amplifier humming & recording room ground looping

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balanced cables will have 3 conductors, 1 black, 1 red or white and 1 bare wire. If it is an XLR type connector (the plug end is round with 3 pins in the center) the shield solders into pin 3. you can simply cut the shield where it terminates onto the connector. If it is a TRS type connector (looks like a larger version of a headphone jack) the shield is soldered onto the metal frame of the connector. You do the same thing, cut the shield at that point. when I used to install large sound systems, we always lifted the shields on every line level input. just make sure you don't do it on the microphone input.
 
Grounding Tutorial -Hum and Buzz in audio circuits

Grounding Tutorial -Hum and Buzz in audio circuits

bohl1076 said:
hello, i figured this would be a grounding issue! i was talking to a friend who has a recording studio at his house, but can't seem to get rid of the humming and noise interferance from his equiptment. right now i'm scratching my head cause i haven't dealt with much audio stuff before, but am going to check it out next week. can anyone give a suggestion as to what i can do/or look for, that may help?
thanks, bohl1076
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First of all.. Greetings! I am a part time audio professional at night, and full time electrician / instrumentation process control technician by day.

Secondly, I highly recommend downloading and reading Bill Whilock's Grounding Tutorial from here:

http://campuspa.com/downloads/grounding_tutorial.pdf

It's geared for audio professionals, but highly useful for electricians as well.

A couple of quick notes:

There are usually racks and stacks of audio gear in clubs, concert halls and studios, etc. It is important to note that the audio equipment frames, racks and chassis grounds are tied together, not only to the equipment grounding conductors, but through a myriad of shielded/grounded audio cables as well.

It is highly recommended that you use a dedicated, local, or portable distribution subpanel, and fan out ALL audio power from there. The grounding bus MUST be at the center of the fanout of ALL audio gear, to insure a zero volt potential between all receptacle grounding prongs.

In other words, avoid plugging into existing in-house receptacles from multiple locations. Variances in gound potentials, such as from the stage versus the front of house location, can cause noisy current flow to travel down shielded audio cables.

The "ground loop" occurs when a voltage or noise source develops across the power grounds between different chassis' of audio gear, and they find parallel paths with the grounding conductors and shielded audio cables. To simplify the concept, imagine a parallel conductor... Now pull it apart in the middle... You have a "loop."

One thing about audio equipment, they amplify millivolt signals and convert them to high sound pressure levels. Normal ground leakage, say through an old MOV of a power conditioner, wouldn't be an issue to a toaster oven, but when it comes to audio, noisy grounds get amplified, and you can hear it.

The key here is the fact that so many pieces of audio gear is grounded from multiple receptacle outlets, and that all this gear also gets tied together through shielded audio cables, forming many "loops" or parallel paths.

There are many NORMAL sources of noise that develop across grounding conductors of multiple circuits in large venues. And I wouldn't be able to cover all of them here in this post, such as an unbalanced cable picking up EMI from a stomp box's wall wart, or noisy MOVs, death cap shorts in vintage equipment, leakage, etc., but there is ONE phenomenon that occurs, and it is highly undetectable with normal test equipment, until AFTER the fact, when all the gear is hooked up...

So I'll mention it here... Neutral/ground reverse wired in a receptacle.

Note: The link I provided on the Grounding Tutorial covers this problem towards the end...

Think of what would happen if you had a ground/neutral reverse at a receptacle...

With just a toaster oven plugged into it, it would appear to work OK. But of course you'd have neutral return current travelling down the grounding conductor illegally, with parallel paths from the yoke, metal box, and conduit creating many more return loops throughout the building. But still, the toaster over would "work."

Whats worse is, a reversed ground/neutral will test good with a three prong neon tester (the kind you?d get from the hardware store).

The real problem is what happens when you have a bunch of audio gear plugged into at least one reversed neutral/ground receptacle, mixed in with properly wired receptacles. Under these circumstances, audio will provide a number of parallel neutral return paths though the assortment of audio cables and their shields. And yet, reversed wired neutral/ground receptacles will test good.

The only way to test for abnormal neutral current in audio shields is ONLY after everything is hooked up and powered up. Only then you could use a clamp around ammeter, and go searching for the needle in a haystack, by dividing and conquering, hopefully finding which receptacle(s) is miswired. But, at least if you're aware of the possibility, it'll help with troubleshooting.

One more thing... Portable distributon panels or "distros."

There are basically two types: 1) Typical breaker panels (usually feeder supplied wired as a subpanel) and 2) What we call "The Poorman's Distro."

Click here for more information: http://www.whiskey-creek.net/pwrdist.html

The Poormans distro is good for small night clubs, where basically you DO use multiple receptacle outlets, from multiple locations. It ties all the grounding conductors from multiple circuits together, inside the poorman's distro grounding bus. All hots and neutral circuits remain paired, and pass through the distro, isolated from each other.

As with any type distro used, ALL audio power, including backline, stage and front of house equipment, MUST FAN OUT from the same grounding bus, centrally located forming a single point grounding bus, ensuring zero volt potentials on the all the grounding conductors fanning out from there.

So, for now, this post is getting too long...

I hope the downloadable tutorial and poormans distro link helps.

See ya around...

Lee Patzius
Hazelwood MO
 
justin59 said:
balanced cables will have 3 conductors, 1 black, 1 red or white and 1 bare wire. If it is an XLR type connector (the plug end is round with 3 pins in the center) the shield solders into pin 3. you can simply cut the shield where it terminates onto the connector. If it is a TRS type connector (looks like a larger version of a headphone jack) the shield is soldered onto the metal frame of the connector. You do the same thing, cut the shield at that point. when I used to install large sound systems, we always lifted the shields on every line level input. just make sure you don't do it on the microphone input.
Click to expand...

The newest standard is:

XLR Pin 1 is shielded.

Pin 2 is hot

Pin 3 is cold

But you'll find on older equipment, pin 2 and 3 may be reversed hot and cold. (Pin 1 is still shielded.)

Reversed hot and cold mixed with proper hot and cold is usually no big deal audio wise, especially if all the systems are unbalanced.

But if you mix quazi-balanced (unbalanced) or UN-balanced XLRs found on older equipment, where the cold pin is tied to shield through a low impedance path in the gear, then you can run into problems. Mixing older pin 3 cold unbalanced XLRs equipment with older pin 2 cold unbalanced XLR equipment will short the signal out through their low impedance paths to ground.

On a different note: Lifting shields is bad practice. But if the need arises, especially in troubleshooting, most equipment manufactrers today have provisions for lifting "pin 1" in XLRs.
 
Lee, welcome.

I also do audio, and own a sound company, among the very many things that I do.
We can provide up to 32x8 FOH @7800watts
Plus 12x6 monitor mix with 2000w plus in-ear channels.
Although our monitor board is in for repair, so right now we only provide up to (4) monitor mixes.
But for most, a 16x4 mix is all that is needed and typically 5800w FOH.

The Tutorial link you provided has some good info. But when it gets into the IG receptacles, I'm not sold.

Also, the 'Poorman's Distro' I don't think will help the OP.
He is talking about a home recording studio.
 
ty said:
Electronics wizzes want to over-ground most of the time. They usually want to drive their own seperate ground rods.


Now that's funny.
They Should, but do they?

ohmhead, have you seen this done, or is this jobsite talk?
I have a hard time believing that they would not be using isolation transformers if needed.
Maybe you just didn't see them being used.

I could see their engineers lifting a ground to find the problem, but not leaving it.
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Just to be sure...

Small signal isolation transformers, used at audio signal levels, and as found in DI boxes etc., with pin 1 lift capabilities, are for low voltage instrument and audio signal cables, and can be switched to open any grounding loop, or temporarily put a "bandaid" on equipment with pin 1 problems.

OTOH, heavy power isolation transformers are only good for isolating the secondary power from the source by breaking the grounding bond from the secondary "neutral" side.

Infortunately, like all transformers, they pass AC. That includes the 60Hz primary signal right along with any noise or spikes right onto the secondary side.

And, isolation transformers are usually wired to pass the grounding conductor, unless you break it, and turn it into a separately derived system.

To actively get rid of power line noise, you could use a generator, or the kind of UPS's that invert full time.

Or if passively, with in-line line reactors, which are series wound with the load, but they'll have to be sized properly.

I've tried a number of Topaz 2.4KVA power line isolation transformers in real world stage settings, and they never did seem to work quite right at getting rid of power noise, especially from noisy dimmer packs and lighting controls.

The best cure for hum and buzz was keeping lighting controls separate (on the house system), and using a portable distro subpanel, or poormans distro, which tied all the grounds together at a single point (grounding bus) remotely, at the stage location, and fanning out all audio power from there. (And Not using in-house receptacles from multiple locations.)
 
bohl1076 said:
this may be a stupid ?, but would over sizing the neutrel help with any of this?
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See 520.53(O)(1)

Oversizing (doubling) the neutral is needed when it is required to carry twice the ampacity of the largest phase leg conductor. This usually occurs when over the road tour groups paralell two (out of three) hot legs of their 3 phase 4-wire switchboard onto one hot leg of a single phase service.

Other than that, it is possible for nonlinear loads to overload a common neutral on multiwired loads, when connected to "Wye" services. But oversizing the neutral doesn't rid the system of noise.
 
ohmhead said:
<snip>
Point is the sound tech geeks at disney or universal have the ability and electronics back ground .

May you should apply for a job there and let them know what there doing wrong because iam not we like there projects its income .

Did you know disney has there own building dept and its own electrical rules & regs .

There a city on there own they do not use your local or county electrical inspectors .
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They are still stupid and are needlessly placing performer's lives in extreme danger. How can you possibly in good conscience defend their actions? How would you like to be the one whose actions injures or kills a performer?

I have worked at another major So Cal theme park and we FIRED techs who compromised safety grounds to eliminate hum!!

I don't care if it's Disney or the President's screening room, THERE IS NO EXCUSE FOR COMPROMISING SAFETY, EVER!!


ohmhead said:
What do you do when you have a insulated tool install a ground on it ?
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That is such a stupid thing to say I'm not even going to grace it with an answer.:roll:

ohmhead said:
Relax think about it i think there poeple are highly able and know what there doing .
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I do not relax in the face of life-threatening stupidity, and it is patently obvious they DO NOT know what they are doing!! I have worked literally hundreds of shows and have NEVER had to resort to ground lifting to stop hum.

ohmhead said:
we did not take ground off they did !
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Neither I nor anyone else ever said or implied that you did take the grounds off.

ohmhead said:
I dont think they use radio shack equipment at Disney or universal studios.
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Again I never said or implied they did, I was referring to the OP's situation.

ohmhead said:
Example orange county convention center does it also the sound crew takes it off during a big show like during a concert you need to ask them why .:roll:
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Another example of people who have no concept of either electrical safety or the proper way to set up a sound system.

Finally, while I agree that income is always good, I will not knowingly work in a situation where the client's incompetent people are removing the Code-compliant grounds that I installed.

You really need to consider this: If someone does get hurt or killed, and investigation reveals that the grounds were removed from YOUR installed circuits/recepts, do you honestly think that your client is going to fess up and say they did it? They're going to point fingers at YOUR company and you're going to have to defend yourself from the liability!! Good luck with that...and if it ever gets out that your company KNEW of the practice and did nothing about it, you're really gonna be in deep!

Simply not worth it. There is enough work out there with people who care about quality work.
 
ty said:
Lee, welcome.

I also do audio, and own a sound company, among the very many things that I do.
We can provide up to 32x8 FOH @7800watts
Plus 12x6 monitor mix with 2000w plus in-ear channels.
Although our monitor board is in for repair, so right now we only provide up to (4) monitor mixes.
But for most, a 16x4 mix is all that is needed and typically 5800w FOH.

The Tutorial link you provided has some good info. But when it gets into the IG receptacles, I'm not sold.

Also, the 'Poorman's Distro' I don't think will help the OP.
He is talking about a home recording studio.
Click to expand...

Awesome! Are you a member of PSW?

As far as the poormans, it works great in noisy clubs. But I see your point, in a quiet home studio, as compared to commercial/industrial settings, it may not fix whatever his problems are, but I can't see how it would hurt to try!
 
The first thing the poor hummy chap should do is to run everything off one outlet, using lots of multiboards. That provides a single ground point, which as noted above, is the most important factor in getting a random sound system to stop humming.
 
leepatzius said:
Awesome! Are you a member of PSW?

As far as the poormans, it works great in noisy clubs. But I see your point, in a quiet home studio, as compared to commercial/industrial settings, it may not fix whatever his problems are, but I can't see how it would hurt to try!
Click to expand...

If you are referring to the web forum, No, I am not a member.
 
Coming into this late, it seems...

Can we ditch the "use only one phase" theory, please? I doesn't make sense either theoretically or actually, and just muddies the water. In every case where someone has insisted that it helped, rearranging the wiring to get to only one phase also rearranged the grounds to remove loops. It's all about the grounds, not the hots.

dbuckley said: "The first thing the poor hummy chap should do is to run everything off one outlet, using lots of multiboards."

I think the chap's time would be well spent reading up on grounding techniques and plotting out the signal flows. Then, redesign the connections to eliminate the loops and recable everything.

Also, IIRC the old Shure M-series mixers called pin 3 as "hot".
 
zbang said:
Coming into this late, it seems...

Can we ditch the "use only one phase" theory, please?
Click to expand...
I agree. it is totally not necessary.

dbuckley said: "The first thing the poor hummy chap should do is to run everything off one outlet, using lots of multiboards."
Click to expand...

It might be necessary for troubleshooting the problem.
plug a cord into one wall outlet and run everything from it. see if there is ground noise.
Then move to another outlet. continue until found.

In a home recording studio, it is quite possible to run everything off of one circuit.

I have a pretty large 'home recording studio' and I could run everything off of one circuit. I don't, but I could.
 
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