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Old 12-10-2008, 05:44 PM   #1
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Strange electrical power situation


Perhaps an electrical expert could shed some light on the cause or solution to this.

I build and maintain sound/lighting systems for a company that supplies disc jockeys for weddings. There is one venue, a large hotel, where we regularly have issues and I can only guess it has to do with their power. They provide the DJ with a large portable sub-panel run off a 220 volt wall socket. The power checks out good as far as ground, polarity and voltage but anytime one of our DJs plays there, the electronic equipment acts very strange, and this is the ONLY place where there are any problems with the equipment. The same problems happen if they plug into a wall receptacle as opposed to the sub-panel.

I've been there myself and one of my amplifiers sent a loud buzz through the speaker, and not your typical ground loop hum, but more like a buzz-saw. They have a number of these portable sub-panels and provided me with a different one, and I still had the same problem. My equipment is run through a voltage regulator which shows the voltage to be in the 120 range.

Does anyone have any idea what might be the problem here, or a piece of equipment I could use to make sure the power is completely clean to avoid possible phantom electrical problems?



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Old 12-10-2008, 06:12 PM   #2
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An oscilloscope will reveal whether power is clean (sine wave). IMHO minor noise; the sine wave still looks clean on the 'scope, will not cause problems with most electronic equipment.

You cannot run 120 volt equipment off of a "plain 220 volt" receptacle without a transformer in between. Without a transformer, if the receptacle supplies "220 volts", it must be a 120/240 volt or 120/208 volt supply with a neutral as well as a ground; the plug will then have four prongs. The subpanel should then have 120 volt receptacles each connected to one of the hot lines and the neutral (and the ground).

Did you verify that the equipment was getting approximately 120 volts when it was running? Improper neutral connections can cause voltage problems, more so if different pieces of equipment are connected to opposite sides of a supposed 120/240 volt circuit.

Also your equipment might be sensitive to air borne electrical fields (radio frequency interference). There might be equipment in the next room or above or below creating such fields. Proper grounding of equipment will sometimes alleviate this. You might want to have a long bare wire on hand to daisy chain from one piece of equipment to another, screwed to each metal chassis or cabinet, with the far end connected to a known ground.


Last edited by AllanJ; 12-10-2008 at 06:26 PM.
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Old 12-10-2008, 06:36 PM   #3
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Is this noise continuous? Or does it come and go?

The portable panel will have recepticles fed from both the 220/208 poles. ( I would expect 208 in a hotel)

Does the noise appear on any particular recepts?

Universl motors such as are used in drills, vacuum cleaners and such, can generate all sorts of hash on the power line.

A scope would show this type of interference. If the power is clean, then perhaps its radio frequency interference!

If it R.F. an AM radio tuned to a dead spot on the dial, may give a clue to this!
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Old 12-10-2008, 07:03 PM   #4
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Neutral and ground swapped, perhaps?
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Old 12-10-2008, 08:07 PM   #5
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Originally Posted by Richo View Post
not your typical ground loop hum,
So it's not a sine wave, at least not when it comes out of the speakers.
Originally Posted by Richo View Post
but more like a buzz-saw.
Check the freq. with a scope.
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Old 12-10-2008, 09:13 PM   #6
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Are the power cords used to feed the subpanels 3-wire or 4-wire? In other words, does the plug have 3-prongs or 4? I ask because if they are 3-wire, then the ground is being provided by the neutral, which is carrying a current; a current which may be feeding back to cause the noise on your system. You need a separate equipment ground that is isolated from the neutral at your subpanel. I also like the idea of jumping your equipment together with a jumper. This will serve to equalize the potential across all of your equipment that may be connected together but fed from different circuits.
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Old 12-10-2008, 10:14 PM   #7
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When I used to do live sound there was always one hotel that gave us a buzzing sound(not a ground loop) in our system. I was always told it was because there was a casino in there before and there was some type of security or something in the power. Now that I know more about electricity it does not make sense to me.

They also say that this hotel was haunted so maybe it was one of the ghost comeing after us.
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Old 12-10-2008, 10:28 PM   #8
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Are there fluorescent lights?

Anyway if there is "noise" in the electrical lines, you might want to try a surge suppressor power strip which also has an "EMI RFI Noise Filter". Like this one...
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Old 12-11-2008, 07:06 AM   #9
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I agree with the other posts on recommending an oscilloscope. Make sure you are using the correct probe, and the ground clamp is connected to the ground. If any of your equipment is 208/220V, you may need two probes, one in each of the hot legs of the 208/220V circuit, then set the scope for differential mode.

Check for dimmers. They give off lots of noise, especially when at a low setting, and it would be a sharp waveform rather than sine wave, since they are phase angle devices.

Definitely check whether you have a 3 or 4 wire plug. You will want a separate equipment ground.



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