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Old November 13th, 2008, 03:59 AM   #1
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Wiring problem

I'm trying to work out why a fairly recent audio installation in a hall is so noisy.

Briefly, there are 2 XLR sockets, twin phono sockets and a stereo 3.5 jack socket wall mounted in surface boxes and wired with individual screened cables. So far so good, but unfortunately there is also a VGA connection for a projector in an adjacent (metal) box and all the the cables are routed through this box and the audio and VGA cables seem to be in the same conduit thereafter until the VGA heads off to the projector. The audio cables continue to similar plastic surface box mounted sockets in the lighting control room.

The audio connections have a high level of mains hum and buzz, and while they can be used for PA with a reasonably strong signal, without any input the level of buzz and hum is loud enough to make people disconnect what should be a permanent connection to a computer soundcard output through the stereo mini jack connection. Apparently the computer is on its 4th soundcard and the IT contractors will charge for any future blown cards.

Basic question - is it OK to run VGA and audio cables together? Should the presumably balanced audio cables take care of any potential interference?
The VGA cable seems just to be a multicore without screening.

Last edited by Colin McDonald; November 13th, 2008 at 02:38 PM.
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Old November 13th, 2008, 04:21 AM   #2
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Sounds like an earth grounding loop, I would get a qualified audio installation engineer to come in and sort it out for you. I think the computer output needs isolating with balancing transformers to stop earth leakage and to buffer it from the PA system.

It is a typical problem when going from unbalanced to balanced equipment and as said an installation engineer will be able to sort it out.

It should be OK to run VGA with audio cables and balancing will help but it sounds like the forementioned earth loop if the hum goes when the computer is disconnected.
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Old November 13th, 2008, 08:53 AM   #3
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I'm with Gary on this, but here's a suggestion.

Break the ground loop by using a battery powered mixer or just a microphone at the head end of the audio. The end you first described with the two XLRs, etc..

If you still get buzz and/or hum, then the problem lies elsewhere. If the buzz/hum goes away, then there's a difference in ground potential between where you have the mixer plugged in and where the gear on the other end of the conduit is plugged in.

Try operating battery powered devices on both ends of the conduit. If it's clean then you can begin to presume it's a power grounding problem. If it's not clean and you are off the grid, then something is radiating in or into the conduit system. Maybe the conduit is not grounded properly or runs past a big AC mains transformer or some other noise source.

You can suss that by running cables instead of using the conduit. PITA, but you may find it that way. I was recording audio for video at a site a few years ago and SOMETHING was under the floor in certain parts of the room. Whenever my balanced cables were there, I got buzz. In fact, even though my cables were balanced, someone accidentally moved a power cable so that it was parallel to my cables for about 10 feet. That's all it took to get the AC into my audio.

Or, there may be Silicon Control Rectifiers (SCR) dimmers for lighting control. If the lights are dimmable, listen for that noise while you turn on the lights and fade them up. Dimmer noise is usually worst when you're dimming a lot. It tends to go away when the lights are up full. SCRs may be in other rooms or even on other floors of a structure and they will still leak (radiate) into your audio.

Or, there could be something wrong with your house wiring. That screen may not be grounded after all. Regular electricians are not always aware of the "best practices" required for AV. Fun stuff. I love ghost hunting.

Regards,

Ty Ford
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Old November 13th, 2008, 02:36 PM   #4
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Thanks, Gary and Ty. I will take some time to try out your suggestions, although funding for a consultant engineer is not possible at present. I know the audio connections are noisy even when the lighting is not fired up but ground loops (earth loops to us poms) are certainly a suspect. I've got some battery powered gear available to try out.

I have had to run my own XLRs (by a more direct route) for recording in this hall a few times due to the probs with the installed cabling and it worked fine. I've also used radio links which gave much better audio than using the suspect cabling (an exception to prove the rule if ever there was!) but I know that this could bite me in the backside at any time so I'd rather not have to rely on wireless if possible.

Your help much appreciated, gentlemen.
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Old November 13th, 2008, 03:10 PM   #5
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Quote:
Originally Posted by Colin McDonald View Post
Thanks, Gary and Ty. I will take some time to try out your suggestions, although funding for a consultant engineer is not possible at present. I know the audio connections are noisy even when the lighting is not fired up but ground loops (earth loops to us poms) are certainly a suspect. I've got some battery powered gear available to try out.

I have had to run my own XLRs (by a more direct route) for recording in this hall a few times due to the probs with the installed cabling and it worked fine. I've also used radio links which gave much better audio than using the suspect cabling (an exception to prove the rule if ever there was!) but I know that this could bite me in the backside at any time so I'd rather not have to rely on wireless if possible.

Your help much appreciated, gentlemen.

Colin,

We can't know about the terms of the original installation contract, but I do have to point out that any installation done in a fashion so incompetent that it results in unusable audio is a sub-standard installation regardless of how long it's been in use.

There still might be recourse to the original contractor if you can demonstrate that the installation has never worked to industry standards.

Don't let the contractor off the hook until and unless they can demonstrate that they did the work competently in the first place and that something has been damaged via user error subsequently.

If you let poor contractor work go unchallenged, then there's no incentive for that contractor to amend their practices so that the next customer doesn't have to face the same incompetence. And the GOOD contractors who know how to wire things correctly (and who must charge accordingly for their actual knowledge and expertise!) will suffer and have trouble staying financially viable leaving all of us with just the cheap knuckleheads as vendors.

NOT a comforting thought.

Good luck.
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Old November 14th, 2008, 08:53 AM   #6
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hum is almost always a grounding problem. you stated that the conduit and wall box are metal. is the plate the connectors are mounted in also metal? if so, that would be the first place to start. unbalanced connections use a common ground for the audio, but this ground should be isolated from earth ground, or the mains ground. mounting the connectors in a metal plate would ground that audio to the conduit, which is surely earth grounded eventually. this is probably why your own cables worked fine, they did not connect to any grounds along the way.

if you have the infamous Radio Shack stores there, try using something they call the Ground Loop Isolator. it's basically a stereo isolation transformer with pre wired RCA phono connectors-it also has a stereo 1/8" adapter handy for soundcard outputs. this will isolate the audio ground for you, as most computers share audio and power grounds. it's under $20 and will help if grounding is the problem. if not, there are other things to try, but that's where I would start.
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Old November 14th, 2008, 10:26 PM   #7
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I'm with Ty on this for the most part. another possibility is the A/V lines run right next to a AC power line. it would be just like a contractor to wire tie the AV lines to the AC lines and let them use the same run to save time and labor. You would not believe what I've seen contractors do to save literally 2 or 3 dollars, never mind $50. in theory conduit ( metal ) should shield the AC line, and so should BX but - if they used plastic conduit, romex ( illegal for most commercial construction ) or the metal conduit isn't well grounded, it could be carrying some voltage potential and inducing hum

if this is the case, it means re- running the A/V wiring. If this is new construction, I'd go back to the original contractor and make them fix it either way. clearly they did not really test the system.

one possible fix is isolation transformers until it can be redone the right way. if its a ground loop, they can be tricky to find.
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