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):
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!