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Bad dimmer noise (interference?)

Discussion in 'Lighting and Electrics' started by SweetBennyFenton, Dec 3, 2007.

  1. SweetBennyFenton

    SweetBennyFenton Active Member

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    So, my theater has an old(ish) dimmer rack in the booth. I went into tech this weekend and found a horrible buzz in the sound system and headsets whenever I power up the lights. If there are few lights up or they are at full, the noise is almost gone. When I have a lot of instruments up (especially at low levels) it buzzes worse than any interfierence I have ever heard.

    It seems to me that any cable running sound signal that comes anywhere close to the dimmers picks up this hum. Problem is, all of my sound and clear-com cables have to run through the booth.

    I remember reading once (from Van maybe) how to build a wire box that could help with this kind of thing.

    Has anyone had similar problems and does anyone have any advice?

    Thanks!
     
  2. icewolf08

    icewolf08 CBMod CB Mods

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    Re: Bad dimmer noise (interfierence?)

    First off, make sure all of your sound and comms cables are properly shielded. Make sure your comms cables are properly terminated. It is the case though that sound and comms cables can pick up EM interference from power cables and dimmers. You get better performance if your sound/comms cables don't run parallel to power cables for any distance.
     
  3. JD

    JD Well-Known Member

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    Re: Bad dimmer noise (interfierence?)

    OK, there are few things to know, and a few things to check out. Dimmers (almost all) work by chopping the AC waveform leaving a hard edge on the wave. For those who know a little math, a square wave is a sign wave with every possible harmonic added. Well, it isn't quite a square wave, but it ain't far from it! This is true especially between 33 and 50%. SO, with the dimmers in the booth, there is a lot of cable to "broadcast" this signal. Now, on the other side of the coin, dimmer packs use choke coils and caps to minimize the amount of upper harmonics the pack puts out. If you only had one channel that was producing this noise, I would think blown cap or shorted choke, but I get the feeling all the channels are noise sources. Next think on the list, have a qualified electrician check to make sure that your neutral and ground have not found an unexpected way of talking to each other (short.) Often, if this happens there will be no difference in performance, but there will be "sneak" currents with a lot of harmonics traveling in some unexpected places! Once you have eliminated the source as having changed, you now have to examine the receiving end. Ground loops are common in sound systems and can be started up by having the board plugged in one location and the amp rack in another. Sound systems need a common point ground, usually at the distro, and should be a "star" topology from there out. You said it was showing up in the headsets as well. I assume you are talking about your com system. This is the wildcard, and why I ran through the concept of ground/neutral shorts. Com sets usually have only one power pack and therefore tend to be pretty immune from this problem. Are you looping back your com through your sound snake? If so, the shield pin (usually #1) may be commoned into your sound system. (bad) Last but not least, the cable runs should avoid power and lighting runs. I hope some of this has been helpful!
     
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  4. SweetBennyFenton

    SweetBennyFenton Active Member

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    Re: Bad dimmer noise (interfierence?)

    Thanks for the tip JD.

    The TD before me in this space put a lot of time, effort and money into the sound system in the hopes of eliminating this hum. I don't think it's a ground loop issue (I had some of those too a while ago, but was able to trouble shoot them).

    And while we are on the subject... many of the sound cables running through the booth are old and hardly shielded at all. They come right out of the wall and run to my amp rack, so I can't easily replace them. Is there a way I could run them through something that would shield them better? (PVC? Conduit?)

    The clear-com cables are better shielded, but because the clear-com base station is sitting in the booth with the dimmers, I think it is picking up the noise too.
     
  5. fredthe

    fredthe Active Member

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    Re: Bad dimmer noise (interfierence?)

    You should also have said qualified electircian make sure that the Neutral is appropriatly bonded to Ground at the correct point. (For both the dimmers, and whatever is powering the sound.) A floating neutral can definatly cause the problems you are experiencing.

    Hum on the intercom seems a bit suspicious... assuming the intecom cables are completely independant of everything else (dedicated cables runs, not in snakes, no "installed" wiring.) If there are permanantly installed cables (such as in conduit) as part if the sound/intercom systems, that may cause additional problems.
     
  6. avkid

    avkid Not a New User Fight Leukemia

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    Re: Bad dimmer noise (interfierence?)

    Somebody punched a whole in the wall, or are they in conduit?
    If they are in conduit all you need is a really long fish tape.
    (might be able to rent one from a local tool place.)
     
  7. SweetBennyFenton

    SweetBennyFenton Active Member

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    Re: Bad dimmer noise (interfierence?)

    When the building was constructed, they had all the speaker and mic outlets from the stage run to a panel in the booth. This panel was ripped out at some point, so all the cables come into the booth through the original conduit and just spill out onto the floor.

    And if you are suggesting I re-run the speaker cables from the booth to the stage, that's really beyond my abilities. Anything serious like that and I have to call in the college's facilities crew.

    To add to all of this crazyness, the mic and speaker cables come out of that hole in the wall on one end of the booth and travel in front of the dimmers to the other side of the booth where the sound racks live. I think that was done a while ago... but I'm not really sure why.
     
  8. derekleffew

    derekleffew Resident Curmudgeon Senior Team Premium Member

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    Seems like this is a recent occurrence? If so, start troubleshooting by what has recently changed. Any new cables (lighting or audio) in the system(s)? Is the audio system patched the same way as it usually is? Any new audio components? I agree with the above posters that it could be a loose ground or neutral somewhere (anywhere). If lighting cables must cross audio cables, make sure they do so at 90°, the worst is for them to run parallel to each other for any distance. Speaker cables are generally immune to such interference, look more closely at line-level and mic-level cables. But since the noise is in both the sound and com systems, I would suspect an electrical issue. The $3.99 outlet tester can do wonders in isolating a problem. (You probably don't need the "expensive" GFCI tester, but I couldn't find a picture of the basic model.)
     
  9. fredthe

    fredthe Active Member

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    Derek's suggestion for a circuit tester is a good one... check all the outlets that anything sound related is plugged into.

    In addition, some quick troubleshooting steps...
    1. Disconnect the inputs from the Amps... does the noise go away?
    If the noise is still there, then it's quite likely electrical.
    1b. Connect an iPod (or any BATTERY POWERED device that generates audio) directly to the amp... Noise?
    2. Reconnect the Amp. Unplug all inputs to the mixer. (Where is the mixer located?) Noise?
    If there is noise here, then it is electrical or cabling.
    2b. As above, with an iPod or similar.

    Hopefully, this would isolate in (or between) what components the noise is being introduced.
     
  10. JD

    JD Well-Known Member

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    Re: Bad dimmer noise (interfierence?)

    Please don't tell me they ran the mic and speaker lines in the same conduit (?!) Although not relevant to your current problem, that would be a recipe for something called ultrasonic oscillation. This is a type of electronic feedback that can not be heard, but should it occur could do a lot of damage to your sound equipment!

    One other thing to check would be the insulation on the mic cables where they come out of the "hole", "conduit" or whatever. If one of the cables has been cut into by abrasion, the shield on the cable could be in contact whit the building frame or conduit creating a new ground loop.
     
  11. SweetBennyFenton

    SweetBennyFenton Active Member

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    Re: Bad dimmer noise (interfierence?)

    Oh, I wouldn't doubt if the mic and speaker cable were run together. For all I know, it was run on the same path as the lighting circuits.

    I will try unplugging stuff to try to isolate what is being affected. Thank you all so much.
     
  12. STEVETERRY

    STEVETERRY Well-Known Member

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    I have had this experience on a number of occasions during the past 30 years.

    I would look for a neutral-to-ground short somewhere in the lighting system.

    1. Unplug every fixture and cable. Does it go away? If yes, plug them back in one at a time until the buzz comes back. Then, mark that cord and fixture, unplug it, and keep plugging in the other cords/fixtures. If the buzz re-appears with any cord/fixture, mark it and unplug. There could be a short on more than one fixture/cable.

    2. If the buzz is there with all fixtures and cords unplugged, you have a tougher troubleshooting problem. Is this a portable dimmer system or a permanently installed system? Please advise and I will give you the next troubleshooting chapter.

    ST
     
  13. SweetBennyFenton

    SweetBennyFenton Active Member

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    I wouldn't be able to start unplugging fixtures untill after my show closes on Saturday. This would be a great trouble shooting project though.

    Oh, and the dimmers are a perminent install. I think it was put in somewhere between 1977 and 1985 or so.
     
  14. SweetBennyFenton

    SweetBennyFenton Active Member

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    HA HA! Success!

    It was a ground loop problem.

    We have our normal clear-com base station sitting into the booth. One cable comes out of it to the wired beltpacks, the other runs out of the booth to our wireless base station.

    The wireless base station had been plugged into a different power source than the regular base station. Once I unplugged it, the whole buzz went away.

    (feels sheepish)

    thank you all for your help.
     
  15. JD

    JD Well-Known Member

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    Ah yes, for the 4,304,290th time in my life, the ugly Ground Loop monster raises it's ugly head!

    Glad to hear!
     
  16. miriam

    miriam Active Member

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    Why is it a problem if sound cables and light cables are run next to each other? And how do you avoid it? Do the different types go along opposite sides if the room?

    Thanks.
     
  17. gafftaper

    gafftaper Senior Team Senior Team Fight Leukemia

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    How about someone writing up a ground loop entry for the wiki? I'm not exactly expert on the topic or I would do it myself.
     
  18. derekleffew

    derekleffew Resident Curmudgeon Senior Team Premium Member

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    All cables that carry electricity, (lighting, audio, and video, etc.) generate EMI (Electro-Magnetic Interference.) The shielding on audio and video cables "protects" the inner conductors from this interference, but does not eliminate it. Thus the "rule" that lighting cables should be at least 18" away from other cables if they run parallel and if they have to cross, do so only at 90°. Likewise, live lighting cables should never be coiled, but stacked in a figure 8, as the double loops will "cancel" (counteract) the EMF.

    For rock&roll tours in my venue, Lighting almost always goes SR and Sound SL. Video generally goes "backstage" on the same side as Sound.
     
  19. icewolf08

    icewolf08 CBMod CB Mods

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    It is possible when you run sound cables parallel to power cables that the sound cables can pick up EM interference. The principle is magnetic induction, which is what makes things like transformers and electro magnets work. The oscillating AC current creates an electromagnetic field. In theory the hot and neutral in an AC power cord are 180˚ out of phase, thus producing 100% destructive interference and canceling each other out. This is why you don't get a reading with a clamp on ammeter if you clamp it around all the conductors of a power cable at the same time.

    Magnetic inductance (in a very oversimplified explanation) is when the electro-magnetic field of one device excites the electrons in another device and induces a current. In terms of sound cables this means that an AC power cable can induce a current in a sound cable and cause a hum.
     
  20. JD

    JD Well-Known Member

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    The key to the whole thing is that mic lines generally carry a signal that is measured in millionths of a watt, and lighting cables carry ac power in the thousands to tens of thousands of watts range. So, a leakage ratio of one billionth can be noticed! (Fraction = 1/1,000,000,000)

    To make matters worse, lighting cables usually carry waveforms that are very choppy and therefore the EMI they produce carries a greater distance then good clean 60cps sign wave. (If there is such a thing!)

    As for the subject of ground loops, that would be a long subject to cover. Basically, it all stems from very small voltage variations that occur across the ground system in a building. If two parts of a sound system are grounded to two different locations (say, the house mixer and the monitor mixer), these voltage variation can be superimposed on the signal that is traveling between the two locations, and end up in the audio mix. Remember, it doesn't take much! On a not-so-light note, ground loops can be dangerous to equipment and people if a ground fault is present in the building. It's covered in many other threads, but all systems need to use a "star" topology when it comes to grounding. Basically, there is only one true ground point, hopefully right at the main service panel of the building. From there all ground paths spread out like branches from a tree. Under no circumstance should any of these branches cross paths once they have split off, and that is exactly what does happen when two pieces of equipment that are case connected end up being plugged into two random outlets in a building. In the perfect world, the ground lines should carry no current or voltage. (That is the job of the neutral return conductor.) In the real world there are many things that introduce, or induce current in the ground system. One of these things it that any long stretch of wire has another name: Antenna!
     
    Last edited: Dec 4, 2007

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