The Curtis CEM3320 VCF IC brought relief to analog synth makers in the ’80s, especially polyphonic instrument builders. It was used in the Oberheim OB-8 and OB-Xa, PPG Wave 2, Sequential Circuits Prophet V and Prophet 10, Elka Synthex and many others. This implementation is an almost literal clone of the filter circuit in Sequential Pro-One (the poly Prophets’ little mono brother), with some control resistor values adjusted for a 12-volt supply and the expected control voltages in a modular system.
It features a 3 channel input mixer and 3 frequency modulation inputs, same as the original, and a resonance modulation input. A trim pot has been added to keyboard CV input so it can be adjusted to follow V/Oct (when the pot is at maximum). This way the filter can be used as a sine VCO when self-resonating. Another trim pot is used to set the lowest cutoff frequency (when the cutoff pot is at minimum).
3320 is wired as a 24dB low pass. The datasheet example uses 300pF filter caps (C2 to C5), Pro-One uses 150pF caps. I had only 220pF ones at hand, so I built the circuit with them and it sounded good. Later I tried with 150pF but decided that 220pF sounded better.
The actual IC I used was ALPHA RPAR AS3320, and I expect it will work just as well with the CoolAudio clone or the original CEM if you’re lucky enough to have one.