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 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 header connections provide a signal path, but there’s no ground. The user manual suggests cobbling interconnects together from hacked cables, taking a ground from the cable to a screw on the case (or the ground on the audio 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.
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 the problem still remains of the lack of buffering – the inputs and outputs are taken straight to the signal point, which may cause issues when mixing signals and/or there is a resultant loading of a signal in either direction. The mods here avoid these troubles.
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:
Here is the mod in situ:
The handiest solder points for connecting the extra components to the existing circuit are TP14 and TP10. Either will do:
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:
Drilling the hole in the case is simple and quick, and a label finishes the job:
The accompanying Gate Input mod is also detailed on this site.
68k 1/4W 1% MF resistor
1/8” panel mount socket
3mm solder tag
3mm washers (x2)