audio mixer diagram

3 Channel Audio Mixer Circuit

This audio mixer circuit uses an LM3900 IC but is not a profesional audio dj mixer. The IC houses four integrated Norton amplifiers. The advantage of using the four op amps is that they only need a single power supply. Since this amplifier circuit is current controlled, the DC bias is dependent on the feedback coupling.

The schematic diagram shows inverting AC-Norton amplifiers. The DC output must be set at 50 percent of the power supply. In this case, a maximum output can be achieved without distortion (also called symmetrical limitation through overdrive).
In designing this mini audio mixer circuit diagram you can freely choose the value of the resistor R2 (100k in the mixer schematic). Set the AC voltage amplification factor through the ration of R2/R1. To set the amplifier gain correctly, choose the value of R4=2R2 (double the value of R2).

LM3900 ping designations

Diagram 1.0 shows the 3-channel sound mixer circuit using three Norton-opamps. The input levels can be set by potentiometers P1 or P3. Furthermore, each input level can be trimmed with the help of trimmers pots P4 to P6 to adapt each input to the source. The resistors at the non-inverting inputs of the opamps work as DC bias and set the DC output at 50 percent of the power supply for this powered audio mixer. All three input signals are summed by the fourth opamp A4 through the resistors R3, R7 and R11. The commom volume level is cotrolled through the potentiometer P7.

You can switch an input channel on or off through the switches S1 and S3. An input channel is turned off when its switch is closed. It is also possible to replace these mechanical switches with transistor gates. By doing so, you can build an analog multiplexer circuit that can be easily expanded by several inputs.

For more audio mixers check the list bellow.

3 channel audio mixer PCB and Parts layout

3 channel audio mixer pcbaudio mixer parts placement

Audio Mixer external wiring layout

audio mixer external wiring

Audio Mixer circuit diagram

audio mixer diagram


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  • Steve Lawson

    The following comments will answer some of the questions posed here:

    Regarding the questions/comments about how the LM3900 is biased at the non-inverting inputs (Brett, Peter, Jose). This is not the type of op-amp that most people are familiar with. It is a “CURRENT differencing” amp, NOT a “VOLTAGE differencing” amp.

    Most people, who are quasi-into this stuff, are familiar with voltage differencing op-amps, such as the LM741 or LM340. These op-amps take the difference of the input voltages (i.e. the non-inverting voltage minus the inverting voltage) and multiply that by the open loop gain of the op-amp. The LM3900 is a “current differencing” op-amp (also called a “Norton Amp”). So, to answer Dharz’ question, no, the LM741 will not work in this circuit. The circuit, however, can be modified to work with the LM741 – check out page 9 (2.3 Summing) in the following PDF:

    • Daniel

      Which one is better for a car audio application. The current diff amp or the voltage diff amp, or maybe something else all together.

  • Chris`

    Can anyone tell me what type of capacitors to use in this circuit for the 0.4uF caps. I am having trouble finding 0.4uF caps, can I use 0.47uF ceramic disc caps? Thanks

  • Jose

    Like said on a previous post, the non inverting input needs to be set a half the power supply voltage, so I don’t think this circuit works.

    Instead you need a voltage divider, of 2 resistors of equal value and connect that to each of the opamps non inverting input.

    I don’t see where the gain is higher than one, the DC gain is one because of the coupling capacitor at the inverting input.

    • Miomir

      Please look at the datasheet of the LM3900. Circuit is properly biased and it’s working fine.

      I build two circuit for stereo mixer and I using it as input stage for my computer 2.1 audio amplifier. Now I can connect my phone, tablet and PC to one audio amplifier, and turn of my PC if I want to listen music (to save power and lower room temperature during summer).

    • Chris`

      Can you please tell me what type caps you used for the 0.4uF caps, were they ceramic disc, can I sub in 0.47 caps? Thanks

  • Peter

    There’s no way this can work.
    Simple reason. Supply Voltage applied to non inverting input has a gain higher than one. Means saturation

  • Riyas

    is this circuit is realy working? need urgent reply…tnx..

  • Dharz

    is this circuit is realy working? need urgent reply…tnx..

  • Dharz

    i just want to ask if i can use another IC (LM741) rather than to use the LM3900?

  • Gito

    I found exactly,the same circuit in elector july/aui\gust 1976.
    the use of the switch (S1,S2,S3) is completely to block any unwanted input when switch to ground.nd the value of R4,R8,R12 must have halve the value of R2,R6,R12 that,s 47 Kohm.


  • hassan sharif


    how can i itrface this circuit with my laptop? where i can find the software for controling it from my laoptop?


    • marius

      You cannot do this, this is not a PC controlled audio mixer, it is just a regular, analogic, mixer.

  • Brett

    This circuit didn’t look right to me and I’m quite sure it doesn’t work because the bias is wrong. I’ve breadboarded one input section and it doesn’t work. It requires an extra 220K resistor from the bias point to earth and as I see it the PCB cannot be easily modified to work. The PCB needs to be completely re-done.