How Can Changing AC Sockets Cure Hum?

[Stevens R. Miller]

I'm working with a local community theater group. We've rented a middle school auditorium for our show. Last night was our first tech rehearsal in the space. We brought our own audio mixer. Setting up, we found two pairs of AC sockets. One had a sticker on it that said, "For Harmony Middle School Audio Only." Dutifully, we plugged into the other one.

Well, we had a nasty hum, even when all of our inputs were muted and all of our faders were down. After trying everything else we could think of, we unplugged from the socket we had used and plugged into the one "For Harmony Middle School Audio Only." The hum went away (not entirely, but almost).

What could be different about two AC sockets that would cause one to cure the hum the other allowed?

 

[sk8rsdad]

They're probably running on different phases or through different distribution panels. Google ground loop for hours of learning fun.

 

[RonHebbard]

They're probably running on different phases or through different distribution panels. Google ground loop for hours of learning fun.

All of the above and additionally one of the receptacles could be a proper IG device c/w a legitimate insulated isolated ground conductor clear back to the common (and grounded) point of the transformer supplying the breaker panel. Initially IG receptacles were always orange but it wasn't long before many other colors came on the market with only a tiny, green, IG symbol to differentiate them. Unlikely perhaps but still a possibility.
Edited to add: I suppose I should mention it's not unheard of for painters to paint right over receptacles removing all traces of IG identification.
Toodleoo!
Ron Hebbard.

 

[BillConnerFASTC]

Or they lifted the ground on the receptacle - really isolated it. They may have gotten tired of breaking off the ground pin from the plug and getting fired for it by inspectors.

 

[RonHebbard]

Or they lifted the ground on the receptacle - really isolated it. They may have gotten tired of breaking off the ground pin from the plug and getting fired for it by inspectors.

I fully agree with you Bill, especially when they purchase a proper IG receptacle to avoid grounding via the mounting strap and then do as you suggest to ensure it's totally ungrounded. Some people never learn while some never ground.
Toodleoo!
Ron Hebbard.

 

[cdavisnyc]

In our space, audio only AC outlets are on a separate conditioned power supply (to prevent spikes & drops). We only use it for audio & video equipment. Thus, no hum.

I suspect that the label meant "audio only" instead of "Harmony Middle School only"

 

[AlexDonkle]

In our space, audio only AC outlets are on a separate conditioned power supply (to prevent spikes & drops). We only use it for audio & video equipment. Thus, no hum.

I suspect that the label meant "audio only" instead of "Harmony Middle School only"

Unfortunately, power conditioners don't really prevent group loop hum like is being described here. If you're powering all your audio gear from a single outlet, that's the easiest method of having no hum (since there's only 1 ground), with or without a power conditioner.

 

[BillConnerFASTC]

I'm use to isolation transformers with isolated ground feeding all av power receptacles, usually identified by color but also within a panel that is all av.

 

[cdavisnyc]

Unfortunately, power conditioners don't really prevent group loop hum like is being described here. If you're powering all your audio gear from a single outlet, that's the easiest method of having no hum (since there's only 1 ground), with or without a power conditioner.

No, you're right about that. The power conditioner doesn't prevent the ground loop hum. I didn't mean to imply that it did.

Keeping audio power separate just means we don't have to spend time hunting down the source of the ground loop.

 

[Fountain Of Euph]

I have to run 80ft of power directly from the audio rack on the third floor boot to FOH on the first floor to specifically prevent ground loops...

Sent from Taptalk for Android, this was.

 

[Wheezy]

What could be different about two AC sockets that would cause one to cure the hum the other allowed?

The short answer is no two ground points are exactly the same potential.

There could be other causes as well, such as a lighting circuit near the other ground, causing power influence. But, maybe by accident or intention, the school found the ground on this outlet is more direct.

If, for instance, your mixer is better grounded than your amp in another outlet, the amp bleeds some of its return through your XLR cable.

 

[BillConnerFASTC]

Steven - next time you are there - plug in a simple outlet tester and see if there is a ground. I did interpret your comment about the label to suggest it was a homemade label - like a building maintenance person might do.

 

[Aaron Becker]

I agree with what everyone else said.

Are the outlets labeled by panel/circuit number by chance? You would significantly reduce your chances of having hum (ground loops, as others have mentioned) by using power off the same circuit, and further reduce it by pulling from the same outlet.

I've been in a couple schools that were on the older side where dedicated "sound system power" was pulled through the crawl space or ceiling (by a qualified electrician, after getting the appropriate permits) for this exact purpose so the FOH equipment could be tied to the drive racks and on-stage sound. It virtually eliminated the hum issues that venue was having.

I agree with Bill, the label seems to be a homemade style, curious to see if the ground is present on that outlet or not...

As a general practice, I now test all outlets (including those on power strips) before plugging ANY of my equipment in. I've found too many places with hot/neutrals reversed (in 120 applications) or missing neutrals that could potentially cause issues to my equipment racks.

 

[Stevens R. Miller]

Steven - next time you are there - plug in a simple outlet tester and see if there is a ground. I did interpret your comment about the label to suggest it was a homemade label - like a building maintenance person might do.

Yes, the label is just a sticky paper thing with handwriting on it. The outlet must have been an afterthought, as it is just inches away from another one. Well, maybe they were installed at the same time, but it just doesn't look that way to me (and there is no such outlet in the other middle school where I've done tech, and the two schools are nearly identical, down to the nuts and bolts).

Am I reading this correctly: some of you are suggesting we make sure the mixer is not using the ground wire in the socket? Is that safe?

Regarding running a power line from the rack in the wings: I thought of doing that, on the theory that, if I could be sure the rack mixer and my technician's mixer (the one whose output is going into a channel on the rack mixer) had a truly common ground (or as close to one as the laws of physics allow), there couldn't be a ground loop. Any downsides to that? Safety issues? Fire code? What's the longest run of a power cord I should consider?

The connection from the tech's mixer to the rack mixer is via a single XLR-3 cable. We did swap that with another, to no effect. Regardless, if there's enough 60-cycle energy in the house, is it possible the cable is playing a role? FWIW, when I dim the house incandescents to 50%, the hum gets worse.

 

[BillConnerFASTC]

But did you test it? Toodbox staple. (I unfortunately have a too addiction disorder - TAD - and have many tools of his sort - and have my eye on a megger, without a real need for one.)

 

[Stevens R. Miller]

But did you test it? View attachment 13330 Toodbox staple. (I unfortunately have a too addiction disorder - TAD - and have many tools of his sort - and have my eye on a megger, without a real need for one.)

Not yet. I have one of those things, somewhere...

I've long had the fantastical dream that my workshop will someday look like this:

But, for now, it looks more like this:

 

[RickR]

The question is whether you have an isolated ground or no ground at all. Not grounding things is NOT good! It's unfortunately become a solution by sound techs that don't understand the dangers involved. I was recently in an AV tech seminar and some of the best sound operators in town all admitted they sometimes break the ground. Performers have been killed by broken grounds! See this Richard Cadena article.

A true isolated ground bypasses the grounding buss at the breaker panel and connects to the bonding point (transformer, source ground, etc) where the neutral and ground meet earth ground. Noise can't travel back up the "iso-ground" so any unintended ground loops or leakage in the power system will be avoided. Even a separate ground to the panel is better than sharing one with whatever is on the other circuit, especially if it's a dimmer or motor.

 

[BillConnerFASTC]

The question is whether you have an isolated ground or no ground at all. Not grounding things is NOT good! It's unfortunately become a solution by sound techs that don't understand the dangers involved. I was recently in an AV tech seminar and some of the best sound operators in town all admitted they sometimes break the ground. Performers have been killed by broken grounds! See this Richard Cadena article.

A true isolated ground bypasses the grounding buss at the breaker panel and connects to the bonding point (transformer, source ground, etc) where the neutral and ground meet earth ground. Noise can't travel back up the "iso-ground" so any unintended ground loops or leakage in the power system will be avoided. Even a separate ground to the panel is better than sharing one with whatever is on the other circuit, especially if it's a dimmer or motor.

Not only performers but a few pastors in the the midst of a baptism as well.

 

[Stevens R. Miller]

The question is whether you have an isolated ground or no ground at all. Not grounding things is NOT good! It's unfortunately become a solution by sound techs that don't understand the dangers involved. I was recently in an AV tech seminar and some of the best sound operators in town all admitted they sometimes break the ground. Performers have been killed by broken grounds! See this Richard Cadena article.

That's a great article. I never knew how GFCIs worked before. It's also full of pretty sobering stuff.

A true isolated ground bypasses the grounding buss at the breaker panel and connects to the bonding point (transformer, source ground, etc) where the neutral and ground meet earth ground.

Okay, help me keep up with you here, if I can: By "neutral," I believe you mean the side of the two-wire pair that provides the 120 VAC to whatever is plugged into them that is not one of the two phases delivered to my house (and that, when used together, provide me with 240 VAC). The ground provides a way for a chassis or enclosure to be connected to earth ground. What I've never understood is why there is a need for the ground wire if the neutral wire is connected to ground. For example, we have a counter-top mixer (Kitchen Aid, five-quart, definitely what you want when baking cookies). It has a three-prong plug and an all-metal enclosure. If the neutral and ground on its plug are both, by virtue of being plugged into a three-hole socket, electrically connected, how is that different from simply connecting the neutral pin on a two-wire plug to the mixer's enclosure?

(I have to say I feel really dumb, asking this. I've been an electronics hobbyist for a long time, but I admit I can see that "electronics" and "electricity" are not the same things. Regardless, I feel like I ought to know the answer to my question, but I just don't.)

 

[Aaron Becker]

Steven,

I'll attempt to shed some light on this without crossing the dangerous "do not enter" territory of controlbooth I've seen before for liability reasons (mods, I'm new, so forgive me if this crosses the line)...

Neutral - in non-scientific terms (electrons and atoms), the neutral wire is the "return" path for the electrical current serving any given device.

Your house (and most residences in the US) have 240 volt service from the "street" (whatever means that may be), with 3 conductors - two "hot" and 1 "neutral". (4 if you count the ground).

The purpose of a ground path to earth (ground) (in layman's terms) is if somewhere in the electrical path/device/wire/etc electricity is getting lost somewhere-and not making it's way back to the neutral/ground bus bar at the tie in, breaker panel, etc. It provides a safe path for electrons instead of taking the shortest path to ground, which is what, by nature (science, actually) they want to do. The shortest path could be a priest, performer, or technician, as mentioned above. The ground prevents this. Removing it, although solving a sound problem, creates many safety issues. The ground wire is also connected to the metal on the device (on your counter top mixer, any metal you can touch on the frame is grounded, on a sound board, the frame (if metal) is grounded to earth ground) to discharge stray electrons. The GFCI, as mentioned above, also helps by shutting off the flow of electricity in the event that electrons are not following the prescribed hot-neutral path within the circuit.

Best of luck,

PS- those of us above who were inquiring about the ground on the outlet were not suggesting it SHOULD be removed, I think we were curious from a standpoint if that's why it "solved" the hum problem or if it was something bigger.

PSS- the fact that the lights being at 50% created a louder hum could be a variety of issues. Grounding, unshielded lines run in long, close, proximity to AC lines or lighting lines, etc. Hard to diagnose without knowing more details.