We found this circuit in an old American publication from the 1980s. Transistors can be general-purpose types like the BC548, and the tuning frequency is given by the L and C components.
We found this circuit for musical instruments in the March 1992 Radio Electronics magazine. The...
This circuit is from an American documentation of 1975. The FET may be the BF245 and the relay is of...
The figure shows three circuits for using headphones in amplifiers with speakers that do not have...