I'm trying to do a simple audio mixer.

For that I tried 2 different circuits:

Circuit 1:

which uses LM3900 chip:


enter image description here

Circuit 2:

which uses PN2222A transistor:


enter image description here

This is my implementation of both circuits:

enter image description here

where I connected as inputs my smartphone and laptop, and the output is my headphones.

My problem is: Even though I can listen both inputs mixed, there is lot of noise. So, I'm wondering if there is another circuit I need to connect at some point, maybe to amplify something. Both circuits has the same output with same mixed audio and same noise, etc. I would say I cannot distinguish between the output of both circuits.

Any idea on how to get a clear output with no noise?


  • \$\begingroup\$ What signal sources are you mixing? Line level signals from phones, or microphones? What kind of microphone? Dynamic mics will need a very low noise amplifier stage ahead of the mixer. \$\endgroup\$
    – user16324
    Commented Mar 8, 2020 at 13:02
  • \$\begingroup\$ as I said on my post I'm using the sound output from my smartphone and my laptop. So, there are no microphones involved. \$\endgroup\$
    – davidesp
    Commented Mar 8, 2020 at 16:17
  • \$\begingroup\$ Then I expect you're hearing clipping, as well as the input signals' noise. And JRE's circuit is a good starting point. If it doesn't satisfy, describe what else you need it to do. \$\endgroup\$
    – user16324
    Commented Mar 8, 2020 at 16:19

2 Answers 2


You circuits all amplify the signal a great deal.

For your first circuit, the gain is set by the resistance of the potentiometer (maximum of 1k) and the feedback resistor (1M.) The minimum gain is like 1000. You're probably not getting that much gain, but it will amplify the noise from your signal source quite well.

Your first circuit has different gain for some inputs. I/P 1 and 2 are the same, and I/P 3 and 4 are the same.

Your second circuit doesn't do anything to explicitly set the gain - it amplifies for all the transistor can give you. It'll happily amplify any noise on the inputs to a crazy volume.

I expect the real problem isn't noise. It probably sounds like crap when there's music or voice on the inputs. That'd actually be distortion, but a lot of folks call it noise.

What you need is a simple mixer with no amplification.

You want something more like this:

enter image description here

Mixer with optional offset function and inverted/non-inverted outputs

Inputs J1 ... J3 are standard inputs. Input J4 has offset function provided that no plug is inserted into J4 as the switching contact of J4 is connected to the positive supply voltage in this case (via the protection resistor R8).

At J5 the inverting sum of all inputs is available. J6 outputs the non-inverting sum. P5 allows an additional attenuation of the complete signal (affects only J6).

Typical values for the parts used: O1, O2 = LM1458, TL082, NE5532 P1...P4 = 47k linear (CV) or log (audio) R1...R7 = 100k (for overall amplification 1) To obtain a higher overall amplification R5 has to be increased (e.g. to 220k for overall amplification ~ 2 or to 1M for overall amplification ~ 10).

The value of R8 defines the offset range (about 0...+6V for R8=33k, a lower value of R8 will increase the offset range and vice versa).


It recommends a 1458, which is a double version of the grotty old 741. Don't use that chip. Use something more modern.

You don't need P5, O2, or J6. The given circuit assumes a dual voltage power supply.

Alternatively, you build your mixer like in your first circuit, but lower all the feedback resistors to something reasonable, like 1k or 10k at the most. That'll lower the gain and avoid clipping.

  • \$\begingroup\$ thank you @JRE, that did the trick! \$\endgroup\$
    – davidesp
    Commented Mar 9, 2020 at 9:12
  • \$\begingroup\$ @davidesp: Which did the trick? \$\endgroup\$
    – JRE
    Commented Mar 9, 2020 at 10:24

Whenever high value resistors are used, there is going to be a lot of noise, generally thermal noise. Try reducing the values by a factor of 10x. Also, a gain of 1000 is very high for a mixer. The signals coming in should already be pre-amplified. A gain of (say) 50 is more than adequate normally.

As usual - do some experiments.


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