Dual AS662D-based audio VCA.
This is a simple dual VCA for audio purposes with an input overload knob . The inspiration for this, came from SH101/MS-1 modding, just like my Model F filter. U
In due time there will be a more feature packed version with non-/inverting inputs and outputs, VC panning and Xfading, bipolar CV and offset. This Model A+ is going to be also 8HP, and thus a tad less luxurious in terms of space for knobs etc. If you’re after a plain “vibey” dual VCA, this “A nonplus” (lol) might do just the trick. Schematic and Gerbers are below, and I designed this for SMD parts (1206 for resistors, so not too tiny).
NB that the final design as linked below differs from the picture in that GAIN is renamed OLD (overload).
There are two VCAs in this module, vertically aligned. As for I/O, you have one CV input, one audio input and one audio output.
The OLD knob can overload the VCA by reducing the audio signal input attenuation. When this knob is at the leftmost setting, +5V CV give you unity gain (output level = input level). Higher CV input results in greater amplification. When you turn the OLD knob towards the right, you can make up for very quiet input signals (this reduces the input impedance from 100k to 50k) or, which is niiice, overdrive the VCA and add sweet overtones. I derived this from what is IMO the only sensible modification for the Roland SH101 – drive that VCA!!!
Here are some sounds from an earlier iteration. Listen what happens to the TRI wave when we pull volume up:
Schematic and Note on DC Coupling
This VCA is AC coupled but you can easily turn it into a VCA that is also able to process CV signals by soldering in wire in place of the four 10uf capacitors (C C2, C9, and C10).
As always, double check for errors before ordering PCBs and such. PCB is for SMD version and 1206 resistors/caps. BOM will follow.