This circuit was found in a Hands On Electronics from the 90s. It turns on an LED to indicate that the phone is in use.
This circuit was found in an Electronics World magazine from 1990. The circuit can be mounted...
In the figure, from a documentation from the 70s, we have a few-watt amplifier for general use....
This circuit is from 1968 and consists of an amplifier for turntable of the time. The CA3020 was...