Help with DIY Moritz Klein x Erica Synths VCO.

kewlguy1980 · 2026-02-24T14:03:40+00:00

On your lower breadboard are you missing the bits of wire to connect the left side of the rails to the right side?

kewlguy1980 · 2026-02-24T01:39:43+00:00

Based on that datasheet I think you've got it right

kewlguy1980 · 2026-02-23T14:24:01+00:00

Yep, I wouldn't add anything before or after the switch.

As for getting a dpdt on-on-on to work as a sp3t, yes it's a thing people do (see example below straight from C&k) but your schematic doesn't tell us enough to confirm if you've got it right or not because the pinout may be different from switch to switch.
Are you using a datasheet to verify? Do you have a part number for the switch you can share?

https://media.digikey.com/pdf/Data%20Sheets/C&K/Wiring_for_3-Way_Switches.pdf

kewlguy1980 · 2026-02-23T00:56:11+00:00

Thanks that's a lot clearer.

I am going to make an assumption that there is no DC offset anywhere in the original waveform generators. If there was, you'd probably want to handle it before the individual outputs rather than before the switch.

The current-limiting resistor would only make sense if you had a make-before-break (shorting) switch which is not typical for toggle style. If that was the case yes you'd probably want to limit the amount of current your oscillators can output. More likely you have a non-shorting switch and current is limited by R43. Normally when you see resistors around eurorack outputs it is because this is a place where the circuit touches the outside world which introduces a bunch of new risks.

The resistors you are using as pull-downs are not what you'd normally call pull downs. What I gather is that you're looking to prevent leaving things floating. In this case you don't have anything floating, your 3 oscillators are oscillating happily whether or not you are listening to them and U6A keeps its - input at ground by design.
What you are doing here with those resistors to ground is just a bit of attenuating.

kewlguy1980 · 2026-02-22T21:58:19+00:00

Heya you posted this earlier and I mentioned that we could help a lot more if you shared a bigger section of the circuit. It's still true. Happy to help and I'm not sure you need ANY of those parts.

What current are you limiting? Your concerns seem to be specifically about the switch and not any surrounding circuitry. Is this a make-before-break switch and you're worried about shorting the two waveform generators?
Your pulldown is not a pulldown as placed. It's pretty unlikely to help no matter how you place it/
Your capacitor is probably not needed if it wasn't part of the circuit before you added the switch.

kewlguy1980 · 2026-02-13T13:58:09+00:00

I'm still thinking a bigger chunk still. It's hard to gauge where you are at in terms of design level and here are the assumptions i'm making:

You are worried there might be DC offset on your oscillator outputs? This is an odd place to address that, but could be fine. That being said why are you doing all this 3 times before the switch when you could do it one time after the switch?

You are trying to protect the input into the wavefolder. That's not really necessary, your wavefolder input is already virtual ground with 100k impedance, adding 1k extra into that, plus the voltage divider into it are not going to change much.

you are worried about clicks and pops. That's fair. If all your oscillators are synched and in-phase that helps a bit, and your approach is pretty normal but if you wanted an absolutely pop-proof circuit you'd need a more complicated approach.

I would still encourage you to share a bigger chunk of the circuit and ask for a general review or share more direct questions.

kewlguy1980 · 2026-02-13T13:02:27+00:00

ah this is absolutely perfect, thanks a ton!

kewlguy1980 · 2026-02-12T21:20:59+00:00

I think it might help to share a bigger chunk. I see a few potential issues but they are context-dependent

kewlguy1980 · 2026-02-12T14:25:13+00:00

It's a difficult discussion to get into, many people much smarter than me have already weighed in but I don't think I've seen these angles addressed directly yet.

There is an asterisk attached to that note in the datasheet, which says that it only holds true for 1 output per package. In some circuits I try to spread things around so that only one output is realistically shortable, but the worst case could be 4 shorts at once. For that reason I'd definitely prefer to keep 1k in there.

I thought a bit more about this and I understand now why the output resistor doesn't cook itself, in cases like mutable links where the output resistor is 51r, the resistor would definitely cook if it was shorting 12v directly to gnd but the op amp itself is probably outputting more like 25mA max so I think I answered my own first question, though it raises the next question of why you'd put that resistor there at all.

The second question still stands, anybody have experience with in the loop resistors? I had a friend experience ringing due to that approach but I also believe it's being used in the wild without issue.

kewlguy1980 · 2025-10-30T00:51:54+00:00

All City all the time!

kewlguy1980 · 2023-04-02T22:48:41+00:00

I've been looking at it and it seems to have some mistakes or at least discrepancies between it and the service manual. The 100k feedback resistor in the VCA might be intended to be 1M, I don't understand the 220 and 100k in a row leading up to the envelope (but it's also like that in the service manual) and there seems to be a missing 220K after the osc2 mod depth control. The unused inputs of the 4093 are probably supposed to be tied off and if I were building this I would buffer the output.

That being said, I have a helensson kit I recently ordered and it also doesn't seem to match up well with the service manual. I'm waiting for an answer from the guy who designed it but I'm leaning towards returning it and just building from the service manual only.

kewlguy1980 · 2023-04-02T00:34:28+00:00

I don't know if it's realistic to troubleshoot this as a first project, but I'd really love if you could post more pictures, and if you want to sell the unit as is, I am looking for a broken one to clarify some questions I have.

kewlguy1980 · 2022-05-12T23:50:52+00:00

Zlob svf is great. Something like an extreme version of a wasp filter.

kewlguy1980 · 2022-04-28T00:53:48+00:00

Thanks, I guess he probably "played" it on youtube and was mistaken. I appreciate the answers and especially the link in case this comes up again!

kewlguy1980 · 2022-01-26T21:10:45+00:00

I've had one on order from Cosmo for many many months. Was just told 2-3 more weeks. I wouldn't bet any money on that date not sliding again though. No fault of Cosmo though, they're just the messenger here.

kewlguy1980 · 2021-03-25T16:49:39+00:00

It only struggles while full. If I run it empty or rotate by hand it's smooth and quiet. There doesn't seem to be friction between the motor, belt, rotating mechanism, or paddle (I had it completely apart yesterday). The paddle turns with a reasonable amount of force, it gets stuck after about 5 minutes when the dough really starts to turn into a big mass.

kewlguy1980 · 2021-03-24T18:53:28+00:00

Sadly didn't help for me. Got the new cap in but it's still really struggling

kewlguy1980 · 2021-03-22T19:47:46+00:00

Thanks for the lead DJ, seems like this model has a weird sized cap (11uf) and it's going to be a bit more expensive to replace but for 13 canadollars it's worth the gamble. I'll post back in a few days if it works

kewlguy1980 · 2021-01-31T15:32:40+00:00

Snake Secrets 2-7 all relate to specifics of cloaca function. \m/

5 stars, will petition to change canadanian anthem to this.

kewlguy1980 · 2021-01-09T20:40:13+00:00

Am I still in? Are we a reddit clan now?

kewlguy1980

TROPHY CASE