This circuit for frequencies of 20 to 2 kHz was obtained in English documentation from 1966. The transistor can be any general purpose and powered by batteries.
Found in American documentation from the 90s, this circuit can use a BC548 instead of the original...
This circuit acts on the dynamic range of an audio signal, which can be useful for recording,...
This circuit doubles the frequency of a signal using a varicap. In this case, the input is 28 MHz,...