This circuit makes use of a low impedance microphone. The relay is according to the supply voltage which can be between 9 and 15 V. The operational gain is approximately 100.
This circuit is from an Electronics Now from August 1995. The triac is according to the load that...
The figure shows an application of the LM377 in a small inverting stereo amplifier with output...
Found in an American 80's documentation, this circuit can use common FETs like the BF245 and cover...