This circuit operates from 20 Hz to over 20 kHz and is easily tunable. We found it in a 70s documentation. The circuit can be implemented with modern operational amplifiers.
This circuit was found in Japanese documentation from the 1960s. It is a commercial circuit with...
This tone control was obtained in the Audio Handbook of the National Semiconductor of 1977. The...
This simple receiver for listening on the phone was obtained in a 1960 documentation. The circuit is...