[sdiy] 4-Quadrant VCA?

[Olivier Gillet]

> I know I could use AD633s but they are really pricey. Any other clever ways
> of pulling this off?

Add enough DC offset to one of the signals to make it unipolar.
Subtract the other signal from the result. Choose wisely which branch
of your circuit is labelled as the "control" path and which one is the
"audio" path.

https://forum.mutable-instruments.net/t/blinds-how-does-it-work/8287
https://mutable-instruments.net/modules/blinds/downloads/Blinds-v60.pdf

Pros: no distortion, no odd behaviour at zero-crossings, doesn't cost
more than a normal VCA circuit.
Cons: need a couple of precision resistors, non-ideal behaviour at
high frequencies.