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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.

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Here's some background information that might be useful to understand what's going on.

The bare physics answer to “why is this happening” is: two wires running close together form an unintended capacitor. This capacitive coupling allows the signal in the headphone wires to cross over to the microphone wire. The longer the cable, the larger the capacitance, and therefore the stronger the effect.

The coupling effect can be reduced by careful design of the cable, such as placing separate shields around the microphone wire and around the headphone wires. I do not know if any commercially available TRRS extensions have this design.

However, you could assemble such a cable out of parts by using a pair of quality TRS extension cables (each of which has its own shield), then using TRRS-to-2×TRS adapters at both ends. This might solve your problem at the cost of having lots of bulky inline plugs.

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!

The switch probably operates by disconnecting the microphone from the cable. This means the microphone signal wire is “floating”, not connected to and not driven by any intended source, and therefore nothing is competing with the capacitive coupling. It's the same idea as if you attached an unshielded wire to a microphone input, or touched the signal contact on a plug: you'd suddenly hear a lot of hum and other EMI. But in this case, instead of general fields in the air, you're picking up specifically the adjacent wire.

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.

Same idea as the switch — any time you have a wire that doesn't make a complete circuit it will mostly pick up interference. (If the wire is shielded, then the shield also picks up the same interference and the net effect is zero — or, looking at it from another angle, the ground connection of the shield drives the voltage on it, and therefore the local electric field, to zero from the perspective of the attached equipment, so the inner wire sees that zero instead of interference.)

(I don't understand why the mic uses a TRS jack if it's an unbalanced mono signal. Is the sleeve used for power?)

Yes, there is power, but it is on the ring contact if distinct at all. In practice, many PC-style microphones short together the tip and ring contacts (since electret microphone elements have only the 2 pins anyway and want bias power on them) and the later-developed TRRS connections enforce this since they only have one contact for mic audio and mic power. The sound card handles this by a capacitor on the audio line (to pass the audio and block the DC power) and a resistor on the power line (which prevents the power supply from suppressing the audio, and also provides current limiting in case the power is shorted out by e.g. connecting a TS passive microphone).

Because of this conflation, some PC microphone inputs may, for example, be stereo, with both the tip and ring providing power and (separate) microphone input.

(The name given to "PC-style" microphone powering is "plug-in power" — presumably originally some sound card manufacturer's marketing term.)

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.

That's interesting! That suggests that perhaps the impedance of the audio interface is relevant (because I can't think of anything else that would make such a difference).

If you're trying to pick up a weak signal, you can get the most power out of the signal source (here the microphone capsule) by impedance matching the receiver ("load") with the source. (However, audio connections are often impedance bridged instead, which simplifies design and makes it easier to achieve flat frequency response.)

The relevance here is that impedance matching causes the microphone capsule to put out the most signal power it can. Thus, it becomes relatively louder than the interference.

I don't know whether the Apple adapter actually does this. It would be potentially as simple as wiring a resistor of the right value across the input signal, causing current to flow, which the FET amplifier inside the electret capsule can supply better than the weak capacitive coupling can. (This description is a little weird to think about because the power source is actually from the adapter/computer and not the microphone, but the math works out the same as if the microphone itself were powered and driving the line.)

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  • Wow, thank you for the detailed and very educational comment! In response to your suggestion: "...you could assemble such a cable out of parts by using a pair of quality TRS extension cables (each of which has its own shield), then using TRRS-to-2×TRS adapters at both ends." I did try this, but it still had the same amount of leakage into the microphone signal as using a TRRS extension cable. I can't necessarily vouch for the TRS cables I used being high quality -- I'm not sure what level of quality would be necessary here. – GuyGizmo Jun 28 at 3:52
  • @GuyGizmo That suggests I'm wrong about some part of the situation; just having two separate cables for most of the length of the extension should make an audible difference, if the problem is coupling along the length of the cable as I thought. Does using the adapters plugged directly into each other also cause leakage? Some trouble could be caused by poor connections somewhere along the line (particularly on the ground contacts/wires). – Kevin Reid Jun 28 at 4:15
  • Having the adapters directly connected to each other causes slightly more leakage, but not nearly as much as using an extension cable. By my observation there's a correlation between the length of the cable and leakage, regardless of how it's lengthened (be it using a TRRS extension cable or two TRS cables connected using splitters). Another thought: given the mic and stereo signals share a ground, could this be caused by a poor quality ground on the TRRS jack in my computer? – GuyGizmo Jun 28 at 5:38
  • @GuyGizmo If the ground wiring/contacts have high resistance anywhere along the cable setup, that would happen; that's what an inadequate shared ground does. But you said it happens on several devices... – Kevin Reid Jun 28 at 13:54
  • Yes I've reproduced it on three devices, two Thunderbolt 3 hubs and a MacBook Pro. Forgive me if any of these are newbie questions since I'm more of a software guy than a hardware guy, but could it be possible that these devices weren't designed with audio in mind, and the ground has enough resistance to cause this problem? Also, wouldn't extending the cable increase the resistance of the ground? – GuyGizmo Jun 28 at 14:41
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Try using a usb or firewire "sound card". Run a long USB cable with a "soundcard" or audio adapter at one end and the PC/Mac/Phone at the other. This might require a micor usb to full sized USB or a firewire equivalent. I Use a full sized Keyboard, Mouse, and external Harddrive on my tablet VIA a standard hub with a micro USB to full sized USB adapter, so it is possible to use external device with phones and tablets. Don't know how well this solution would work with your equipment, but my laptop's internal sound card died, I bought a little hub that had a built in sound card and have had no problem since, The laptop's built in volume control even operates the external sound cards volume level... *which I would like to disable since I accidentally hit the volume control all the time, but, "Oh Well!".

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