This circuit consists of a stereo audio compressor. We found it in Radio Electronics magazine from October 1968. The circuit uses an opto-coupler made with a neon lamp and LDR.
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...