AnalogSteph
Major Contributor
Most importantly, unplugging the DBX from the wall removes the mains PE connection as well. So now, you have eliminated the ground loop that I was suspecting was there:
Your audio cable is a plain old' balanced 1/4" TRS to TRS cable, not an instrument cable (TS to TS), right?
If so, it may be time for the soldering iron:
In theory, this sort of thing should not even be happening when using a balanced connection. In practice, it is possible for shield currents to impair the signal if internal ground routing does not follow AES48-2005 guidelines, effectively reducing the valuable common-mode rejection to much less than the input should normally be capable of. As the shield connection is carried by XLR pin 1 (which is the sleeve on a TRS plug), this is known as a "Pin 1 Problem". It is often easier / cheaper to connect pin 1 to local circuit ground compared to giving it an independent ground connection all of its own.
I suspect what's effectively a Pin 1 Problem that is uncovering the ground loop between the DBX (an IEC Class I device) and the PC.
Your audio cable is a plain old' balanced 1/4" TRS to TRS cable, not an instrument cable (TS to TS), right?
If so, it may be time for the soldering iron:
Are you able to modify your TRS cables? If I'm right, the problem should go away once the shield connection is disconnected at one side. Likewise when adding a line isolator like a Behringer HD400.
In theory, this sort of thing should not even be happening when using a balanced connection. In practice, it is possible for shield currents to impair the signal if internal ground routing does not follow AES48-2005 guidelines, effectively reducing the valuable common-mode rejection to much less than the input should normally be capable of. As the shield connection is carried by XLR pin 1 (which is the sleeve on a TRS plug), this is known as a "Pin 1 Problem". It is often easier / cheaper to connect pin 1 to local circuit ground compared to giving it an independent ground connection all of its own.
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