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I am trying to interface a theatre Comms unit with a consumer TRRS microphone headset.

Comms headset outputs consist of a 4-pin XLR with the following pinout: 1. Mic input; 5V phantom for electret 2. Mic ground 3. Speaker output 4. Speaker ground

These units separate the microphone ground from the speaker ground. If the two grounds are bridged (i.e. pins 2 & 4 shorted), a very loud buzzing occurs when the microphone is enabled.

My problem is that consumer headsets with TRRS connectors have a common ground between microphone and speaker L & R. I assume there must be some passive method to take the speaker output from the comms unit, and couple it to the headset common ground, while leaving the two grounds separate.

Below is a guess at a circuit to do so — would it work? What transformer spec would be appropriate? I want the impedance to work for 30-300 ohm headphones.

NB. the Microphone must remain a complete circuit due to the DC phantom power for the electret preamp.

proposed transformer circuit

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I've tried this and it worked just fine, I used an audiograde transformer 1:1

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