This simple circuit has good gain and can be implemented with any general-purpose transistor. The circuit is from Electronics Now, June 1997. The coil determines the frequency range.
This circuit generates sinusoidal signals between 300 and 3000 Hz. We find a 303 Circuits from the 90's....
This amplifier for audio transmission lines was found in a 1961 American publication. The critical...
Old phones with dials can be used in a simple home intercom system, as this circuit from Popular...