This circuit was found in a Popular Electronics from the 90's. The output signal can be applied to a headphone or speaker. The microphone can be a piezoelectric tweeter capsule or other type.
This symmetrical 1.2 to 35 V source was found in a 1994 documentation. The integrated circuits are...
Found in a 90s Elektor, this mixer makes use of integrated circuits of the time. The supply must be...
This amplifier is specifically designed for portable radio and recorders with a 4 ohms load....