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.

Circuit diagram.

3 input microphone preamplifier circuit


  • 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.



    • Seetharaman

      Hi Saha the problem is due layout and shielding problems. To avoid this add a ceramic disc capacitor of any value from 220PF to 1KPF in parallel with R2 R12 and R16. Add a 0.1 ceramic disc capacitor from IC pin 4 to 11, as close as possible to the IC. If you are using a medium impedance Magnetic Mic Reduce input resistance R1, R11 and R15 to 10K. If you are using high impedance Condenser mic R3, R10 and R13 to any value from 22K to 47K till you get a clear sound(may be your preamp input stage is getting over loaded with high input signal from the sound source)

  1. Seetharaman

    Hi Emmi approximately 100 ( 101 1 + Rf / Ri ) First stage gain is 101 and second stage gain is 1.
    Input and out put will be out of phase by 180 deg.

  2. Abdul Rashid

    I am pleased to get all the Electronic Circuits and the Communication Circuits ,I Need Following Lab.Manuals & Lab.Sheets for my Electronics Lab.1,at Riphah International University.I/14 Golra More,Islamabad Pakistan.
    (1) Electronic Circuits. (2) Electronic Devices (3)Linear Integrated Circuits (4) Network Analysis (5) Linear Circuit Analysis.

    Kindly if you can provide.
    Hoping a favourable Reply.
    Engineer Abdul Rashid.Electronic Lab-1,Email:-, Mobile No;- 0300-5048640. Office 92-051-5469641-6, Ext 240