This circuit was found in Sgnetics documentation from the 80's. The circuit also has loudness control. The power supply must be symmetrical.
Using SCRs, this circuit from a 1967 General Electric manual allows full wave load control with SCRs....
This interesting circuit from a publication from the 1970s, measures the intensity of the sound...
This simple NPN and PNP transistor tester configuration can use the low current range of a common...