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I have a Behringer X1622USB mixer which I am trying to set up with my PC. I have my PC's motherboard line out connected to a stereo line in on the mixer, which is fine for the most part. On each line in on the mixer, there is a switch for changing between +4dBu and -10dBV input levels. As far as I can tell from the manual and online research this is for switching between professional (less sensitive) and consumer (more sensitive) line levels, respectively.

This is all fine and good on the +4dBu setting, but not so when I try to use the -10dBV setting. When set to -10dBV, after a minute or two of audio, the volume will suddenly drop out, crackling is introduced, and often the left or right channel will cut out completely.

Initially I though perhaps the PC audio output was set too high, and was somehow overloading the mixer at that higher sensitivity level - but this was not the case. Even at 10% volume from the PC - which results in a very quiet output from the mixer - the same issue occurs.

I then decided to dig into it more to see if it was more complex then "the volume's too high".
Checking the mixer's manual, I see that its input impedance for the line inputs is 10kOhms, and the maximum input level is 30dBu (about 24V RMS). Then checking my motherboard's specifications, the op-amps are quoted as 2V RMS maximum output. Further investigation on the manufacturer's site quotes 100mA typical output.

Doing the math, it doesn't seem possible that it's an overloading issue at either the PC's output or the mixer's input. 2V RMS output from the PC would be far below the mixer's maximum input of 24V RMS. Furthermore, with an input impedance of 10kOhms, the mixer would draw at maximum around 0.2mA, a fraction of the PC op-amp's maximum current.

So why does my PC's audio become distorted and quiet after a while going through the mixer with -10dBV level setting?

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After further experimenting with an oscilloscope, I discovered that when set to the -10dbV level, a large offset voltage (up to about 6V) was accumulating between the audio channels and ground. Such a large offset must have been causing issues with the mixer's input channels and/or the PC output.

I do not understand why this would occur, but it was simple enough to fix. Since it seems to be some sort of static-like accumulation, placing a large resistor (considerably higher than the mixer's input impedance, to avoid attenuation of the signal) between the channel and ground fixed the problem.

(If anyone has any knowledge of what phenomena was occurring here, I would love to hear it.)

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