This circuit was obtained from a Radio Electronics issue from January 1989. The indicator is a ceramic transducer, and the frequency of the emitted sound is given by C3.
In the figure we have the typical circuit of a Baxandall tone control applied to an operational...
This circuit is from a Japanese publication in 1967. The circuit uses a Geiger-Muller valve and...
Found in a 90s 303 Circuits, this circuit has an output power of 100 mW. L1 consists of 890 turns...