Passive mixers - stereo collapse?

Hey guys,

I recently made a passive mixer for my live setup (OP1, Circuit, Microgranny, KP2Smini, Restyler, RE20), and I’ve noticed I get substantial collapsing of the stereo field - when I A/B two channels hard-panned L-R through the mixer vs straight to my interface the difference is huge. Through the passive mixer it seems like instead of hard-pan (100% L or R) it feels like about 20%-30% pan.

FWIW the mixer has 6 stereo inputs, 4 mono inputs, and I used 10k resistors for the summing network. The stereo image doesn’t seem to change when other devices are plugged in, it just always seems collapsed. Wiring all looks good too.

Any ideas?

Maybe @darenager has some ideas, having designed passive mixers too.

You say it doesn’t seem to change when other devices are plugged in; are you sure about that?

Specifically, if all 4 mono channels are plugged in to things, does it change?

My first thought is on your mono channels: they’re each just a single input that splits off thru two 10k resistors, one to each of the output channels, right?
If there’s nothing connected, then you just have a crosstalk path between L/R. (2*10k on each of the 4 mono branches, all those in parallel, so overall you’ve got effectively a 5k resistor connecting the L/R channels)

If you have a source connected to all the mono inputs, then you should be able to break the crosstalk path. (an external source would typically have an output impedance on the order of 1k, so that’s gonna control what’s happening on that crosstalk branch, not the bleeding from the 10k resistors)
Alternately, if you’re using switched jacks, you could normall the mono inputs to ground. You’d lose a bit of gain on the output, but there shouldn’t be any crosstalk.

@beefinator ahhhhh that makes sense. I had a feeling it might be the mono inputs - they are connected as you say, with 2*10k’s, which yeah, would create cross-talk… Damn!

Foolishly, I’d tested it by just plugging cables into the mono inputs, rather than actual sources. Will give this a test tomorrow. I don’t have any switching sockets on hand, so might just convert the mono inputs into stereo sources instead. Or, maybe using dummy 1/4" sockets that are shorted to ground can give me the semblance of normalled for the time being…

OK so I gave it a test - the mono inputs were definitely the main culprit. That said, I’m still getting quite a bit of cross talk compared to going direct - it now feels like the panning is maybe at 70-80% rather than the 30% previously. Is this amount of crosstalk normal in a passive system, or should it be reasonably close to 100% panning? I might break this one down and start again, maybe I cocked up some wiring somewhere…