LM13700 VCA help

[fredrik]

If that’s not what’s happening, there’s something wrong.

I’m not sure how that circuit can even work (EDIT: with reasonable CV signals), since the IABC input is current driven and for the LM13700 the reference voltage is two diode drops above the negative rail, not 0 V. See the discussion here:

Simple but good dual vca

@Dud le Bricoleur built another copy of this, which reminded me of this thread and that strange power supply connection, and after skimming through some 70s opamp literature I think I’ve figured it out. Here’s the Iabc input on the CA3080: [image] The Iabc input (pin 5) controls the current through the differential input pair that handles the input signals (pins 2 & 3), via a transistor connected to the negative supply rail (pin 4). Note that this means that Iabc is held one diode drop abov…