Found in a 1968 American documentation, this circuit uses an operational amplifier that is uncommon today. The feed must be symmetrical. The gain is 40 dB.
The oscillator shown in the figure keeps its frequency constant in an active cycle adjustment...
For bench use, this circuit functions as a bistable safety switch. The circuit is from a Radio...
In fact, this is the circuit also known as “vox”, that is, a relay triggered by sound. This...