In the figure we have a good rectangular signals generator with several frequency bands. The circuit is based on old logic but can be implemented with modern equivalents.
This is a possible setting for a Colpitts oscillator operating in the audio track. The shunt coil...
We found this circuit in documentation from the 1950s. The circuit basically applies to germanium...
This excellent 3-transistor audio preamplifier is suitable for use with high-impedance...