Audio signals are sensed by the condenser microphone and amplified by two op-amps built around 741 ICs. The first IC 741 (IC1) is wired as a high gain preamplifier, while the second IC 741 (IC2) is used as a low-gain, medium-power amplifier to drive the earphone. A 470-kilo-ohm preset (VR1) is used to control the gain, while a 1-kilo-ohm preset (VR2) is used as the output volume controller.
Assemble the circuit on a small general-purpose PCB and house it together with a 9V PP3 battery inside a cabinet the size of a cigarette pack.
The article was originally published in June 2010 and has recently been published