[sdiy] Help with simple single supply VCA

[Olivier Gillet]

> Oliver , looking at that design trying to understand! what do you mean
> its referenced to 0v? you mean the + input is at half voltage? so if I
> connect the LM13700 to 0v without a diode and used this circuit , when
> my envelope hits 4.5v it will switch off?

Could you please post a schematic of your envelope generator?

What do you want the "rest" voltage of your envelope generator to be?
- If it is 0V (this is the case in my circuit since the envelope is
generated by a DAC powered by 0-5V), the bit of circuit I have given
works as is.
- If it is 4.5V, and the envelope swings between, say, 4.5V and 7.5V
(well inside the supply rails), you simply need to remove R3 in the
circuit I have given. In this case, Igain = (V_envelope - 4.5) / 10k,
so you get Iabc = 0 when V_envelope = 4.5V.

A cheaper and simpler solution for generating the control current, if
you don't care about having a "knee" in your response is to use a PNP,
emitter connected through a 10k resistor to the envelope generator,
base connected to whatever voltage you consider a "rest" voltage for
your envelope (4.5V if your envelope circuit is designed so that it
"rests" at 4.5V), collector to the OTA. The non-linearity is OK for
the resonance control of a VCF, but probably not acceptable for a VCA.
But you're aiming at cheap and low part count, right?

Olivier