This circuit is from 1964 documentation but can be easily implemented based on current components. Input waveforms and detection are shown next to the diagram.
This circuit of a voice-controlled switch, Vox or sonic switch was obtained in a Popular...
I found this circuit in a 1992 Electronics Experimenter Handbook. The circuit can be easily...
This is the circuit of a commercial amplifier sold in Japan in the 1960s. The circuit for hearing aid...