This hearing aid circuit offers the advantages of high sensitivity, low current consumption, low cost and light weight.
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.