Hum gone with no ground.

Discussion in 'Sound, Music, and Intercom' started by alime, Aug 22, 2009.

  1. alime

    alime Member

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    We have hooked up our system and are getting a hum until we switched to a non grounded connector. I understand using this connector is not good for our system. Can anyone tell us where to start?

    The connector looks like this.
    [​IMG]
     
  2. Footer

    Footer Senior Team Senior Team Premium Member

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    Odds are its a bad cable that is not grounded properly. There are hum eliminators out there that will take care of the issue as well. 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. Don't run your system with a lifted ground. Not only is it dangerous for you but it can blow your gear too.

    It also could be bad power. Do you have any dimmers or lighting equipment nearby? If so, move it.
     
  3. museav

    museav CBMod CB Mods Departed Member

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    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.

    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).

    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.
     
  4. derekleffew

    derekleffew Resident Curmudgeon Senior Team Premium Member

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    The Whirlwind Lifter
    [​IMG]
    Whirlwind / Specialty Interface Devices
    or
    Sescom IL-19 Isolation Transformer
    [​IMG]
    Whirlwind / Specialty Interface Devices
    (or their equivalents by another quality manufacturer) are much recommended over defeating the EGC.

    See also Rane Note110: Sound System Interconnection.
     
  5. TimmyP1955

    TimmyP1955 Active Member

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    Temporarily, run extension cords so that all of the gear is plugged into the same outlet.

    If the system is silent, then the problem is with one of the AC outlets that you have been using (outlet mis-wired, outlets not fed from the same breaker box = grounds not at the same potential, ....).

    If the system still hums, there may be a problem with the wiring of a rack's AC (mis-wired cable or power strip, leaking surge suppression component, "pin one problem" with the gear, ....).
     
  6. FMEng

    FMEng Well-Known Member Fight Leukemia

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    Actually, lifting the AC ground is a SAFETY hazard. It makes it possible for an electrical fault within the equipment to electrocute someone. Lifting the AC ground is a tempting, fast fix, but don't do it.

    I'm assuming you have amps on stage, and console some distance away. To prevent a ground loop from causing hum, you can drop the shield (XLR pin 1) connection at one end of LINE LEVEL cables. 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.

    You are also far less likely to have hum issues if the interconnections are balanced , not un-balanced. In order to acheive noise reduction, the console output, the amp input, and the cables all have to be balanced.
     
  7. Chris15

    Chris15 CBMod CB Mods Premium Member

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    I've seen ground lift cables marked with green, I like the idea. Green being the colour associated with Earth / ground in mains wiring...
     
  8. museav

    museav CBMod CB Mods Departed Member

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    Or just carry of few of the audio shield lift adapters like those Derek noted. In permanent installs I like to see the shield bent back and under heat shrink rather than being cut off as if the ground potential problem is resolved or Pin 1 compliant equipment later used it would then be preferable to be able to attach the shield at both ends of the run.

    I do think that referring to lifting shield versus lifting ground avoids some of the potential confusion and is more accurate when addressing audio cabling as audio cabling does not have a 'ground', it has a shield.
     
  9. TimmyP1955

    TimmyP1955 Active Member

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    The solutions mentioned are like using Maalox to treat an ulcer - they give temporary relief, but they don't solve the problem.
     
  10. museav

    museav CBMod CB Mods Departed Member

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    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.
     
  11. TimmyP1955

    TimmyP1955 Active Member

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    They solve the hum problem, but they do not find the cause of the problem. It is first necessary to ascertain the cause in order to insure that one does not have a potentially hazardous situation. Once the troubleshooting has been done, and it has been determined without any doubt that there is no hazard, it is perfectly acceptable to float shields or install transformers.
     

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