[sdiy] OTA Iabc feeding

[Olivier Gillet]

The voltage at the Iabc pin is V- + 2 diode drops.

So if you want to only use a resistor between your CV source and the
Iabc pin, your CV source must be offset so that its minimum level (the
value that will, for example, completely shut off your VCA) is exactly
V- + 2 diode drops. That's not a very convenient voltage to deal with!

It's more convenient to provide a current to the Iabc pin with a
current source circuit - a circuit that is able to source a current
proportional to the voltage at its input, irrespectively of the
precise value of the voltage at the Iabc pin.