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noise/buzz induced by stage prop on dimmer

Discussion in 'Sound, Music, and Intercom' started by jkowtko, Jan 26, 2008.

  1. jkowtko

    jkowtko Active Member

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    There have been a couple instances where cheap electrical props used in our performances induced a buzz in the audio when they are faded up or down.
    One of these was a cheap $10 fire unit with a small light and a fan that blows a piece of silk upwards like a flame. The transformer might be a diode(?) transformer, evidently very cheap. Interestingly, two of the four units induced the buzz, and two (purchased earlier in the year) did not.
    The second is an old tube TV set being used on stage and is being powered up and down during a scene. When it turns off we get a series of static pops through the PA.
    Neither of these devices had a ground wire.
    The lighting guy and I haven't had a chance to research this yet -- so we're not sure how a small device like this attached to a different 100amp circuit from the sound circuits is getting into the audio.
    If you know what this is and can explain briefly and/or point me to some literature, I would appreciate it. I would like to understand what's going on so we can plan appropriately for these issues in the future when props like this are needed on stage. It's not hugely disruptive to the performances but nontheless I'd like to resolve them whenever possible without having to pull a prop.
    Thanks. John
     
  2. Van

    Van CBMod CB Mods Premium Member

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    what type of dims are you using? Dimming a TV? I'd hate to think what that is doing to your dimmers. A big issue with those fire units is the inductive motor they use. Use of an IPS dimmer system will allow you to control inductive loads as well as incandescent, but thos fire units have everything ganged together, personally I'd seperate the lighting from the motor and run them off different dims. Run the motor of a non-dim, or a dimmer set to non-dim, and the light of of a dimmer.
     
  3. SHARYNF

    SHARYNF Well-Known Member

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    If you have a two wire device, any problems in this device can be fed back on the neutral wire, all the neutrals are common at the panel, so if you feed garbage into the neutral you can have problems. I am guessing that your audio and the feed for this stuff is NOT at the main service entrance where the neutral and the ground are tied together, but rather at a sub panel. This is why on tours etc you triy to get a tie in all the way back to the main panel where the neutral s and ground are connected.

    Old TV's have high voltage power supplies and are know for having leaking into the power, using this stuff without some sort of isolation is not a good idea, you run the risk of damaging the gear you have connected that you really care for.

    Also be aware that in faulty wiring it is very difficult to detect if the neutral and the ground are swapped, the cheapo testers only look for the presence of the neutral and ground and since they are tied together way back at the service entrance they are connected

    Also look for someone wiring in a sub panel and connecting ground and neutral together at that point, so that there are mulltiple ground connections, also be on the look out for wiring there the ground is provided by the conduit and not a continuous connection.

    Sharyn
     
  4. Chris15

    Chris15 CBMod CB Mods Premium Member

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    I agree that neither of the aforementioned devices should be run on a dimmer. The fan will be somewhat unhappy and the TV set won't be getting enough voltage to run the HV section and fire the gun.

    I'm thinking that we could be talking about stray EMF radiating as radio, not necessarily being coupled via the power. But if it is in the power, perhaps a ferrite around the power cable might help.

    Any unbalanced audio in the vicinity? Or any audio running parallel to the power supplying these props?
     
  5. jkowtko

    jkowtko Active Member

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    Thanks for the ideas. Some more tidbits of info in reference to your comments:

    - for these plays (both dramas) there is no audio wiring of any sort anywhere near these devices on stage. However I can always check up in the catwalk and corridors to see if they're feeding these circuits back to dimmers whose cables are close to any audio wiring.

    - to my knowledge we're running the sound circuits directly off a 100 amp breaker, which is tied directly to the 400 amp 3-phase which is coming into the building. The main panel upgrade was done last year so the wiring should be new ... although I wouldn't rule out neutral/ground switches. But I guess this doesn't matter? Noise can still make its way back through the neutral?

    - I have one of those small testers which says it can indicate a switched neutral/ground, and in fact it's found a circuit or two with switched wires in my parents' home. I'll make a note to bring it to the theater and give the backstage circuits a test.

    If this is simply a nasty signal being sent back through the neutral wire, then are standard filters (ferrite, as you say) available as in-line plugs? Will ground-lift adapters do the trick here?

    Thanks. John
     
  6. Chris15

    Chris15 CBMod CB Mods Premium Member

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    Ferrite is those little "beads" you see around power and signal cables on things like computer monitors etc. You can get them in a form that is a plastic "case" that goes around the cable and then clips together with some crummy little piece of plastic. Your Rat Shack I expect would sell them.

    I had thought the general consensus of the forum reached in a previous thread was that ground lifters ought to be illegal because if they fail, you have no Earth. But I thought you'd said that neither device in question have an Earth connection in the plug anyway?

    Could be any number of things and without seeing the place I suspect I can't really diagnose the problem...
     
  7. jkowtko

    jkowtko Active Member

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    Okay, I'll check the local Radio Shack for a filter, put it on the TV extension cord and see if that helps at all.
    Correct, these units have AC two-prong plugs without ground. Is that enough to say we're susceptible to noise problems unless these devices are grounded properly?
     
    Last edited: Jan 27, 2008
  8. Chris15

    Chris15 CBMod CB Mods Premium Member

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    If only it were so simple... Because these are what Australian rules would defines as Class II devices, ie. they are double insulated, they do not require safety earthing as most metal chassis devices do. So in essence there is no place to connect an Earth wire inside the appliance and so there is no point and doing so may in fact compromise the safety of the appliance. My comment related more to the fact that there is zilch point in putting a ground lifter in (presuming here we're talking about some device in the mains line) because there is no Earth connection there TO lift...
     
  9. avkid

    avkid Not a New User Fight Leukemia

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    If you have any throw them out immediately!!!
     
  10. Chris15

    Chris15 CBMod CB Mods Premium Member

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    But only after permanently disabling them. We have a duty of care to ensure that anything we throw out has been rendered safe. Chop off the plug, cut retired rope into pieces 500mm or less, cut retired ladders into pieces etc. so that no one can use them. For something like these, a pair of pliers to the pins of the plugs and snap them off should be sufficient.
     
  11. David Ashton

    David Ashton Active Member

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    Question, is the noise coming through 1 channel, you need to re-create the noise and trace back to which channel the noise is entering.
    My guesses are 1-Gain structure is wrong leading to over sensitivity
    2-Failed screen on mic cable, probably channel specific.
    3-Earth loop caused by something plugged into or touching something on the same circuit as your tv/heater.
    4-Classic fault would be neutral earth cross on that circuit.
    RF transmission is extremely unusual.
     
  12. cutlunch

    cutlunch Active Member

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    jkowtko The simplest solution is the best. Don't run these through the dimmers. Just use ordinary non-dimmed power from the stage and have the SM cue the on/off.

    Connecting this gear to your dimmer can damage it and for the sake of a few props it's not worth it. Get a multiboard with switches on it.

    Do you normally get some noise from the dimmers on the audio side when adjusting light levels?
     
  13. SHARYNF

    SHARYNF Well-Known Member

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    Be aware the the home testers or even the more sophisticated one http://www.tripplite.com/products/static/ct120.cfm
    cannot effectively test for ground neutral reversal, they can test for no ground, not neutral or hot ground or hot neutral reversal but NOT ground and neutral reversed. Problem is that since back at the service entrance they are connected so unless you are testing potential with a meter it is difficult to detect a problem

    as mentioned above NEVER put devices like a tv or any electronic device on a dimmer connection, use a relay pack if you have to but the electric feed to a device via a dimmer is asking for big troubles.

    Sharyn
     
  14. jkowtko

    jkowtko Active Member

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    I checked with our lighting designer and he said we use AC relay boxes on these devices. He also suspects dischard through the neutral wire.

    If this is the case, then can we confirm that this noise is coming from the power supply to the speakers and not the signal cable?

    And if that's the case, then how do I remedy? Put power some sort of power conditioner on the speakers? On the AC relay box?

    Fyi, I never hear noise when lights are dimmed halfway. Occasionally during call when they warm the dimmers with all 80 channels at 25% I can hear a slight bit of hum all around but it's barely discernable and some of it's coming from the lights and dimmers themselves, and nothing like the buzz I get on these few noisy devices.
     
    Last edited: Jan 28, 2008
  15. SHARYNF

    SHARYNF Well-Known Member

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    First thing I would do is disconnect the devices them selves and activate the relays, then substitute a light bulb/worklight for the device and see is you still have the noise problem, this should identify if it is the relay, the relay under load, or the device.

    IF you don't have the problem during the test, then try simply swapping the plug around on the device, and also check to see if the wiring from the edison back to the relay is correct, someone could have wired up the edison incorrectly.

    If you can see if you can find and borrow a filter voltage regulator isolation transformer system (tripp lite make on pretty cheap for 1800 watts) since you are just switching with a relay you will not have any problem with voltage variation. While you are at it move the tv/device you are having problems with around a bit and see if this changes the noise,, it is possible that the tv high voltage is inducing noise

    Shary
     
  16. jkowtko

    jkowtko Active Member

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    We did this the first time we heard the buzz, and tried plugging and unplugging different combinations of the four light pots we had ... two of the light pots had the buzz, two didn't. So I think the relay unit was okay.

    I thought about that this morning, and yes, I'll try it if the plugs are not directional. (If they are directional, I can still try it but it will be a bit more work to do ...)

    Okay, I 'll see if we can get our hands on one of these, and will also experiment with the TV power source and location a bit more to figure out where the problem is coming from.

    Thanks. John
     
  17. museav

    museav CBMod CB Mods Departed Member

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    Are these devices and the audio system served from the same subpanel? Do they share neutrals?

    You may not have been using any mics themselves, but are there any mic or other inputs in the area of these sources? Any actors wearing wireless mics?
    Do any changes to the audio system such as muting or changing input or output levels change the noise? Does the noise go away when you power down the console or certain sources?

    As Sharyn noted, ground and neutral should only be bonded at the service entrance and often the only way to check for some problems is to break that bond for testing, being sure to restore it afterwards.
     
  18. jkowtko

    jkowtko Active Member

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    Okay, more info --

    * separate the two incidents (fire pots and tube TV) as they may be different issues. The fire pots were used two shows ago and have likely since been tossed. The tube TV is in the current production.

    * When we turn the TV on or off we get the static discharge (quick series of pops in about one second) in only the right rear PA speaker.

    * The TV is connected directly to a dimmer channel (#1 bad), and that dimmer is likely one of the ones placed in the upper house right corridor very close to the right rear speaker audio feed cable (#2 bad).

    * The RR speaker will not get the noise if I unplug the audio cable from the back of the speaker (speaker still powered on). If I plug the audio cable back in, I'll get the noise even if there is no signal coming from the board.

    * The TV plug was not directional, but reversing it didn't eliminate the noise.

    Things I should have tested but didn't because I had only a short time in the theater before I had to take off tonight, and therefore I didn't think of them --
    - leave the audio cable plugged into the speaker but unplug from the board.
    - try rerouting the audio cable further away from the dimmer.
    - try a different cable in case this one has shielding problems
    - try plugging the TV into a nearby regular outlet and turn on and off manually

    There are of course a few ways to rectify this ... the most sensible of which for now would be to get the TV off the dimmer and put it on a manual switch that can be thrown from the booth. At this point I'm guessing its the static discharge making it's way back into the dimmer and affecting the audio cable of the nearby speaker. If the above info gives you any other ideas I'm still interested in hearing your thoughts ...

    Thanks. John
     
  19. jkowtko

    jkowtko Active Member

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    Latest update ... checked out a few more things tonight before the show:

    * The TV is plugged directly into a dimmer channel. Leaving the TV plugged into the dimmer, if I route the audio cable away from the dimmer cables I can reduce the noise some but not much.

    * If I pull the TV from the dimmer and plug it into a separate AC outlet in the theater, the noise through the PA is greatly reduced, but is still audible.

    * The only way to completely eliminate the noise is to break the audio connection between the speaker and the sound board. Speaker and sound board are on separate circuits, but both are dedicated sound circuits connected to the same 100 amp primary circuit, and should be separate from either the dimmer or the AC outlet that I tried plugging the TV into.

    In all cases, the TV itself makes a fairly loud static discharge noise when turned off. So it's a noisy old TV. I don't think there will any way to get rid of the static noise coming from the TV itself, but as far as isolating it from the rest of the circuitry in the theater, should some sort of power conditioner between the TV and whatever it's plugged into help?

    Also, do you think the fact the this TV is plugged directly into a dimmer channel could be harmful to the dimmer? I think the lighting guys regularly plug incandescent bulbs (floor lamps, etc) into dimmer channels, but it's these non-trivial electronic devices that seem to have the problems.

    Thanks. John
     
  20. David Ashton

    David Ashton Active Member

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    Get some long power cables and run your powered speakers from the same power points as the mixer, I'm willing to bet that will remove the noise, if it does, try lifting the signal earth at the input to the speaker or feed it through a transformer.You will get earth loop hums on the same circuits, as the earth wire
    loops around the building, by plugging in at the sound desk you are star wiring the supply and can't get an earth loop.
     

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