<div dir="ltr"><div dir="ltr">
Actually, crappy oscillators prone to pitch variations are a feature... don't fuck with it. It's very musically useful and people pay crazy money for stuff like that when it's vintage and has "the sound". I'll be blunt, the only reason you think this doesn't sound cool is because you're not listening to it in a mix. I know two different songs off the top of my head that specifically create that sound that you have in the mp3 and are absolute bangers that were played incessantly and constantly by everyone for a good decade or so. For a musician, listening to VCOs alone in a sterile recording is audio necrophilia, it's very similar to those people who go to hi fi tradeshows, walk up to a $100k hi fi, and put in a cd with full-range sweeps from 0 to 22.050 kHz (the 050 is so the bats can enjoy it too).<br></div><div dir="ltr"><br></div><div>Instead, buy/build a different VCO, which is properly decoupled.</div><div><br></div><div>The value of a drifty VCO becomes obvious only if you have a stable VCO playing together with it.</div><div><br></div><div>You'll want the drifty VCO on bass lines and bass drums as well as on pads.</div><div><br></div><div>I haven't read the whole thread, but when decoupling, there's something that a lot of people forget.</div><div><br></div><div>A decoupling capacitor works because it essentially forms a first-order low pass filter between the rails and its circuits. So what you have is:</div><div><br></div><div>V+ ----> series R/L 1 ----> cap to ground ----> circuit ----> series R/L 2 ----> ground</div><div><br></div><div>If there's no physical resistor in position 1, what you get is just a parasitic. That's a stupid design because it means the low pass filter isn't very effective.</div><div><br></div><div>Or otherwise you can think of it this way: when the node "circuit" is sourcing current, i.e. becomes a short circuit, you want it to be sourcing the current as much as possible from "cap to ground". To do this, it has to go through ESR:</div><div><br></div><div>cap to ground = positive terminal ----> ESR ----> capacitance ----> ESR ----> negative terminal</div><div><br></div><div>That ESR of a single electrolytic is usually on the order of 1-2 ohms.</div><div><br></div><div>If your parasitic resistance is 1-2 ohms too, then from the point of view of "circuit", looking towards "V+", you have two ways you can source current, and they are equivalent in difficulty, because they have the same impedance between where you are, and the charge you want to get. So "circuit" will source current equally from both. That's a mere 50% reduction in noise, meaning merely 6dB! That's shitty. What you want is for the current to be coming from "cap to ground" almost exclusively. That's what you do:</div><div><br></div><div>1. make "cap to ground" actually two capacitors, paralleled. One larger electrolytic, and one smaller ceramic. The ceramic will have low capacity, but what it will also have is very low ESR, on the order of 0.1 or less ohms, at higher frequencies than the electrolytic. This way you get a "compound capacitor" of sorts that has very low ESR on spikes, which are usually where most of the noise comes from anyways. The electrolytic handles lower frequencies which need higher currents. Every capacitor has its lowest ESR at a different frequency, and you want those ranges to be different so they complement each other, like the frequency bands of an equalizer. This is called a "bypass capacitor bank".<br></div><div><br></div><div>2. Add a 50-200 ohm resistor in "series R/L 1". Adjust to taste.</div><div><br></div><div>Now with a 200 ohm "series R/L 1" and a 0.1 ohm ESR "cap to ground" you have a 200+0.1 : 0.1 or 2001 : 1 reduction. That's a 66 decibel reduction, basically below noise floor, using parts that cost $0.01 total. The capacitor will have to charge up before "circuit" sees a voltage rail at full voltage, so don't play your synth within the first 0.1 seconds or so, unless you specifically want to go for that experimental sound, that is. There's value in that too. There should be a collaboration album of people playing synths within the first minute of turning them on, during winter.</div><div><br></div><div>I don't know why no one's telling you to use series resistors.<br></div><div><br></div><div><br></div><div>As regarding the potentiometers being referenced to the voltage rail directly, instead you can build a voltage reference... but this here should be plenty stable for you anyways, at least as a start.</div><div><br></div><div>But again. Just get a different VCO. You'll be missing the shitty one sooner than you think.<br></div><div><br></div><div><br></div><br><div class="gmail_quote"><div dir="ltr">On Mon, Feb 20, 2023 at 6:15 PM A.M. Barrio via Synth-diy <<a href="mailto:synth-diy@synth-diy.org">synth-diy@synth-diy.org</a>> wrote:<div><ul><li>To Mike Beauchamp, Chris McDowell, Roman Sowa: The VCO design comes from Moritz Klein's one. I'm attaching a schematic of the exact circuit I'm implementing. After reading all the responses it is quite obvious to me that it is VERY prone to changes in output frequency: the CV pots are tied directly to the rails, as well as the CD40106 IC in charge of generating the oscillation. Roman, I am taking absolutely no offense from your comment: it is indeed a bad design. I am not willing to improve <i>this</i> particular VCO design as it was just a test and a bad example of me being too excited to have my first panel mounted on my case. Please ignore the wasteful use of the 40106 and the crappy expo converter, much more care will be put into the next VCO I put together. About this: <i>
If you need help deciding which ones those are, post the VCO schematics.</i> Would you mind pointing them out? I mentioned a couple spots above, but I'd like to make sure I cover them all.
That being said, I'd love to hear about the reference voltages. Chris,
thanks for the IC examples, I'll check them out. Besides, I have some
spare 7805 regulators at home. Although they are quite big, would they
do the job for testing before I go ahead and grab some of the examples
you mentioned?
</li></ul></div><div><div><br></div><div>So, to summarize: I must look for a way to improve my bus board (uh consider spending some money on a proper PSU/bus board). I must plug 100uF to 470uF caps between each rail and GND on every module as well as the bus board. 100nF caps between each IC and GND are recommended. Sensitive parts of circuits should never be tied directly to the power rails, but to a voltage regulator instead. Would these measures remove (or at least notably reduce) the problem I'm facing?</div><div><br></div><div>Thanks a lot beforehand,</div><div><br></div><div>A.M. Barrio.</div><div><br></div></div><div>
<br><div class="gmail_chip gmail_drive_chip" style="width:396px;height:18px;max-height:18px;background-color:rgb(245,245,245);padding:5px;color:rgb(34,34,34);font-family:arial;font-style:normal;font-weight:bold;font-size:13px;border:1px solid rgb(221,221,221);line-height:1"><a href="https://drive.google.com/file/d/1EBAwnrlCLTUUEoa8b8sTuSTGzpBY5-kL/view?usp=drive_web" style="display:inline-block;overflow:hidden;text-overflow:ellipsis;white-space:nowrap;text-decoration:none;padding:1px 0px;border:medium none;width:100%" aria-label="VCO.png" target="_blank"><img style="vertical-align: bottom; border: medium none;" src="https://ssl.gstatic.com/docs/doclist/images/icon_10_generic_list.png"> <span dir="ltr" style="color:rgb(17,85,204);text-decoration:none;vertical-align:bottom">VCO.png</span></a></div>
</div></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><br><div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Sun, Feb 19, 2023 at 5:21 PM A.M. Barrio <<a href="mailto:albertomunozbarrio@gmail.com" target="_blank">albertomunozbarrio@gmail.com</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div dir="ltr">
<div>This is the complete email I wanted to send. There's another thread
where the attached files and additional info is missing. My
bad, pressed CTRL + Enter by mistake whoops. Please ignore that one! <a href="mailto:pata@ieee.org" target="_blank">pata@ieee.org</a> and <a href="mailto:mbryant@futurehorizons.com" target="_blank">mbryant@futurehorizons.com</a>, thank you for your replies, I have taken them into account :-)<br><div><div><table cellpadding="0"><tbody><tr><td><table cellpadding="0"><tbody><tr><td><br></td></tr></tbody></table></td></tr></tbody></table></div></div></div><div>------------------------------------<br></div><div><br></div><div>Hello,<br></div><div>I'm having an issue where the frequency of my
VCO is being affected by the operation of other modules. I'll give a
broader explanation:</div><div><br></div><div>I'm a beginner on synth
DIY, right now I have my home made case, a PSU, two VCOs, an LFO, a
clock module and an amplifier/speaker (built in the case, not
externally). Everything has been arranged by me, I haven't purchased any
module yet. The clock and the LFO are the latest modules I have built
(pretty much both at the same time), and when I tested them on their
own, they worked just fine. However I noticed that when they are
powered, the frequency of the VCO would stutter following the operation
of the clock and the LFO. <br></div><div><br></div><div>That would be: <b>without connecting the clock or the LFO anywhere</b>,
just having them powered up, whenever the clock is up or down, or the
waves generated by the LFO reset, the frequency of the VCO varies a tiny
bit. It gets more noticeable the higher the pitch of the VCO.</div><div><br></div><div>After
some head scratching I've realised that the signals generated by any of
the modules I have (both VCOs, the clock and the LFO) are somehow
leaking into the +12 and -12 rails of my PSU. Measuring any of the rails
on my scope (on AC mode, so only the noise is shown), I can see that
the waves generated by the modules I mentioned above are there as well
in the form of noise (around 20mVpp each of them). That explains the
little variations in frequency of the VCO. However I have no idea why
this is happening or how to fix it. I have tried with different PSUs I
have around:</div><div><br></div><div><ul><li>11V 0.750A SMPS into DCDC to get +12 and -12</li><li>Two 12V 1A SMPS together to get +12 and -12</li><li>Linear PSU with 7812 and 7912</li></ul><div>I
have the same problem<span id="m_-1581733828203025329m_-7389626369345500697goog_1466379976"></span><span id="m_-1581733828203025329m_-7389626369345500697goog_1466379977"></span> with all of them. I'm positive there's an issue
somewhere with a lack of filtering, but I don't know where, or why.</div><div><br></div><div>I'm
attaching a sample of the VCO output where the stutter can be heard and
the schematic of the LFO. The way the LFO is set up in terms of caps
and isolation can be extrapolated to every other circuit I have made, I
always place those 100nF caps between each rail and ground.</div><div><br></div><div><br><div class="gmail_chip gmail_drive_chip" style="width:396px;height:18px;max-height:18px;background-color:rgb(245,245,245);padding:5px;color:rgb(34,34,34);font-family:arial;font-style:normal;font-weight:bold;font-size:13px;border:1px solid rgb(221,221,221);line-height:1"><a href="https://drive.google.com/file/d/1ts1WGcyl2y1Ypp5eP79Ptb5fHXawetZh/view?usp=drive_web" style="display:inline-block;overflow:hidden;text-overflow:ellipsis;white-space:nowrap;text-decoration:none;padding:1px 0px;border:medium none;width:100%" aria-label="LFO (square).jpg" target="_blank"><img style="vertical-align: bottom; border: medium none;" src="https://drive-thirdparty.googleusercontent.com/16/type/image/png"> <span dir="ltr" style="color:rgb(17,85,204);text-decoration:none;vertical-align:bottom">LFO (square).jpg</span></a></div><br><div class="gmail_chip gmail_drive_chip" style="width:396px;height:18px;max-height:18px;background-color:rgb(245,245,245);padding:5px;color:rgb(34,34,34);font-family:arial;font-style:normal;font-weight:bold;font-size:13px;border:1px solid rgb(221,221,221);line-height:1"><a href="https://drive.google.com/file/d/1RrUNX_Of_RGLAwn2CyV7I5fxRrlHvfHL/view?usp=drive_web" style="display:inline-block;overflow:hidden;text-overflow:ellipsis;white-space:nowrap;text-decoration:none;padding:1px 0px;border:medium none;width:100%" aria-label="VCO stutter.mp3" target="_blank"><img style="vertical-align: bottom; border: medium none;" src="https://ssl.gstatic.com/docs/doclist/images/icon_10_generic_list.png"> <span dir="ltr" style="color:rgb(17,85,204);text-decoration:none;vertical-align:bottom">VCO stutter.mp3</span></a></div><br></div><div><br></div><div>In case you have any idea what could be wrong, your reply will be greatly appreciated. Thanks beforehand!</div><div><br></div><div>Kind regards,</div><div><br></div><div>A.M. Barrio.<br></div><div><br></div><div>
------------------------------------
</div><div><br></div><div>In response to pata and Mike Bryant: <br></div><div><ul><li>Yes,
the current PSU I'm using can provide 8W in total. I'm only powering
one VCO and one LFO with some LEDs, power shouldn't be the issue in this
case. </li><li>I always place 100nF caps between each rail and GND on
every circuit/board. However I hadn't heard of placing caps on each IC. I
suppose it should be between their power pins and GND? I will have a
look at capacitance multipliers as well as I haven't heard about them
before.</li></ul></div></div>
</div>
</blockquote></div>
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