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I am building a cable to connect a guitar (or a piezo) and headphones to the TRRS connector of a smartphone through 4-wires cable splitting at ~1 mt from the connector.

I am experiencing a significant leak of the output signal back into the microphone output. After a few tests (also with a cheap headset affected by the same issue), I came to the conclusion that most of the crosstalk must be due to the shared ground wire resistance, which is not negligible.

I've reduced the circuit to the one in figure (there's only one output channel for simplicity) where Vo is the output signal from the phone, Ro is the headphone resistance, Vs is the input source (coil/piezo), Ri is the microphone input resistance.

Simplified headset schematic

Keeping aside all other aspects not considered by the simplification, I'd like to know if there is any way to counterbalance the effect of Rgnd, without having to run two separate ground wires. I was thinking about some op-amp based solution, or any other off-the-shelf chip (since it's a hobbyist project).

Edit: for further reference, here's how I wired the actual cable to the TRRS connector (image taken from https://source.android.com/accessories/headset-spec.html)

TRRS connection schematics

Edit 2: Ok, I performed additional tests with a resistance connected instead of the guitar. I'll try to sum up the results of the tests:

  1. Nothing connected (no headphones, no input) -> very low crosstalk
  2. No headphones, guitar only -> very low crosstalk
  3. No headphones, resistance only -> very low crosstalk
  4. Headphones only, no input -> very low crosstalk
  5. Headphones + guitar -> sensible crosstalk
  6. Headphones + resistance -> very sensible crosstalk

The crosstalk is almost the same for the first 4 cases, in case 5 it becomes very distinguishable and invalidating for any practical purpose, case 6 is even higher than 5.

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  • \$\begingroup\$ Can you post a schematic of how you've actually wired things to the TRRS plug? I'm pretty sure you got it right, but it's good to verify. \$\endgroup\$ – AaronD Jun 11 '15 at 22:32
  • \$\begingroup\$ I followed the android specifications at source.android.com/accessories/headset-spec.html You may refer to figure one for the wiring (without all the buttons-related stuff, of course) \$\endgroup\$ – athos Jun 11 '15 at 22:39
  • \$\begingroup\$ Hey, I didn't know that existed. It fills in a few gaps in my knowledge as well. But it's still a good idea to collect all relevant information here in case a link dies and so we don't have to go look for it. \$\endgroup\$ – AaronD Jun 11 '15 at 22:43
  • \$\begingroup\$ First, you need to be sure that the source of the crosstalk is the ground wire. Check by connecting 1k resistor instead of mic. \$\endgroup\$ – ilkhd Jun 12 '15 at 3:01
  • \$\begingroup\$ Thanks for the suggestion, I updated the question with some additional experiments \$\endgroup\$ – athos Jun 13 '15 at 13:18
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through 4-wires cable splitting at ~1 mt from the connector.

One meter of thin cable has indeed some resistance. But you can eliminate this almost altogether by splitting close to the connector, which makes the circuit look like this:

schematic

simulate this circuit – Schematic created using CircuitLab

Note: R1-R5 are the resistances of the copper cables.

In other words, a shorter common wire will have less internal resistance and thus less crosstalk.

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  • \$\begingroup\$ Thanks, that was actually the solution I mentioned in the question. I was wondering if there was another solution that would let me avoid using a 5 wire cable. \$\endgroup\$ – athos Jun 13 '15 at 13:20

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