This circuit was found in 1980s American documentation. The transducer is a piezoelectric capsule and the power supply voltage can be between 3 and 18 V.
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...