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I just bought a QYT KT-8900D mobile radio. One of the first things I've noticed is that I cannot use my mixer to completely lower the volume of the radio's 3.5mm TRRS output. When the fader is all the way down, there's still a good amount of audio coming through.

After closer inspection by my friend on his identical radio, an audio signal is being sent over just the ground pin somehow. Tested it on 3 radios with the same result.

Does anyone know why this is happening and how to fix it?

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    $\begingroup$ can you elaborate on your mixer's input and the radio's output? Balanced? Unbalanced? What's on these Tip, ring, ring, sleeve? My guess is that there's something DC-coupled that shouldn't, or vice versa. $\endgroup$ – Marcus Müller Jul 16 at 20:06
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    $\begingroup$ When you write, "Just a ground pin," you must be measuring that voltage - whether with a meter, a transducer (speaker, headphone), oscilloscopte, etc. - with reference to something else. Please describe the measurement technique. $\endgroup$ – Brian K1LI Jul 16 at 20:06
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    $\begingroup$ Good questions. I will have the answer shortly. $\endgroup$ – Will Freeman Jul 16 at 20:14
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    $\begingroup$ I'll try to run some tests tonight to see what's going through each wire $\endgroup$ – Will Freeman Jul 16 at 20:49
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    $\begingroup$ Just to check — could you edit your question to include the model of (and preferably a link to the manufacturer's site for) the mixer you're using? $\endgroup$ – Kevin Reid AG6YO Jul 17 at 2:26
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More info: the mixer takes 2 stereo unbalanced 3.5mm inputs. maybe the extra ring on the radio's output jack is actually for stereo? Some weird behavior I experienced with the mixer: When both faders for radio and bluetooth audio receiver are at negative infinity, there is no sound coming out. However, when I fade up only the BT audio receiver's level while keeping the radio at negative infinity, it increases the level of the radio's audio, and I can hear it in the background. It's not as loud as it would be if its fader were up, but it's definitely noticeable and only happens w/ radio.

Aha!

So, you've got cross-talk between the radio input and the other channels :)

Cross-talk between audio channels at a level this high is typically not because audio-frequency signals somehow couple into other conductors, but because higher-frequency components couple more easily.

My guess is that your radio might be leaking some IF signal into its audio outputs, which get transported along wires towards your mixer, influence some non-linear circuitry inside, and thus get mixed down to baseband involuntarily by your mixer acting like a radio receiver.

If you have a ferrite clamp lying around (old computer cables?), that might be something you can try adding to your audio cable.

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  • $\begingroup$ "clamp-on ferrite thing" -- oh how I love your terminology ! LOL $\endgroup$ – Edwin van Mierlo Jul 17 at 10:56
  • $\begingroup$ sorry, the name eluded me :) I'd be very open for a correct term, though :) $\endgroup$ – Marcus Müller Jul 17 at 11:25
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    $\begingroup$ And that is a good question, possibly "ferrite clamps" or "ferrite beads". Both will yield a good selection when searching online. $\endgroup$ – Edwin van Mierlo Jul 17 at 11:48
  • $\begingroup$ Thanks! I've used ferrite clamp. $\endgroup$ – Marcus Müller Jul 17 at 12:17
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Audio isn't send just over the ground pin, or the other pin. It's sent over both. If both weren't necessary, then why not just make audio connectors with one pin and audio cables with one wire?

The audio signal is represented by a voltage at the input. And the source of the signal manipulates that voltage by adding or removing electrons, what we usually call "current". And since electrons aren't just magically created, but must come from somewhere, where are they obtained?

The answer is the signal source doesn't create or destroy electrons: if it wants to increase the voltage then it takes electrons from the "audio" pin and deposits electrons in the ground pin. And to decrease the voltage it does the opposite.

So naturally, if you try to measure "just" the ground pin, you're going to see something. Only what you see is going to be highly dependent on how you've connected the test equipment.

The more likely reason your mixer can't totally eliminate the signal is probably just because mixers don't work like that. The faders only attenuate the signal, perhaps to the point where it's no longer audible. Or perhaps not, depending on how much amplification follows the mixer output, how good your ears are, and how quiet the room is.

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    $\begingroup$ In general, I'd slightly disagree with the last part: in any audio equipment I've encountered so far, pulling a slider fully down means "attenuate enough to make reliably impossible to hear", and a mixer's job is also to isolate the output characteristics from the input characteristics. $\endgroup$ – Marcus Müller Jul 17 at 7:22
  • $\begingroup$ Also, I just read op's last comment: He's got crosstalk to other channels! So, my guess is, it's not an audio frequency electrical current but an EM wave getting transported here. $\endgroup$ – Marcus Müller Jul 17 at 7:29

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