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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  • gsavas

    Hi, I made this circuit and it seems that the switches don’t do anything. Did anyone else make this?

  • dannyelectro

    Hi, I’m looking to use bits of this circuit in a mixer design, would you recommend log taper or linear pots for the level inputs? I assumed they were linear and breadboarded this which worked well. Thanks.

  • Ajit Shinde

    I want to combine tow signals… one is my cell phones audio output and other will be mic and i want one output at end. how it is possible ?

  • Ajit Shinde

    I want to combine to signal… one is my cell phones audio output and other will be mic and i want one output at end. how it is possible ?

  • PHH

    Hello ,
    I need to mix some audio sources but my audio source has to run a long cable so It has much noise. Please recommend which Op-amps should I use CURRENT differencing amp or VOLTAGE differencing amp?

  • Tyler

    This works great. I used .47 electrolytic caps where the schematic calls for .4 caps. Thanks Steve for the helpful comments I am getting a hiss when using the LM3900 so I may look into some of the other op-amps you listed for a better quality and will probably try to make a 4 input mixer based on this concept for two stereo signals.

  • Partha

    one thing I have to ask..can I connect microphone into this mixer? and can I make it with 741 op-amp?please reply

    • Steve Lawson

      Partha, also take a look at my other replies (above) for slightly more detail.

    • Steve Lawson

      I’m not qualified to address the microphone portion of your question, Partha, but the 741 op-amp is a Voltage differencing amp whereas the LM3900 amps are Current differencing. A mixer can be made using the 741, but it’s a slightly different circuit and best done with a dual supply. Also, the 741 is rather noisy and thus will introduce hiss into the audio. There are other voltage differencing op-amps far better suited for audio design, such as the TL074C or NE5532P or OPA2340 or OPA4340 etc.

  • Steve Lawson

    …To answer Chris” question about the type/value of cap for the 0.4uF. Yes, you can use a 0.47uF capacitor. In fact, any value above 0.4uF will work. Polyester (Mylar) is the best choice. Polypropylene is good, too (except for their physical size), also Tantalum will work (be sure the get the polarity right – Neg towards the op-amp). Electrolytics will also do the job, but they will “dry out” over time and thus become high-pass filters. Ceramics can suffer from what is called the “microphone effect” because they can actually act like little microphones!

  • Steve Lawson

    …A Norton amp takes the difference of the current on the input and multiplies that by the open-loop gain. Thus, by connecting a resistor between the non-inverting input and the positive supply line (lets call that Rb) and then connecting another resistor, at half the value of Rb, from the output to the inverting input (lets call that Rf), the output is set to half the supply voltage. This happens because the output voltage rises until the current flowing into the inverting input is equal to the current flowing into the non-inverting input, which occurs when the voltage on the output is at ½ the supply voltage, because Rf is half the resistance of Rb and thus it takes half as much voltage to cause the same current to flow.

    This refutes Gito who claimed that Rb should be ½ the value of Rf (in which case, the op-amp output would need to rise to twice the supply voltage to cause an equal current to flow through Rf – clearly this is impossible, so the output will only rise as high as it is capable, which would be slightly less than the supply voltage (in other words it would be driven to the positive rail and stay there, as most of the signal levels that would be applied to the input, would not even come close to being able to drive the output below the positive rail, and even if there were such a remarkably high input level, the output signal would be SERIOUSLY clipped.

    So, to answer Dharz and Riyas: yes, this circuit really works, but only for Norton type op-amps (i.e. NOT the LM741 😉

  • 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.

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