Moog Werkstatt: adding a proper CV input

Note: I make reference to the Moog Werkstatt schematics throughout. Copyright prevents me reposting them here; they can be found on Moog’s website.

The existing header on the Werkstatt allows for a VCO pitch CV to be patched in. Although the pitch can already be modulated by either the LFO or the EG (selected using a panel switch), the patch header input means you can use both modulation sources simultaneously – or an external CV, if you can cable it up.

When you start wanting to connect control sources to the Werkstatt, one problem is pretty obvious: the patch pin header provides a signal path, but there’s no ground. The user manual suggests hacking cables together, taking a ground from the cable to a screw on the case (or the ground on the audio output jack), but this isn’t a very neat solution. Better to add a proper CV input jack so you can directly and simply hook up your external CV source using standard cables.

Moog themselves (at the time of writing) do sell an add-on jack board, which provides both a row of minijacks and a signal ground, but I decided against buying it for two reasons: 1) it still doesn’t offer a true Gate input, which I felt necessary; 2) the jack board replaces the patch pin header – adding mods like mine means you can use them and the patch pins simultaneously, giving more possibilities.

How it Works

The circuit is very simple. Looking at p.2 of the official schematic, we can see the existing header CV input is mixed in via a resistor R46 and trimmer VR5. This trimmer can be carefully adjusted to give a 1V/octave response for your external CV.

It would be super-easy to simply wire a jack to the CV point on the header, but this has the disadvantage that inputs are not isolated from each other. Better (and still easy) is to replicate the two passive components and route them to the same mix point.

Here are my additions to the circuit:

Werkstatt CV modification schematic

Werkstatt CV modification schematic

Here is the mod in situ:

Werkstatt CV input mod (top)

Werkstatt CV input mod (top)

Werkstatt CV input mod, rear

Werkstatt CV input mod (rear)

The handiest solder points for connecting the extra components to the existing circuit are TP14 and TP10. Either will do:

Werkstatt CV input mod routing

Werkstatt CV input mod routing

The jack is wired to be brought to the side panel beneath the header. In this photo the Gate mod jack is also in place. The jack grounds are wired together, and then to a solder tag that connects to the nearby screw post. The existing screw is long enough to accommodate a washer or two:

Werkstatt mod ground point

Werkstatt mod ground point

Drilling the hole in the case is simple and quick, and a label finishes the job:

Werkstatt with CV and Gate mods

Werkstatt with CV and Gate mods

The accompanying Gate Input mod is also detailed on this site.

*

Parts Used

68k 1/4W 1% MF resistor
100k trimmer
1/8” panel mount socket
3mm solder tag
3mm washers (x2)
wire

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8 responses to “Moog Werkstatt: adding a proper CV input”

  1. boomfred says :

    As I said, I did this one, and the Gate input mod too, and it works perfectly. Thanks a lot. 🙂

  2. SteinAldermann says :

    Hey, i know I’m a couple of years too late, but i had a few questions.
    I did the mod, but i can’t seem to get it in tune. I have about one and a half octave per volt.

    I was going to do the gate mod too, but I didn’t have any transistors lying around so I set my sequencer s/trig instead (because that what the mod did right? made a short?)

    Could the way I did it affect the tuning to such a degree?

    • synthnerd says :

      The pitch CV input mod, as described, gives correct 1v/oct scaling for the VCO. If it’s built correctly and you feed it a correct voltage, it will work. Have you used the correct parts, and have you correctly adjusted the trimmer?

      As for the gate input mod, yes the mod does short the circuit to ground, but I’d be wary of just hooking up a sequencer directly to this point without that transistor.

  3. Martin says :

    Works well! I also added USB-support. Done with a small IC fron dspsynth.eu.

  4. Will says :

    I have a beginner question about this mod, if you don’t mind:

    “Moog themselves…do sell an add-on jack board, which provides both a row of minijacks and a signal ground, but the problem still remains of the lack of buffering”

    I’ve been looking over the schematic for both the werkstatt and this CV input mod, and I can’t figure out how/where the CV input mod adds a buffer that the add-on jack board doesn’t have. I don’t fully understand which circuitry constitutes a “buffer” here, though.

    Unfortunately I haven’t found a schematic for the add-on jack board, nor do I own one, but it kinda looks like it just extends the existing EXP_VCO_CV_IN pin to a jack, and also grounds that jack.

    So I can see how this CV input mod adds an isolated jack, but how exactly does it add a buffer that would be lacking from the add-on board?

    I hope that makes sense. Thanks for your time!

    • synthnerd says :

      Excellent question! Also excellent spotting of an oversight on my part. You’re absolutely correct that in the case of the CV input, my mod does not add a buffer at all. Actually the various pitch CVs are combined using an inverting op-amp summer, which means the different inputs don’t affect each other anyway, so an extra input buffer would be overkill. I’m no longer quite sure what I was referring to when I wrote that paragraph, so I’ve edited this page for clarity. Thanks! 🙂

      There are two reasons I decided to mod my Werkstatt rather than buy the jack board: 1) the jack board still doesn’t offer a true Gate input, so I needed to add that anyway; 2) the jack board effectively replaces the patch pin header, so you can’t use them simultaneously – adding mods like mine means you can still use the patch header as well.

      • Will says :

        Thank you! I’m planning to do both the CV and gate mods, and this is will help me better understand what’s going on in each. I definitely want to leave the pin headers accessible for arduino/breadboard projects and whatnot, and it’s good to know that I’m not missing something about how the pin is wired.

        Also good to know about how the inputs are combined, since I’m trying to learn about what the op-amps are doing at various stages in the signal path.

        Much obliged!

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