take a look at passive matrix mixers, there is a good schematic (for a 3 x 3 matrix) in "the art of hardware hacking" book. i could maybe scan or photograph it for you, if the relevant googling fails!

Howdy all, I don't have much knowledge of building things like this, but I'm trying to design something to re-route signal in or out of external effects.
I use a two Gameboy setup, and want to be able to direct either of them between one, both, or neither FX boxes while playing a song. (So I don't have to plug in/unplug anything during a performance or recording. Just a flip of a switch or two.)

Everything needs grounded of course, and it's also going to need to maintain stereo sound throughout. This is just to get an idea of how to wire things.  So here's what I've drawn up.

Which resistors should I be using for the passive mixing? I have made a passive mixer before using 1K resistors, and noticed signal was louder when one side of the audio was playing, but much softer with both sides of the audio playing... I need to avoid that if possible, I don't know if that's due to the resistors I used or how I wired it.

Edit: I'd also like to be able to optionally run one signal source through both FX, that won't work with what I've drawn. Should I add a switch to mix the two FX at the end, or do something else?