This amplifier circuit with a low power MOSFET was found in Popular Electronics from the 90s. The circuit uses a small transformer of the type found in old transistor radios.
This power supply may have 5, 6, 9 or 12 V outputs depending on the 78XX and 79XX series...
This simple amplifier uses a microphone attached to the telephone. The 3-transistor circuit was...
This 80's circuit can be made with current comparators. Rectification accuracy is 1% DC up to 100...