Do It Yourself

Passive Summing Mixer

by eric_B

original article on  •  July 10, 2016

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 (click on these images to see a larger, clearer version of the same photo):

parts: TRS jacks and resistors

jacks and resisters

19" rack plate with custom holes drilled

plate with drilled holes

TRS jacks mounted

mounted jacks

cut and painted 2 wooden clamps to hold the copper wires

wooden wire-stand-offs

first wire in place (ground connection)

first wire in place

soldering the resistors between the TRS jacks and the copper wire

soldering a resister

all soldering done

all wires and resisters soldered

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

shunt resisters and wiring

some labels and the mixer is ready to go

finished panel

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:

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.

schematic diagram:

schematic diagram