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I ordered an iPhone cable off eBay a couple weeks back, and then realized I'd ordered one that was compatible with the wrong model numbers of AKG headphones. No problem, I figured. I'll just solder on a mini-XLR connector instead of the 2.5mm TRS and all will be well.

 

So I lopped off the 2.5 mm TRS, and I can't get it open to see what they did at the headphone end with all the wires. I've got red, blue, green, and two coppers -- so four wires, assuming the two coppers are merged. But whether it's a TRS or a mini-XLR connector at the headphone end, that's only 3 pins.

 

So what happens with that fourth wire (which I haven't tested yet with a multimeter, but assume is the sleeve on the TRRS end that plugs into the iPhone (i.e., the mic wire) at the headphone end?