How to select resistor values for passive mixer
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.




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