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DIY Passive Summing Mixer

Discussion in 'Miscellaneous [BG]' started by eric_B, Jul 10, 2016.

  1. eric_B


    Apr 19, 2015
    The Netherlands
    Note: this thread is not directly bass related but might be of use to some of you nonetheless.

    As I always seem to run out of stereo inputs on my home studio mixer, I was looking into getting a cheap simple stereo mixer to mix multiple inputs into 1 channel. It turns out those mixers are not so cheap.
    Then I ran into a DIY schematic by New York Dave - hey, thanks Dave!
    It states how you can connect multiple (balanced) stereo inputs through resistors into one stereo output which you then feed to a mic pre-amp to make up for gain reduction.

    So I ordered some parts (blank 19" rack plate, a bunch of TRS jacks and resistors) and got into building my custom passive summing mixer, here are some pictures:

    parts: TRS jacks and resistors

    19" rack plate with custom holes drilled

    TRS jacks mounted

    cut and painted 2 wooden clamps to hold the copper wires

    first wire in place (ground connection)

    soldering the resistors between the TRS jacks and the copper wire

    all soldering done

    two 'shunt' resisters between the tip and ring wires and those connected to the TRS jack

    some labels and the mixer is ready to go

    The mixer works perfectly, in my version I can connect 8 stereo and 4 mono inputs and they are summed into 1 stereo output. That goes into 2 mic inputs on my mixer, so only taking up 2 channels for all those 12 inputs. You can also use a separate mic pre-amp and feed a stereo signal into your mixer.

    A couple of notes:
    - use Metal film, 1/4 watt, 1% tolerance resistors
    - I used 6,8K resistors (instead of 10K on Dave's schematic) to have a bit less gain reduction
    - for the mono inputs you have to connect the tip and ring to both the left and right copper wires. Because you're doubling the signal, it gets 3 dB louder. You compensate this by using 1,4 times higher resistors, so I used 10K (1,4 x 6,8K = 9,5K, almost 10K).
    - to calculate to value of the shunt resistors, use this formula:
    Rx= (II/N)*OI / ((II/N)-OI)

    Rx = value of one shunt resistor. You need 2, one for the left and one for the right channel. You solder them between the tip and ring.
    II = input impedance. 13,6K in my build - 2 x 6,8K tip and ring.
    N = number of channels. I had 12.
    OI = output impedance. I've read to aim for 150 to 200, I went for 150.

    So the formula for me was Rx = (13600 / 12) * 150 / ((13600 / 12) - 150) = 172 Ohm.
    Get two resistors that are around this value.
    - if you don't need balanced inputs, get TS jacks and you won't need halve the resistors and 2 wires
    - you could add an input pot for volume control and also a mute (on/off) switch, I might add those in version 2.
    - I added Dave's schematic as well - hope you don't mind, Dave!

    Attached Files:

    Last edited: Jul 10, 2016

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