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.
This circuit was found in an American publication from the 50's. The transistors are made of...
The circuit shown is an amplifier for medium impedance headphones including bass and treble...
This circuit for an optical audio link using LED or LASER was found in a 90s QST magazine. The circuit...