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Total newbie question: I wanted to buy a passive audio mixer, and I wanted it to be cheap. I picked up one of these in a hurry, only to realise that it's a switch. But hey, it's cheap, so let's make it do what I want.

I figured out the routing in/out of the switch, de-soldered it, and dropped some resistors in place on the pathways. This way, the two L channels pass through their resistors then combine on the PCB, and the same on the two R channels. Those paths run to the potentiometer/volume knob; maximum resistance on the pot is about 42 kOhm.

For the resistors that I put in place, I'm following a couple of things I've read online, including this document which suggests 10 kOhm resistors. Another document suggested 4.7 kOhm. But I don't know how to select this value.

I've dropped in four 5.1kOhm resistors, and it does the job. But really, what should I be measuring (on the incoming line audio, or the destination amplifier) to determine what these resistor values should be? What are the risks if I go too low? What is the impact if I go too high? If I adjust the volume knob so that the resistance through the pot is > 5.1 kOhm, would that be bad?

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    \$\begingroup\$ There is no easy answer without knowing the impedance characteristics of the devices that you are connecting to. If the value of the summing resistors is too low, you could stress the source outputs. If the value is too high, you will lose amplitude and you will be more susceptible to noise. If 4.7k works OK, I would go with it. \$\endgroup\$
    – Mattman944
    Commented Dec 28, 2020 at 1:28
  • \$\begingroup\$ So what should I be measuring on the devices that I'm connecting through this? \$\endgroup\$
    – Stephen
    Commented Dec 28, 2020 at 10:18

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If the devices that you are connecting to are typical, the summer resistor values are not that critical. As an example, here are the impedance values of my Stereo receiver, which I will be connecting a passive summer to the right and left Tape outputs to feed a spectrum analyzer. I haven't picked a value yet, I am working on other parts of the design.

What is important to remember is that each summing channel will have it's amplitude reduced considerably. A three input passive summer will have the amplitude of each input reduced to 1/3. The load of the destination will further reduce it. Here, assuming that the destination is the Tape input, one volt will be reduced to 0.267 V.

A DMM won't necessarily give you an accurate impedance measurement of your device input and output impedances. With a function generator and a scope/DMM, you can determine them experimentally. This is beyond the scope of your question, you should ask another question if you want to do this.

You want the summer values to be low to minimize the loss, but high enough so you don't stress the outputs on the left. For my case, the output can probably withstand a continuous short, so I could use a summer of zero (directly connect the signals). I won't do this, since maybe someday I will want to connect my spectrum analyzer to a different device.

The algebra to solve this is tedious, I used the simulator. You can use it too, it isn't that hard to learn.

enter image description here

schematic

simulate this circuit – Schematic created using CircuitLab

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