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DIY Passive Summing Mixer

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


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  1. eric_B

    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
    20160626-IMG_1295.

    19" rack plate with custom holes drilled
    20160626-IMG_1297.

    TRS jacks mounted
    20160626-IMG_1299.

    cut and painted 2 wooden clamps to hold the copper wires
    20160626-IMG_1300.

    first wire in place (ground connection)
    20160626-IMG_1301.

    soldering the resistors between the TRS jacks and the copper wire
    20160626-IMG_1303.

    all soldering done
    20160626-IMG_1304.

    two 'shunt' resisters between the tip and ring wires and those connected to the TRS jack
    20160626-IMG_1305.

    some labels and the mixer is ready to go
    20160710-IMG_1425.

    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!
     

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    Last edited: Jul 10, 2016

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