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Author Topic: Source of AC-induced buzz ?  (Read 3968 times)

Mark McFarlane

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Source of AC-induced buzz ?
« on: December 14, 2012, 11:06:49 pm »

Did a gig last night and had a fairly loud buzz in one powered speaker cabinet (TT22A)

As I started unplugging signal cables the behavior was very strange, buzz level 'changed' as I plugged and unplugged mains and subs from the DSP.

In the end, the noise was isolated to a single powered speaker, and the buzz remained even when the 65' XLR was disconnected (from the DSP end).  Moving that speaker to a new 120V outlet solved the problem.

Curiously, what kinds of things might be going on in that circuit that would cause a continuous buzz?  A ground loop explains the behavior of the buzz changing as I re-patched at the DSP (physically bypassing, disconnecting cables,...), but doesn't seem to explain the speaker buzzing when it wasn't connected to anything else (except an open-ended 65' XLR).  Unfortunately I didn't test the outlet, it's a venue I've used before with generally good power...
« Last Edit: December 16, 2012, 10:34:19 pm by Mark McFarlane »
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Mark McFarlane
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Tim McCulloch

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Re: Source of AC-induced buzz ?
« Reply #1 on: December 15, 2012, 12:21:00 am »

Did a gig last night and had a fairly loud buzz in one powered speaker cabinet (TT22A)

As I started unplugging things the behavior was very strange, buzz level 'changed' as I plugged and unplugged mains and subs from the DSP.

In the end, the noise was isolated to a single powered speaker, and the buzz remained even when the 65' XLR was disconnected (from the DSP end).  Moving that speaker to a new 120V outlet solved the problem.

Curiously, what kinds of things might be going on in that circuit that would cause a continuous buzz?  A ground loop explains the behavior of the buzz changing as I re-patched at the DSP (physically bypassing, disconnecting cables,...), but doesn't seem to explain the speaker buzzing when it wasn't connected to anything else (except an open-ended 65' XLR).  Unfortunately I didn't test the outlet, it's a venue I've used before with generally good power...

The ground and neutral are tied together at some point where they should not be.
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John Roberts {JR}

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Re: Source of AC-induced buzz ?
« Reply #2 on: December 15, 2012, 11:07:30 am »

Low voltage...

JR
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Tim Perry

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Re: Source of AC-induced buzz ?
« Reply #3 on: December 15, 2012, 03:41:55 pm »

Possibly a lamp dimmer or other non linear load on that circuit.
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Geoff Doane

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Re: Source of AC-induced buzz ?
« Reply #4 on: December 16, 2012, 08:47:36 pm »

Possibly a lamp dimmer or other non linear load on that circuit.

I've seen this problem in the building I work in.  The power was nice and quiet until we installed a large (80KVA) UPS. Certain equipment (not all) has a residual buzz that wasn't there before.  It's slightly worse on the line side ratehr than the load of the UPS.  Ironically, running the equipment from a smaller, double-conversion UPS cleans it up.  I tried every trick know to man to get rid of the buzz (including "illegal" things like lifting the ground, and more conventional methods like an AC isolation transformer), and the only thing that would work was the second UPS.

The buzz isn't really loud, but it's noticeable (and unacceptable) in a quiet studio environment.  So far the only equipment that has had the problem is some db Technologies powered wedges.  They have conventional (not switching) 2-rail power supplies.  But other equipment with conventional supplies doesn't seem to suffer from the noise.

The problem seemed to be caused by harmonics on the AC waveform.  I had a post about it on the old forums with pictures, but I can't find it now.

GTD
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Tim Perry

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Re: Source of AC-induced buzz ?
« Reply #5 on: December 17, 2012, 02:40:37 am »

I've seen this problem in the building I work in.  The power was nice and quiet until we installed a large (80KVA) UPS. Certain equipment (not all) has a residual buzz that wasn't there before.  It's slightly worse on the line side ratehr than the load of the UPS.  Ironically, running the equipment from a smaller, double-conversion UPS cleans it up.  I tried every trick know to man to get rid of the buzz (including "illegal" things like lifting the ground, and more conventional methods like an AC isolation transformer), and the only thing that would work was the second UPS.

The buzz isn't really loud, but it's noticeable (and unacceptable) in a quiet studio environment.  So far the only equipment that has had the problem is some db Technologies powered wedges.  They have conventional (not switching) 2-rail power supplies.  But other equipment with conventional supplies doesn't seem to suffer from the noise.

The problem seemed to be caused by harmonics on the AC waveform.  I had a post about it on the old forums with pictures, but I can't find it now.

GTD

I have a similar problem with a 130K generator. 5th and 7th harmonics from a 60K non linear load reflect back and drive the regulator nuts.  It checks fine into a dummy load.
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Art Welter

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Re: Source of AC-induced buzz ?
« Reply #6 on: December 17, 2012, 11:28:35 am »


In the end, the noise was isolated to a single powered speaker, and the buzz remained even when the 65' XLR was disconnected (from the DSP end).  Moving that speaker to a new 120V outlet solved the problem.

Curiously, what kinds of things might be going on in that circuit that would cause a continuous buzz?  A ground loop explains the behavior of the buzz changing as I re-patched at the DSP (physically bypassing, disconnecting cables,...), but doesn't seem to explain the speaker buzzing when it wasn't connected to anything else (except an open-ended 65' XLR).  Unfortunately I didn't test the outlet, it's a venue I've used before with generally good power...
A 65' unterminated XLR makes a good antenna for noise, regardless of the AC used ;^).

As Tim McCoulloch said: "The ground and neutral are tied together at some point where they should not be", or the outlet could have a hot/neutral reverse, or even a ground/neutral reverse.
The latter won't be detected by the usual "plug in light" devices, and can be a real headache.

The fact that the problem cleared up when the cabinet was plugged in to another outlet would indicate either a ground loop or a miswired outlet.
Next time in you should check the outlet carefully, hot/neutral reverse, or ground/neutral reverse are dangerous.
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Mark McFarlane

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Re: Source of AC-induced buzz ?
« Reply #7 on: December 17, 2012, 01:54:59 pm »

A 65' unterminated XLR makes a good antenna for noise, regardless of the AC used ;^).
...
Next time in you should check the outlet carefully, hot/neutral reverse, or ground/neutral reverse are dangerous.

Thanks everyone for the help.  I usually check all outlets with a DMM and a non-contact tester but I've used this room several times,... Something changed since the last time I was in it, or I never used that particular outlet.  I shall be more careful in the future. I should have checked it when I had the problem but the band was walking in for sound check,...

Excuses, excuses...
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Mark McFarlane
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Tim McCulloch

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Re: Source of AC-induced buzz ?
« Reply #8 on: December 17, 2012, 02:00:38 pm »

Thanks everyone for the help.  I usually check all outlets with a DMM and a non-contact tester but I've used this room several times,... Something changed since the last time I was in it, or I never used that particular outlet.  I shall be more careful in the future. I should have checked it when I had the problem but the band was walking in for sound check,...

Excuses, excuses...

Mike Sokol posted over at SFN about the reverse neutral/ground phenomena and his comments are well worth a read.  The bad news is that this kind of problem is pretty much not detectable with a meter or outlet tester.
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"Practicing an art, no matter how well or badly, is a way to make your soul grow, for heaven's sake. Sing in the shower. Dance to the radio. Tell stories. Write a poem to a friend, even a lousy poem. Do it as well as you possible can. You will get an enormous reward. You will have created something."  - Kurt Vonnegut

Greg_Cameron

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Re: Source of AC-induced buzz ?
« Reply #9 on: December 17, 2012, 04:26:25 pm »

Mike Sokol posted over at SFN about the reverse neutral/ground phenomena and his comments are well worth a read.  The bad news is that this kind of problem is pretty much not detectable with a meter or outlet tester.

Yup, visual inspection only. If the wiring at the outlet looks correct as far as color coding and the problem is upstream, then you've got a bigger headache to deal with.
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Re: Source of AC-induced buzz ?
« Reply #9 on: December 17, 2012, 04:26:25 pm »


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