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.
The transformer of this unit found in an old Japanese documentation can have 22 to 25 V of...
This JFET amplification stage was found in the April 1993 issue of Electronics Now. The input...
This circuit with two power transistors was found in Radio Electronics magazine from October 1960....