This circuit was found in a 90s Popular Electronics. It generates tones given by the oscillators and can be changed. The circuit already includes an amplification stage.
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...