This audio oscillator is from a 1957 Raytheon publication. The tone generated depends on C2 and C2 and the handset must be high-impedance magnetic. Equivalent transistors can be tested.
This circuit came out in an American magazine that no longer exists. It's the Electronic...
This circuit originally came out in a Budget Electronics from 1981. The American magazine no...
This is the traditional audio preamp configuration with the National Semiconductor LM382. The...