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I've got a Bluetooth device that has a ten-pin mini-USB output port (a Sena GP10 GoPro bluetooth audio receiver). Two of those pins output a mic-level microphone signal, but I don't know which ones. I want to figure out a way to adapt the mini-USB output port into a standard microphone TRS connector so I can plug the receiver directly into an audio recorder.

Unfortunately I don't have an oscilloscope, only a standard multimeter, so for any kind of analog audio cable the DMM gives me a reading of 0V since it averages out the positive and negative AC voltage readings.

I don't want to just blindly connect random combinations of pins into an audio recording device because I'm not sure what all the other pins do and I don't want to risk perhaps blowing the audio recorder.

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    \$\begingroup\$ How are you certain that the mini-USB has an analogue mic-level signal? \$\endgroup\$ – loudnoises Oct 25 '17 at 8:03
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    \$\begingroup\$ Probably easiest to find someone's reverse engineering work e.g. chargeconverter.com/blog/?p=130 \$\endgroup\$ – pjc50 Oct 25 '17 at 9:24
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If you have access to the microphone whilst it is active and can probe the output, you can make some acoustic sound and see which pins make a voltage that corresponds to this. Hopefully ground is easy to find so you can measure relative to that pin.

Analogue microphone signals are usually very small, so you may want an op-amp stage to boost it. This would be safe to connect to any output as it won't apply any voltage. The following stage has a gain of 100 and should boost the signal enough that your multimeter (which should be on an AC setting) will measure the difference when the mic picks up a signal.

schematic

simulate this circuit – Schematic created using CircuitLab

If you don't have an AC mode on your multimeter, you can add a diode, resistor and capacitor to the output which will detect the envelope (by rectifying) converting the signal closer to DC.

schematic

simulate this circuit

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