The circuit given here is of a three input mic mixer cum preamplifier using the IC LM348. The LM348 is a high gain, internally compensated quad operational amplifier with a class AB output stage. The IC has very low input supply current drain (0.6mA/ opamp) and operates from a dual power supply.
Out of the four op-amps inside the IC LM348, IC1a, IC1b and IC1c are wired as non inverting amplifiers and they serve as the input amplifiers for the corresponding mic channels. The output of these three amplifiers are tied together and connected to the inverting input of the IC1a which is wired as an inverting amplifier. IC1a mixes the signals from each channel and also works as the output stage.
- Assemble the circuit on a good quality PCB.
- Use +12/-12V DC dual supply for powering the circuit.
- The power supply must be well regulated and filtered.
- POT R5 to R7 can b e used for adjusting the gain of individual channels.