This VCA was found in the July 1988 issue of Radio Electronics magazine. It uses an OTA op amp and has its gain adjusted by the voltage at the gain input.
The circuit shown is from an old European publication. In it we have the way to activate a relay...
We found this circuit in a European publication from the 1990s. Transistors are still common types...
This circuit was found in an American documentation and can be added to common amplifiers of any...