MS-20 Filter modified for 9v by Dirtbox Layouts

Hi all, I have question for you! I’m putting this together for a friend and am realizing how much hand-holding other wiring diagrams have done for me. This one is throwing me for a loop on how to wire the all the pots. I understand my output is coming from volume 2, and volume 1 is ground. Can someone help me understand where to go from here? Do I go to lug 2 of all the other pots? Ground their lug 1s? But that wouldn’t make sense considering Freq1 and Res1, and now it’s gibberish.

How to wire this dang thing? #Learning

My interpretation would be:

Hold the pot so the knob is facing towards you. The three legs sticking out below are 1,2, and 3. Don’t wire anything extra unless it tells you to in the text below. You don’t have to wire every leg every time.

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Thank you!!! Drawing this out and that makes the most sense to me. My only other question would be about grounding the pots? I have to do that right? So should I daisy-chain lug 1 from Volume to freq lug 1, then from that to res 1, and then to lfo 1?

Ohp, wait no, that doesn’t make sense. So I’m assuming it still holds to just wire as instructed. I just thought the rule about grounding pots was a thing :confused:

Just build it as it says and don’t do anything extra, it should be fine.


You got it. Thanks so so much!

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The original schematic is here


It shows all pots having all terminals connected. So does the stripboard layout, with 1, 2, and 3 connections for each pot. (See text below stripboard for Volume 2 and 3, as well as Switch 3 and 4.)

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Ahh! This is a big help too, thanks Rich!

Also, try to avoid daisy chaining the ground connections. It is preferable to use the so-called “star grounding” by having all the ground connections routed to a single spot, which in turn goes to the negative pole of the 9V battery.

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As mentioned over there, there are major differences between the stripboard here and the MS-20 filter version by Rene Schmitz, which I posted above. Partly because of the change from ±12 V power to +9 V but not entirely. The whole CV controlled current supply is removed, along with the CV input, there’s just a pot and a fixed resistor connecting +9 V to the LM13700 (which is standing in for the two CA3080s).

Specifically there’s a 100k pot and a 100k fixed resistor, so the maximum current is 9 V / (100k to 200k) = 45 to 90 µA, which is… wow, small. The LM13700 has a maximum control current of 2 mA (more will destroy it) and Schmitz uses 10k with a 15 V maximum voltage so that’s 1.5 mA maximum. But I guess for a guitar processor you want smaller output level so smaller control current. Maybe it makes sense.

On the signal input there’s a 100k/220k voltage divider, compared to 10k/220R in Schmitz’s. You want around 50–100 mVpp maximum on the input pin (see here) so Schmitz’ divider is really not so good for ~10 Vpp synth signals: 10(220/(10k+220)) ~ 220 mVpp is too big. But on this stripboard the 100k/220k divider is only cutting the input voltage by 50%! So the input should be not above 75–150 mVpp. That’s awfully small even by guitar level standards. Nope, I misread it, sorry.

Disclaimer: I don’t design or build guitar pedals. Maybe I’m just failing to understand a few things here.

Added: The schematic reconstructed from the stripboard is something like this, unless I’ve made errors:

The input divider is 100k/1k so maximum signal input is about 5 Vpp or higher, so no problem there.

In addition to the differences with the Schmitz circuit noted above, you can see the resonance circuitry has been reduced to an attenuator. In particular no clipping diodes to give the characteristic MS-20 resonance distortion. Altogether I’m doubting this circuit was based on Schmitz’s, and I’m not even too sure it was based on anything that could be called an MS-20 style filter. It’s more your basic vanilla LM13700 filter with single side supply.


I am so sorry I missed this! I am really glad to know your opinion on this filter as it compares to Schmitz’s. I greatly appreciate you taking the time on this as it has helped me decide to start building the Schmitz circuit. Thanks again @analogoutput

Hi, can someone help with the switch numbers, how do they connect to the dpdt? Is it like this?
Screenshot 2023-10-01 at 11.57.03

From datasheet :

Awesome, thank you! asd