May 27, 2015

Simple passive mono/stereo to stereo audio mixer circuit



A while ago I needed a very simple pasive miker to join one stereo and two mono signals together before amplifying them...

There is some mixed information on how to properly connect a mono signal to a stereo input, but most of it uses active mixing.

I looked at a few schematics, like Tomi Engdahl's simple line mixer, and combiner, and the All Electric Kitchen simple mixer.

For my needs I finally made this simple ciruit that mixes the signals and allows to ajust the levels of each channel independently, before the amplification stage.


mono/stereo passive mixer schematic


mono/stereo passive mixer board
600dpi (aprox. 51x48.5mm)


mono/stereo passive mixer component layout


Picture of the actual board

16 comments:

  1. Replies
    1. No, the stereo (double) pot is used to control the level (attenuation) of the stereo signal, in the same way as each mono pot controls the level of each mono signal.

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  2. are you sure you're not tapping the outcoming signal from the wiper, with the ends of the pots going between the incoming signal and ground? That'd be a standard divider circuit, whereas what you have in the schematic with a grounded wiper is.. well, actually I'm pretty sure that would result in no output.

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    1. You are totally right!
      The schematic showed the pots in the wrong position (just changed it).
      The board was right, though.
      Thanks for the correction!

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  3. how can I add a second stereo channel to this?

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    Replies
    1. Just add two additional 1k resistors and another stereo pot (or two pots one for L and one for R of the new stereo channel). Connect the each of the new L and R inputs through one of the new 1k resistors to the corresponding L and R ouputs. See as reference:
      http://www.epanorama.net/circuits/linemixer.html
      http://www.instructables.com/id/Passive-Audio-Mixer-w-Channel-Volume-Control/

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  4. This comment has been removed by the author.

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  5. shall I do it vice versa? (to convert mono to stereo)?

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    Replies
    1. mono to stereo is straightforward: connect the same mono signal to both stereo channels

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  6. Great inspiration, just what I've been looking for! Any idea how to add pan to the mono inputs? I guess it would need a stereo pot, but connected kind of reversed, so that when left gets turned louder right gets quiter. But I'm not quite sure about the details.

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    1. Hmmm, how about if you put, before the stereo input, a regular mono pot, say 10k, plugging the new mono input to the center pin of the pot, one side's pin of the pot to the left stereo input of the mixer and the other side pin to the right stereo input?
      The higher value of the pot the more noticeable panning, but with more gain loss (you may try also with 1k and 100k).

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  7. Apologies for my lack of knowledge. I am new to this and playing around. The 1u caps, does it matter what voltage it is ?

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    1. Thank you David.

      Im actually trying to make a 3 channel mixer (cell in, landline in, pc in) 1 out to main headphones. I'm also trying to get one mic in from the headphones into the 3 devices. If listening to music and a call comes through I would like to be able to mute the music and take the call. (would be amazing if this could happen automatically - calls take preference and mutes the rest) Has anyone made anything like this before ?

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  8. I still did not have a chance to put it together and I still have question.

    I assume this mixer accepts line level audio signal but I would like use with my modular synth setup which is modular audio level = 10v p-p.

    Do I need to use different value for resistors on mono channels ? Thanks

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    1. As far as I understand, if all the modules use the same voltage the circuit should work. However, I'm not an expert in audio circuits an this is a very simple passive solution, there is no calculation of impedance maching, etc. My suggestion is that an experimental test may give you the answer: try mixing two signals by joining the wires through two resistors. Do the test with a low volume signal and see if increasing the volume saturates the module where the mixed signals come in. If so, double the resistors value and repeat the test...

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