Moog Werkstatt: adding a proper Filter 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.
After the VCO Frequency CV and Gate inputs, perhaps the next most useful control we can modify is the VCF cutoff frequency. The Werkstatt already has switches to select either LFO or EG filter modulation in positive or negative amounts. Many synthesizers also have a Keyboard Tracking control which routes the CV generated by the pitch control source to the filter cutoff, allowing the filter to open up as higher notes are played. The amount of this modulation is often governed by a pot — giving continuous variable control — but is also often implemented with a switch — giving either preset amounts of modulation, or at its most basic just on/off (that is, 100% or nothing). At 100% Keyboard Tracking, a self-resonant filter can be used as a sine wave oscillator, the pitch of which will follow the keyboard.
The Werkstatt’s filter has a CV input on the header, which is fine for simple self-patching, but two problems show themselves when you want to control this parameter from an external source: firstly, the necessity of hacking a cable together as described previously; secondly, the accuracy of tracking. The Werkstatt’s existing filter CV input point does not, in my experience, give accurate 100% tracking from an external V/oct CV, which spoils sounds that require the resonance to boost harmonics that are locked to note pitch.
The mod below overcomes these problems by giving the Werkstatt a separate, tunable Filter CV Input that can be trimmed to give suitably accurate pitch tracking.
How it Works
As with the Pitch CV Input mod, we’re going to simply duplicate the existing control input and make a slight alteration. The existing header input for cutoff control mixes its CV via a 47.5k resistor. In order to be able to give tunable tracking, this mod is going to use a 43k resistor and a 10k variable trimmer in series. 100% tracking should be somewhere towards one end of the trimmer’s range.
Solder the two extra components to the board, take the third leg of the trimmer to TP17 (purple wire in the photos below), and take the outer leg of the resistor to the input jack, which is mounted and grounded exactly as for the CV and Gate jacks.
Tuning the tracking is similar to tuning the pitch (a process described in the manual) — with the Werkstatt open, connect the external CV and play as normal, using full resonance on the filter, with the cutoff tuned so you can hear the pitch of its self-oscillation. Adjust the trimmer so the filter’s resonant frequency scales up the keyboard at the same rate as the note pitch — that is, two notes played an octave apart should give a resonant filter peak an octave apart.
Because the Werkstatt’s VCO cannot be silenced without modification, it might be easier to disconnect the pitch CV control while tuning the filter; alternatively, if you have a way of multing the pitch CV, connect it to both pitch and filter. You might try setting the filter to resonate at an obvious harmonic such as a 5th above the pitch, as any deviation in the tracking will result in some noticeable sonic artefacts.
Of course, you might not want a simple fixed 100% tracking filter. It would be possible to add a pot to allow the user to vary the tracking amount; you could install a switch to select between different resistors to give preset fixed trackings; you could route the pitch CV to a break-contact on the filter CV input jack so that it tracks by default unless a jack in insterted to over-ride it. My own mod is simple and quick and functional, and hopefully will provide a point of departure for your own experimentation.
43k 1/4W 1% MF resistor
1/8″ panel mount jack socket