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I'm kind of stuck, and not really sure where to look, or what to look up.

I need to take the output of a USB audio fongle that has a single 3.5mm jack (TRRS), into a break out cable, and then need to take the microphone and the left and right channels into two different transformers for some impedance matching with another device.

If I take the microphonr + and the ground -, attach them to the transformer pins 4/5 it causes Windows to automatically take the volume and adjust it to zero. I assume this is because it senses a fault.

The transformer is a MET-32t, and the USB dongle is a generic USB audio device.

Computer -> USB dongle -> TRRS breakout cable -> microphone + -> leg 4 (150 ohm) ~~ -> (600 ohm) leg 1

Computer -> USB dongle -> TRRS breakout cable -> GND - -> leg 5 (150 ohm) ~~ -> (600 ohm) -> leg 2

This also happens with the L and R channel. Windows will just adjust the volume on its own.

schematic

simulate this circuit – Schematic created using CircuitLab

I'm just beginning in circuit engineering so I'm not really sure what to do. I have a bunch of different components I've tried and if I increase the resistance on the ground side it stops the volume changing, but then there is no voltage going through the transformer.

JRE
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blckmorning
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  • Why are you trying to impedance match audio signals? Seriously, nobody "in the know" does this except maybe, as an experiment to demonstrate to beginners or doubters that something wonderful, relevant, useful or meaningful doesn't actually happen. Of course if you are trying to maximize the power into a load then it can be useful. – Andy aka Feb 20 '23 at 21:04
  • This USB device is interfacing with a Radio console that will send and receive audio as well. It requires that the input from the PC be 600ohm. The PC is acting as a phone system that is sending its signal through the USB dongle, and the Radio console is receiving them, mixing them with the radio audio as well. When a "Call" in in progress the Radio console knows to output the Radio audio through a speaker instead of a headset, and send the PC audio to the headset. – blckmorning Feb 20 '23 at 21:21
  • Aha, it's a phone system. That makes a big difference then. You should have mentioned this in your question. – Andy aka Feb 20 '23 at 21:55
  • I probably should have given a little more background that was my fault. – blckmorning Feb 20 '23 at 22:06
  • Try putting a capacitor in series with the dongle side of the transformer. Something like a 10uF 16V electrolytic. The value and voltage aren’t critical. You can probably salvage something from some e-waste. – Kartman Feb 21 '23 at 12:49

1 Answers1

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That's a standard feature.

If you take any mobile phone headset, it has the hands-free call answer button shorting the Mic wire to Gnd.

Your audio transformer has low enough DC impedance so it emulates a pushbutton shorting Mic to Gnd, or at least as seen by the device.

Other buttons such as volume buttons are implemented by different resistances between Mic and Gnd, so if the other transformers do change volume too, you may have miswired someting.

It might be a good idea to change to a standard USB sound card with line level inputs and outputs, instead of USB dongle meant for mobile phone TRRS headsets which are also sensitive to standard mobile phone headset button controls.

Justme
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  • I figured it was because the Mic and Ground were shorted that it was acting the same way as the mobile phone when you hit the volume up/down on a headphone set. The problem I have is I'm trying to avoid that. – blckmorning Feb 20 '23 at 22:08