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    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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    22 Responses to "3 Channel Audio Mixer Circuit"

    1. Hello!
      I had just a stupid question. In the PCB diagram of this project I found there are two holes with no apparent function at all, one of them connected to the 14 IC pin and the other to ground. Is there any reason for this?
      Thank you very much

      • Looks like a perfect spot to add an LED w/current limiting resistor for power on indication (panel mount). This may have been the intent – smaller trace size and parallel to the power supply.

    2. 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.

    3. hassan sharif says: on February 18, 2010 at 10:40 am

      hi,

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

      thanks

    4. 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.

      Gito

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

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

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

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

    9. 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.

      @Peter:
      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.

      • 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).

      • Hello,
        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

    10. 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

    11. 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: http://www.eng.yale.edu/ee-labs/morse/compo/sloa058.pdf

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

    12. …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 ;)

    13. …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!

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

      • 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.

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

    15. 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.

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