Fig.1: the circuit is based on two low-cost dual op amps: IC1a & IC2. IC1a functions as a balanced to unbalanced preamplifier, while IC1b functions as a non-inverting amplifier with a gain of 46. IC2a, VR2, VR3 & VR4 make up the equaliser stage, while IC2b provides an out-of-phase signal for pin 3 of CON3.

Whether it is for karaoke, public address or for a band, a microphone connection to an amplifier is a basic requirement. This Balanced Microphone Preamplifier includes a 3-band equaliser and can be used to drive a guitar amplifier, any stereo amplifier or provide an additional channel for a public address amplifier.

Balanced microphones are desirable since they prevent the injection of hum and noise into the sound system. A balanced microphone has a 3-wire cable usually connected via XLR plugs and sockets. XLR pin 1 is the return or ground and the other two terminals (pins 2 & 3) are for the signals. The signals are in anti-phase; in other words when one line goes positive, the other line swings negative by the same amount. Any hum that is picked up along the lead is effectively cancelled because the same level of hum will be present in both signal lines.