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Author Topic: 2-phase hum?  (Read 6565 times)

Mike Sokol

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2-phase hum?
« on: February 10, 2016, 11:25:13 am »

I just read this article posted on PSW about chasing hum in a church sound system. http://www.prosoundweb.com/article/accidental_electrician_eliminating_dreaded_sound_system_hum_buzz/

While much of it was a reasonable tutorial on troubleshooting hum, one particular paragraph seems to be incorrect.

"So the original system installer (OI) had specified that the circuits needed to be all on the same phase. My guess is that some well-meaning electrician had thought it would be smart to group the circuits together during one of the church expansion projects. I returned the circuits to their original spacing.

Having all the power circuits on an identical phase is important for minimizing the possibility of an inter-chassis current in a ground loop.

Power supplies leak small amounts of AC to ground. If there happens to be enough leakage from gear in the ground loop, and its of differing phases, an inter-chassis current will flow through the loop great enough to induce a hum.

By establishing a common system power phase I minimized the ground loop hum potential."


Now I've heard this idea that connecting a sound system across two different phase legs can cause ground loop hum, and it appears to be common industry "knowledge", at least with some of the sound tech's I've talked to over the years. However, I think it's an "old wives tale" that's simply incorrect.

I've seen where connecting a sound system so that it bridges two different sub-panels causing a lot of ground loop hum. But that's because the sub-panel grounds are at two different voltage potentials and the equalization current can be huge (many amperes).

So what's been your observations? Has anyone ever proved the allegation that ground loop hum can be caused by connecting a sound system so that it bridges 2 different legs of 3-phase power?
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Mac Kerr

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Re: 2-phase hum?
« Reply #1 on: February 10, 2016, 11:43:56 am »

So what's been your observations? Has anyone ever proved the allegation that ground loop hum can be caused by connecting a sound system so that it bridges 2 different legs of 3-phase power?

My personal experience is mostly with 3 power, and I make an effort to balance the load across the phases to minimize neutral current, but I have not had any notable experiences of hum caused by multi phase power.

Mac
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Tom Bourke

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Re: 2-phase hum?
« Reply #2 on: February 10, 2016, 12:12:19 pm »

If a properly wired split phase or 3 phase system causes hum then the problem is in the connected gear.  The only exception, as already noted, is spanning several panels or services in a building.
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John Roberts {JR}

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Re: 2-phase hum?
« Reply #3 on: February 10, 2016, 12:15:39 pm »

"Ground loop" (?) hum potential.  If a loop exists that converts a magnetic flux into voltage, it seems drawing  power from both polarities (same phase) could in theory create opposing magnetic fluxes, so logically could reduce ground loop hum not increase it.

"Ground Loops" are used as a catch all phrase for ground potential differences.

JR 
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Chris Hindle

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Re: 2-phase hum?
« Reply #4 on: February 10, 2016, 12:44:16 pm »


So what's been your observations? Has anyone ever proved the allegation that ground loop hum can be caused by connecting a sound system so that it bridges 2 different legs of 3-phase power?

Like Mac, Whether 3 phase, 2 legs of 3, or split 220/110 never a problem for me. I also try to balance the loads.
Chris.
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Mike Sokol

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Re: 2-phase hum?
« Reply #5 on: February 10, 2016, 12:57:09 pm »

Here's where ground loop hum comes from...  ;D

Gear that has the "pin-1" problem will hum if you have current coming in the (pin-1) shield. My bench testing confirms that there can easily be around 1 amp of current per volt of ground potential difference. Since I've found that even 100 mA of ground loop current can make many self-powered speakers hum significantly, that suggests it only takes 1/10 of a volt ground difference to make a sound system hum.

I like to use a clamp-ammeter right around the XLR cables to look for this current. As you can see from the diagram, since the return current is outside of the jaws of the clamp-meter, there's no need to split out the twisted pair from the shield. Just clamp around the cable and you'll see if there's any ground loop current in the shield.

Since there's really no way that connecting across gear across two different phases can cause this current path, I don't see how it can contribute to hum. And you can also see that normal (sub-mA) power supply leakage currents can't be anywhere near the 100mA hum threshold I've noticed on the bench. 
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John Roberts {JR}

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Re: 2-phase hum?
« Reply #6 on: February 10, 2016, 01:52:34 pm »

Perhaps I'm being overly pedantic about the terminology but "ground loops" are a real thing, actual physical conductive path loops typically inside products, even on circuit boards that convert magnetic energy present to voltage like a one turn transformer winding (a real issue inside power amps with huge transformer magnetic fields).

As others have shared these are generally product design (pin 1 problems) issues, while large fault currents flowing in and between sundry grounds can cause problems.

UL specifies that equipment safety grounds can sink tens of amps with only modest voltage rise. OTOH tens of amps in mic cable shields will likely release some smoke (as you know and have seen). The magnetic field to generate that much current in a wire loop would be significant. I expect that much ground current is coming from other sources.

JR
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Stephen Swaffer

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Re: 2-phase hum?
« Reply #7 on: February 10, 2016, 02:15:15 pm »

Our church sound system is installed on a 3 phase system-3 phases sharing a common neutral (probably would do it different today-but have seen no reason to change it).  I have never run into an issue and quite honestly, really don't concern myself too much with what phase we are on as we don't use subs and really not pushing the circuits.  The only issue I have ever fought has been with a ground loop caused by a projector connected to another panel (most subpanels in this building are not wired correctly).

Leakage to ground from different phases would tend to cancel-leakage from the same phase would add thus making the problem worse.  Maybe you just keep changing wires around willy nilly until it sounds better?   
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Steve Swaffer

David Buckley

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Re: 2-phase hum?
« Reply #8 on: February 10, 2016, 02:37:55 pm »

Has anyone ever proved the allegation that ground loop hum can be caused by connecting a sound system so that it bridges 2 different legs of 3-phase power?
This is utter bollocks, so no-one ever will.  Ground loop hum is caused by, well, ground, more specifically, potential difference across what is supposed to be a single ground.  The hots and the neutral can come from anywhere, but if the ground isn't common, well...

The other noteworthy thing is that the first act that happens to power of any phase in a sound system is it gets converted to DC.
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Stephen Kirby

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Re: 2-phase hum?
« Reply #9 on: February 11, 2016, 02:05:04 pm »

My first thought is that everyone running a small California plug distro would immediately have such a problem.  But the neutral to both legs as well as the ground to both legs is at the same potential in the distro since each is on a single wire.  Any neutral to ground current will be common to both legs.  Using only one side of the distro (phase, if you want to misuse that term) kind of cripples why you have it in the first place.
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ProSoundWeb Community

Re: 2-phase hum?
« Reply #9 on: February 11, 2016, 02:05:04 pm »


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