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Control room monitors hummmmmmmmmm

Discussion in 'Sound, Music, and Intercom' started by Tomas, May 6, 2009.

  1. Tomas

    Tomas Member

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    HAHAH GOOD MORNING EVERYONE! Yet another day we must save the day! Okay so at our high school our monitors have a hummmming issue. When ever our lights aren't at 100% we get this humm even at 100% there is still a noticeable humm. This is with nothing even going through the system everything is muted. Any way of getting this done cheep? because I already had to pay $200 for a mic rental. (they are coming later today) And pay for new monitors for our control room $60. and thats not even half it. Add on $80 in gels. $400 soundboard when ours *&^$$^ out. $100 shure SM58 because some smart person stole ours. umm $20 in patch cables also stolen. Needless to say I have a back up of everything with me. And to top it off I'm only 17 and my car fund is almost gone.:(:evil:
     
  2. Madeen

    Madeen Member

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    The school is making you pay for this stuff???
     
  3. avkid

    avkid Not a New User Fight Leukemia

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    They need to be on a separate supply, usually a different panel.

    Try running a long extension cord to another room that you know is on a different panel, if that fixes it you know what has to be done.
     
  4. seanandkate

    seanandkate Well-Known Member Fight Leukemia

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    You can get something called a HumX that will eliminate a ground loop hum. You plug it in to your existing power supply and then plug your audio components into it. It will set you back about a hundred and change, but it's certainly LESS expensive than having a new circuit with run with an isolated ground.
     
  5. Eboy87

    Eboy87 Well-Known Member

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    Are these monitors on stage or in a booth? Are they powered (amp in the speaker) or passive (amp seperate in a rack somewhere)? Where are they plugged in? In otherwords, is it your dimmers on the circuit, work lights in the booth (if the speakers are in a booth), or dimmable lights on the circuit? Is there a computer monitor, or computer plugged into the circuit? All those contribute to buzz. I'd suggest you start troubleshooting with those questions. Also, how are the monitors fed, are they off an aux on the board or a mic in the house somewhere? Are you certain that feed is clean and buzz free?

    And if you're paying for rentals and gear out of your own pocket, you are doing something very wrong. I trust you are getting reimbursed for this? As I learned from Heath Ledger, "If you're good at something, never do it for free."*



    *I really didn't learn that from Mr. Ledger, but it's still valuable advice.
     
  6. museav

    museav CBMod CB Mods Departed Member

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    These often work quite well, but it is not necessarily a good solution for every application. You are not supposed to connect a plug strip or any anything that draws more than than 6 Amps to a Hum-X, so they may not work for an amplifier or multiple pieces of equipment. There is also some concern that the method this device uses to provide isolation under normal operation also leaves it susceptible to failure when most needed. Basically, there are diodes or transistors that electronically isolate the ground path until the current exceeds a certain level, however it is not clear what a very high current might do to this circuitry including it potentially failing and not providing a safety ground path. It is also apparently not UL listed, unusual for any power related device.

    The hum could be a ground loop or it could be induced in some cabling, a little more information and troubleshooting may help determine which.
     
  7. derekleffew

    derekleffew Resident Curmudgeon Senior Team Premium Member

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  8. jsparhwk

    jsparhwk Member

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    I regularly have to do sound setups..mostly powered
    in places I haven't been before, and I carry around a
    handful of ground lifts, pickem up at lowes for 50cents
    because I often can't get the mixer and speakers on same circuit
    rarely had a problem they couldn't fix.
    also not sure of your setup but check to make sure there isn't a
    lighting dimmer within a few feet of the monitors...
     
  9. derekleffew

    derekleffew Resident Curmudgeon Senior Team Premium Member

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    Hopefully I'll get here before others who might not be as diplomatic.:neutral:
    Just a few of the reasons that's a bad idea, [user]jsparhwk[/user]:
    from the same document I referenced above: http://redirectingat.com/?id=41X76&url=http://www.ebtechaudio.com/findloop.pdf.

    See also the threads http://www.controlbooth.com/forums/sound/7027-overheard-today.html, http://www.controlbooth.com/forums/sound/3732-ground-loop-issues.html.
     
  10. Chris15

    Chris15 CBMod CB Mods Premium Member

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    Like, say me?

    Numero uno, I doubt they can be picked up at a hardware store, but a ground lift that lifts pin 1 in XLR is absolutely fine by me.

    If you cannot afford to fix your problem without use of a mains ground lifter, can you afford it when you get sued because someone gets zapped or worse electrocuted because you lifted a mains ground and something faulted?

    There are very few electrical codes in the world that permit the fusing or switching of Earth connections. (We'll ignore substation design here). Basically these hyped up ground lifters are 2 diodes back to back on the Earth line - meaning you impose a 0.7 volt drop across the Earth, effectively blocking all bar the worst noise.
    I would argue that fiddling with the Earth in this way is no different from putting a switch or fuse in. The particularly worrying thing is that you will never know until it is too late if the device has failed. I mean no Active or no Neutral and you don't get power out. No Earth and nothing tells you there's a problem...

    Basically you have a single point of failure - a dry joint and the thing is lethal and the failure mode is catastrophic. I don't know how many of you have any knowledge of formal risk assessment, but you multiply severity of failure by likelihood multiplied by likelihood of detection. Let's see now (scale is 1 - 10, 10 is bad, 1 is good):
    Severity - 9 (death), Likelihood - 5 (at least a 1% chance), Detection - 10 (no way of detecting failure) = 450 = Immediate action needed.

    RA is all subjective, but basically those devices are a law suit on a timer.

    In the case of active speakers on sticks etc where allegedly one can't get common point of supply for power, throw a pin 1 lift in or use an iso transformer. (Oh how we miss the days of everything being transformer balanced...)
     
  11. museav

    museav CBMod CB Mods Departed Member

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    As Chris said, an audio ground lift is a different thing than a power ground lift. If you are talking about an audio Pin 1 lifter, that may be a good option. However, if you are referencing the common three pin to two pin AC adapters like this, Cooper Wiring Devices at Lowe's: Grounding Adapter, then these are not ground lift devices and have to be used improperly, in fact in almost the exact opposite manner as intended, to do so. Instead these devices are intended to adapt modern three prong devices with a dedicated safety ground conductor to older electrical systems that used the metallic boxes and conduit for the safety ground path. The safety ground pin connects to the tab on the adapter which is supposed to connect to the metal plate or box, thus providing a continuous ground path. If you break off the tab or use these in any other way you are defeating the entire purpose and creating a potentially deadly condition. Using such an improperly modified device as a ground lifter may be okay to help determine if a problem is indeed a ground loop, but they should never be viewed as a solution, even temporarily.

    What is sort of ironic in all this is that the susceptibility of audio systems to ground loops is really more a problem with the audio equipment. The problem is that many audio devices allow any signal on the shield, such as the resulting from a ground loop, to be introduced into the audio signal path. Ignoring shield current induced noise (noise induced differentially into the audio signal conductors by their relative physical relationships to the shield or drain wire in the cable), if signal and chassis grounding are properly addressed in the equipment design then ground loops should not be a problem as any ground loop current does not directly affect the audio signal. As such practices become more prevalent the problems should also be less common, but that may be some time and doesn't fix systems using older equipment.
     
  12. TheatreImage

    TheatreImage Member

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    This woun't fix your problem but is a great White Paper on power and grounding system for audio and video applications.

    http://www.surgex.com/pdf/PowerGround.pdf

    Also from this paper - some more reasons not to "lift" the AC ground

    "In our discussion of grounding, we learned that the Equipment Ground must be extended to each outlet, and from there to each piece of connected equipment. It must be a bond – that is, a mechanically robust connection of very low impedance – so that breakers or fusesoperate quickly and reliably to protect personnel and property in the case of a power fault. Interrupting any portion of Equipment Ground (for example, by breaking off the ground pin of an AC plug or using a "ground lift" plug), or adding a series impedance (like an inductor or "choke") that reduces fault current is unsafe, and could cause electrocution. Products that add an inductance in series with the Equipment Ground, in the name of "cleaning up dirty grounds" are unsafe, because they reduce fault current, especially the leading edge of fault current, increasing the time that it takes a fuse or breaker to operate."
     
  13. nd925a

    nd925a Member

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    How do you run your sound board to your speakers...
    I had a similar problem at an elementary school where I had to use cables they supplied to hook in a seperate board through a mic jack in the wall into a rack backstage...
    It turns out the cables that were supplied for me to use were bad so I got this offol hum.
     

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