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Author Topic: AC outlet ground loop hum  (Read 5830 times)

Mike Sokol

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AC outlet ground loop hum
« on: January 21, 2014, 10:20:46 am »

I just tested the outlet out of curiosity and this was the result.  No real problem.  Is this something I should look into further with the utility company?  Is it unrealistic to think that outlets can deliver a clean power ( clean sine wave )?

There's nothing wrong with your power waveform, and calling the power company about this will be a lesson in frustration. They will neither understand nor be able to do anything about it. Modern power supplies for electronic gear are regulated and designed to digest AC power with lots of distortion and voltage variations. Dropouts are a different story, so as long as you don't have equipment resets, don't worry about it. 

Power conditioners are highly overrated as a way to stop hum, and generally will do nothing to fix it. I just did a sound system tune-up over the weekend in Orlando, and they had the most hideous monitor ground loop hum I've heard in a long time. They paid an electrician big bucks to add a second ground rod, installed big copper plates in the back of the sound racks, and #6 wires between the gear chassis. Of course there were also expensive power conditioners added to everything. It took me all of 5 minutes to figure out the ground loop hum and fix it. I just popped out the aux send lines one at a time until the hum stopped, confirmed it with a clamp-ammeter and found around 2 amperes current on the shield of the line feeding that channel, then put a Whirlwind ISO-2 box in the line between the board and the drive rack. Problem solved....

A quick look confirmed they had the mixing console and the drive/amp rack on separate 3-phase power panels. That was stupid since both 3-phase panels were in the same room within 10 feet of each other. I don't know how many thousands of dollars they spent trying to fix the hum, but the original installer was the one who recommended the power conditioners and additional ground rod(s). I'm not impressed with their ground-loop troubleshooting abilities.

So, before you let anyone start pounding in extra ground rods or adding power conditioners to stop a ground loop hum, troubleshoot by using a clamp-on ammeter on each signal line until you find any AC current (it only takes 100 mA to make significant hum) and add a good quality 1:1 audio isolation transformer in the line with the shield lifted. It's lots cheaper in the long run, and should NEVER hum under any circumstance.   

As far as your AC power distortion... fagiddaboutit...
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Greg_Cameron

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Re: AC outlet ground loop hum
« Reply #1 on: January 22, 2014, 01:00:59 pm »

I just popped out the aux send lines one at a time until the hum stopped, confirmed it with a clamp-ammeter and found around 2 amperes current on the shield of the line feeding that channel, then put a Whirlwind ISO-2 box in the line between the board and the drive rack. Problem solved....

I keep a bunch of Whirlwind ISO1 transformers handy for such stuff. The WW transformers can take much hotter levels and lower frequencies than the ubiquitous Sescom barrel style iso transformers without saturation. The Jensen transformers spec out better, but due the nature of them being an "input" transformer, they have to have their outputs connected with a foot or less of wire to the input of the device they're feeding or the signal degrades. So they're not as versatile.

As for the barrels iso's go generally speaking, I don't recommend them if you can avoid it. If you must have them, the Whirlwind versions can take a bit more juice than the Sescoms before saturation and have better LF frequency response.
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Mike Sokol

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Re: AC outlet ground loop hum
« Reply #2 on: January 22, 2014, 02:16:11 pm »

As for the barrels iso's go generally speaking, I don't recommend them if you can avoid it. If you must have them, the Whirlwind versions can take a bit more juice than the Sescoms before saturation and have better LF frequency response.

Many users forget that smaller transformers saturate quicker and poorer low frequency response than larger transformers. And there's just not much room for a transformer of sufficient size in a barrel ISO connector. This is true of DI boxes as well at 1:1 ISO transformers. About 10 years ago I did an experiment and paralleled 6 different DI boxes on a common console output, then fed the individual XLR outputs into 6 console inputs. That way I could run music with a lot of high and low frequencies into everything simultaneously, then listen to each one by un-muting the channel. IIRC my cheap Peavey and DOD DI boxes really lost a lot of bass, my mid-price WW IMP-2 box sounded quite good, and my expensive Jensen DI didn't seem to sound any better than the WW box. I had a few other DI boxes of unknown Chinese heritage that were horrible sounding when you put any significant level through them, so they were given away to my students for parts.

However, there's more to a DI box than just frequency response. So perhaps the Jensen would do better than the WW under controlled CMRR tests or in a nasty RF environment. I've never set up that kind of test, but I have done a few tabletop experiments with DI boxes and RPBG (Reverse Polarity Bootleg Ground) outlet fails testing with 120-volts AC across the transformer windings. Kids, don't try this at home. 

Basically, any DI or ISO box is just a transformer with connectors, so transformer size and quality determines the ultimate sonic quality. If it's got a cheap or small transformer inside, than all the fancy switches and connectors on the box won't help. However, even a great transformer can be degraded by poor connections, so those are important too. I like the WW gear since it's never let me down at a gig, and I know that Frank DeWitt (on this forum) makes a quality DI box as well. Just don't expect a barrel ISO transformer or a cheap import DI/ISO box to sound as good as the big boys...
« Last Edit: January 22, 2014, 03:14:01 pm by Mike Sokol »
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Lyle Williams

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Re: AC outlet ground loop hum
« Reply #3 on: January 22, 2014, 03:29:55 pm »

... it is better to have cheap gaffer tape than no gaffer tape!  :-)

Isolation transformers need to be cheap enough that you can own enough to get yourself out of trouble.  Few people will notice low frequency rolloff on some fill/delay speakers run off different power.  Everyone will notice hum.

The Radial Twin ISO looks really cool, but down here it is 6x the price of the Art DTI.  The DTI seems to have ok specs from the testing published in SOS.
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Greg_Cameron

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Re: AC outlet ground loop hum
« Reply #4 on: January 24, 2014, 08:08:11 pm »

Isolation transformers need to be cheap enough that you can own enough to get yourself out of trouble.  Few people will notice low frequency rolloff on some fill/delay speakers run off different power.  Everyone will notice hum.

The Sescom IL-19 sells at Full Compass for $42 for the basic model. The one with a ground lift switch sells for $69. Rat Sound has the Whirlwind ISO1 for same price as the more expensive IL-19 PRS GLS but it has far superiors specs. It also has both 1/4" and XLR connectors along with the ground lift button. For me, it's a no brainer. the ISO1 is in a small square box to accommodate the larger transformer. It's not that bulky relative to the benefits :)
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Mike Sokol

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Re: AC outlet ground loop hum
« Reply #5 on: January 24, 2014, 08:35:26 pm »

The Sescom IL-19 sells at Full Compass for $42 for the basic model. The one with a ground lift switch sells for $69. Rat Sound has the Whirlwind ISO1 for same price as the more expensive IL-19 PRS GLS but it has far superiors specs. It also has both 1/4" and XLR connectors along with the ground lift button. For me, it's a no brainer. the ISO1 is in a small square box to accommodate the larger transformer. It's not that bulky relative to the benefits :)

I really do like having both TRS and XLR inputs/outputs. You never know when you're going to have a cheapie console with TRS outputs you need for a monitor send. Plus I've paralleled a few of these together using TRS jumpers to act as a poor-mans Distro-Amp feeding multiple power amps plugged into separate wall receptacles. Since this box has a 600-ohm input impedance, and most XLR console outputs should be able to drive down to at least 200 ohms or less, you could parallel three of these together without adding distortion. I've done two channels like this before and it worked perfectly with zero hum.

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Barry Singleton

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Re: AC outlet ground loop hum
« Reply #6 on: January 26, 2014, 12:29:07 pm »

The Jensen transformers spec out better, but due the nature of them being an "input" transformer, they have to have their outputs connected with a foot or less of wire to the input of the device they're feeding or the signal degrades. So they're not as versatile.


  The Jensen DM2 series transformers have no cable length restriction on either side.
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Jordan Wolf

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Re: AC outlet ground loop hum
« Reply #7 on: January 26, 2014, 10:09:10 pm »


  The Jensen DM2 series transformers have no cable length restriction on either side.
And it's better to knock out the noise as far ahead in the signal chain as possible (if that is indeed where the issue lies)...I'd rather use an output transformer at the source end if possible.

I need to get me a DM2...Birthday's comin' up in April - maybe I'll splurge!
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Mike Appleby

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Re: AC outlet ground loop hum
« Reply #8 on: February 15, 2014, 12:36:21 pm »

Mike,

I'm sure you have answered this a thousand times, but I am going to ask you to answer it one more time...here goes..

What is the easiest way to reduce or eliminate hum in your active FOH cabs and wedges and, how can I stop getting my lips fried every time I step to the mic to sing while I'm touching my guitar strings?  How in the hell can you get shocked by your mic while touching your guitar strings when you are using a wireless???? This boggles my mind!!! I have been told hundreds of times to make sure that your mixer is plugged into the same circuit as your active cabs and if used, your outboard gear rack.

If you already have a post on this, please point me to it...I'm eager to read and learn.  It's so frustrating to setup in a venue and find "dirty power" that causes the dreaded "hum" to drive you and the audience nuts.

Mike
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Tim McCulloch

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Re: AC outlet ground loop hum
« Reply #9 on: February 15, 2014, 02:47:06 pm »

Mike,

I'm sure you have answered this a thousand times, but I am going to ask you to answer it one more time...here goes..

What is the easiest way to reduce or eliminate hum in your active FOH cabs and wedges and, how can I stop getting my lips fried every time I step to the mic to sing while I'm touching my guitar strings?  How in the hell can you get shocked by your mic while touching your guitar strings when you are using a wireless???? This boggles my mind!!! I have been told hundreds of times to make sure that your mixer is plugged into the same circuit as your active cabs and if used, your outboard gear rack.

If you already have a post on this, please point me to it...I'm eager to read and learn.  It's so frustrating to setup in a venue and find "dirty power" that causes the dreaded "hum" to drive you and the audience nuts.

Mike

Do you receive the shock without the guitar?

Sokol has several threads here that are relevant, you might want to take a closer look at this forum's index.

It took me years (okay, maybe decades) to determine that there is no such thing as "clean" electricity once it leaves the generation facility.  This is speaking form the standpoint of the line frequency's symmetry of waveform, lack of harmonic content and voltage regulation.  Everything downstream will have some kind of affect on the "quality" of power.

That said, mis-wired outlets/cord sets and sound/musical equipment with dubious grounding schemes (or actual component failures) are the likely suspects in any situation in which a person receives a shock.  Inductive hums are more often a function of instrument pick ups, poorly shielded audio transformers or use of unbalanced audio circuits.

Mike S. suggests keeping a non-contact voltage tester around.  He uses a Fluke 1AC or 2AC (big box home store price is around $25).  He discusses testing various stuff for unexpected voltage presence in another thread, and it's illuminating (pun intended).
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Re: AC outlet ground loop hum
« Reply #9 on: February 15, 2014, 02:47:06 pm »


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