class="">The Open Decay and Closed Decay variable resistors control the rate of<br class="">discharge of C45, going through each variable resistance and through<br class="">the respective diode and then to ground, either through Q1 or through<br class="">pin 3 of the 555, depending on whether pin 3 is high or low. Each of<br class="">these resistances can be replaced with an OTA, with the control input<br class="">current (controlled by an input voltage) determining the discharge<br class="">rate. The Clank Decay pot likewise determines the discharge rate of<br class="">C43, and can likewise be replaced with an OTA.<br class=""><br class="">There's a transistor, op-amp, diode and resistors involved with each<br class="">OTA to do voltage-to-current conversion and switch things, OTAs will<br class="">give linear discharge rates, as opposed to the exponential discharge<br class="">rates the pots give, and I don't offhand how much of a sound<br class="">difference that will make.. With the voltage drop across the diodes<br class="">and Q1, and emitter-follower transistors Q2 and Q4, it appears they're<br class="">not perfectly decaying to zero anyway.<br class=""><br class="">On Mon, Apr 27, 2020 at 9:46 PM Kevin Walsh <<a href="mailto:Kadub@outlook.com" target="_blank" class="">Kadub@outlook.com</a>> wrote:<br class="">><br class="">> Hi all,<br class="">><br class="">> I have never posted a question here but I have been subscribed for several years and I have learned a lot from reading the threads. Thank you all for this.<br class="">><br class="">> I am an sdiy hobbyist who enjoys making modules for my own use. I occasionally make music and I have played a few gigs out locally.<br class="">><br class="">> I have recently finished my own version of the Thomas Henry 'Clangora'. I couldn't find NE566's (VCO's) so I just swapped them out for LM13700's and done the same with the NE571 (VCA's) while I was at it.<br class="">><br class="">> I also added the XOR noise source and VCF's from Ken Stone's Cynare circuit.<br class="">><br class="">> It was looking like a huge module so I decided to not add CV control on version one. Now that it is finished I am really happy with how it sounds and I want to add CV control to the next version.<br class="">><br class="">> I know how to stick in a OTA VCA here and there to add CV control to the Sweep and FM etc but I can't get my head around how to use an OTA to replace the variable resistors that are controlling the open and closed decay times.<br class="">><br class="">> I know I could use a vactrol but these are pricey, bulky and slow etc so I'd rather not have to use them.<br class="">><br class="">> My guess after looking at the LM13700 datasheet would be that I need to use the 'floating vc resistor' configuration.<br class="">><br class="">> I am no EE so I am totally patching bits of circuits from my Thomas Henry books together and hoping it sounds good :D<br class="">><br class="">> Is it the 'floating VC resistor' configuration that I need to use in place of the open and closed decay resistors or is it something crazy complicated that's not in the datasheet?<br class="">><br class="">> Any advice on what I need to do here would be greatly appreciated.<br class="">><br class="">> Many thanks in advance for any advice.<br class="">><br class="">> Here's the trigger logic and envelope generator section of the original circuit which I downloaded in a PDF from Nuts&Volts (Nov2003):<br class="">><span class="Apple-converted-space"> </span><a href="https://i.imgur.com/GXw9KVR.jpg" target="_blank" class="">https://i.imgur.com/GXw9KVR.jpg</a><br class="">><br class="">> My sketchy block diagram:<br class="">><span class="Apple-converted-space"> </span><a href="https://i.imgur.com/absDij1.png" target="_blank" class="">https://i.imgur.com/absDij1.png</a><br class="">><br class="">> A video of my first version of this module:<br class="">><span class="Apple-converted-space"> </span><a href="https://youtu.be/mq05zgHSYso" target="_blank" class="">https://youtu.be/mq05zgHSYso</a><br class="">><br class="">> And my (slightly ridiculous) intended layout for the version two front panel:<br class="">><span class="Apple-converted-space"> </span><a href="https://i.imgur.com/n0LzEoq.png" target="_blank" class="">https://i.imgur.com/n0LzEoq.png</a><br class="">><br class="">> My whole synth (last year):<br class="">><span class="Apple-converted-space"> </span><a href="https://i.imgur.com/lcdscrw.jpg" target="_blank" class="">https://i.imgur.com/lcdscrw.jpg</a><br class="">><br class="">> Best regards,<br class="">> Kevin Walsh.<br class="">><br class="">> -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------<br class="">><br class="">> _______________________________________________<br class="">> Synth-diy mailing list<br class="">><span class="Apple-converted-space"> </span><a href="mailto:Synth-diy@synth-diy.org" target="_blank" class="">Synth-diy@synth-diy.org</a><br class="">><span class="Apple-converted-space"> </span><a href="http://synth-diy.org/mailman/listinfo/synth-diy" target="_blank" class="">http://synth-diy.org/mailman/listinfo/synth-diy</a><br class=""></div></span></font></div></div></div>_______________________________________________<br class="">Synth-diy mailing list<br class=""><a href="mailto:Synth-diy@synth-diy.org" target="_blank" class="">Synth-diy@synth-diy.org</a><br class=""><a href="http://synth-diy.org/mailman/listinfo/synth-diy" rel="noreferrer" target="_blank" class="">http://synth-diy.org/mailman/listinfo/synth-diy</a><br class=""></blockquote></div><br clear="all" class=""><br class="">--<span class="Apple-converted-space"> </span><br class=""><div dir="ltr" class="x_gmail_signature"><div dir="ltr" class=""><div class=""><div dir="ltr" class=""><div class=""><div dir="ltr" class=""><div class=""><a href="https://www.instagram.com/colinmuirdorward/" target="_blank" class="">https://www.instagram.com/colinmuirdorward/</a></div><div class=""><a href="https://www.instagram.com/colinmuirdorward/" target="_blank" class="">-<br class=""></a></div><div class=""><a href="https://www.instagram.com/ssdp_synthesis/" target="_blank" class="">https://www.instagram.com/ssdp_synthesis/</a></div></div></div></div></div></div></div></div></div><span style="caret-color: rgb(0, 0, 0); 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