I've been trying to get an extension cable for my various wired headsets. These headsets have a standard 3.5mm TRRS jack using the CTIA standard, so the cable carries both a stereo out signal and a mono microphone input signal.
However, every single extension cable I've tried has the same problem: using it causes the output signal to leak into the microphone signal, and at quite a loud volume too. It makes voice calls impossible because the person on the other end of the call will always hear their own voice played back to them! And it doesn't matter which headset I use or what device I connect it to -- all the extension cables exhibit this problem.
And given that multiple cables from different manufacturers all have the same problem, I can't just assume they're all defective. I used a multimeter to confirm that there's no continuity between the different contacts on the jacks of these cables.
I even tried using two TRRS splitter cables, one of which was female TRRS jack to two male TRS jacks for headphones and mic, and the other was a male TRRS jack to two female TRS jacks for headphones and mic. It has the exact same problem.
I have a few other clues as to what's going wrong:
One of my headsets includes a switch for muting the mic. When I move the switch into the mute position, the stereo signal leaks even more loudly into the mic signal!
If I split the TRRS cable into two separate cables for microphone and headphones, and then split the TRS microphone jack into separate cables for the ring and sleeve but leave the sleeve disconnected, this causes the leakage to be incredibly loud. (I don't understand why the mic uses a TRS jack if it's an unbalanced mono signal. Is the sleeve used for power?)
What's going on here? Am I getting the wrong kind of extension cable without realizing it, and if so, in what way are these cables no good? And why is it that barely anyone else is reporting this problem in the product reviews for any of these cables?
edit: I just tested a headset on my iPhone 7 using an Apple lightning to 3.5mm adapter, and it doesn't have the leakage problem! But the standard 3.5mm TRRS headset port on my MacBook, iMac, and USB-C hubs all do have the problem.