I would say that it is probably that the cables are properly grounded but there is a ground potential that varies at either end, thus a ground loop.Odds are its a bad cable that is not grounded properly.
I tend to avoid "hum eliminator" devices as while better than lifting the ground, the one I have seen are still a bit questionable in regards to safety and having the proper ratings. The fact is that the problem is typically really an audio device design problem, any current on the audio signal ground should not get into the audio signal path, however less than ideal equipment and circuitry design does allow it to get into the audio circuitry. Because of this, I prefer to always resolve these issues on the audio rather than power side. Be it simply dropping audio ground (not power/safety ground but the ground associated with an audio signal) at inputs or using input transformers, it makes more sense to me to leave power grounding alone (unless it has problems of its own that need to be addressed by an Electrician).There are hum eliminators out there that will take care of the issue as well.
Agreed, try to narrow down where the noise is being introduced. If it is a line level audio run involved, the simplest fix may be dropping the audio shield at the end of that run.The easiest thing to do is to unhook the system and hook it back up one cable at a time. That should pinpoint where the ground loop is coming from.
The Whirlwind Lifter
The best way to do this is to make some patch cords built that way, and mark them in such a way that the special configuration is obvious. Bright red paint or obnoxious tags would help.
Can you clarify this? Lifting the shield is a very accepted approach and has been used by many in the pro audio world for years. Where dropping the shield may result in RFI problems instead of simply dropping the shield you can terminate it through a capacitor. Adding audio transformers has also been an accepted practice for many years. Neither solution may be ideal but they are certainly accepted practice as a long term solution.The solutions mentioned are like using Maalox to treat an ulcer - they give temporary relief, but they don't solve the problem.
Can you clarify this? Lifting the shield is a very accepted approach and has been used by many in the pro audio world for years. Where dropping the shield may result in RFI problems instead of simply dropping the shield you can terminate it through a capacitor. Adding audio transformers has also been an accepted practice for many years. Neither solution may be ideal but they are certainly accepted practice as a long term solution.
Contrary to what many believe "ground" is not the same everywhere, when dealing with long runs, etc. you will always have the potential of some differences in ground potential. Ground loops are somewhat a fact of life and believing that eliminating any differences in ground potential is a viable solution for all cases is impractical. Equipment that is "Pin 1 compliant" with the audio shield to ground path properly addressed so that it does not affect the audio signal path should alleviate the problem. Until then, approaches such as dropping shield at one end (preferably the input side) of audio runs or adding input transformers are generally accepted practices in the audio industry.
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