The Mini-3 VCA

The VCA is built using a OTA as the input resistor in a inverting amp configuration. PIN 7 of the TLO72 is the summing point. PIN 8 of the TLO72 is connected to the constant-current source made up of the NPN and PNP transistors. As the voltage increases the current supplied to the OTA increases and the resistance is lowered raising the gain of the op-amp.

Testing The VCA

Apply +- 12VDC to the curcit, then hook the output to a monitor ampand the input to a audio source.

You should hear the volume raise/lower by ajusting the INIT LEVEL trimmer.