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Discussion Starter · #1 ·
Picture this:

1) Ceiling, front mounted projector

2) Computer, DVD connected to a switch box on an approximate 300ft run (of VGA and composite (no repeater on comp)) to said projector

3) VGA runs through a 4 output multiplier; one to a second monitor, one to a background projector, one to the ceiling projector

4) Line to ceiling projector is split in the video room once more (composite from split runs to a video mixer) just after a 210ft repeater

5) Audio from computer and DVD runs from switch box to Soundcraft GB8/32 mixing console (attaches to 2 crown xti amps 4000 and 1000)

6) Output from console off the 1-2 subgroup to video mixer (soon to be run from the 11 by 4 matrix outputs 1 and 2)

7) 3 Canon cameras (1 xl1 and 2 gl1) S-video cable runs of 100ft, 75ft, and 25ft to video mixer (no repeaters)

8) Power to building (400A single phase) runs to main buss fuse and splits to three legs: one leg to breaker panel A (computer, projectors, DVD, sound booth, cameras, house and stage lights (stage lights run from secondary box, 100A breaker in panel A)), one leg to panel B (60A breaker supplies power to the video booth from panel B) and one leg to panel C runs to the back of the building (6 to 7 breaker panels in total)


Yes sir we have ground loops!!!!


Questions: What's the best way to unify the grounds from 2 sources while one is supplied from another panel and the grounds don't technically coverge until they hit the main buss? Or, any ideas on how to get rid of a ground loop between the sound board and the video mixer and the computer and the video mixer? Should I just install a new panel and put all A/V related equipment in it's own box?
 

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For video, all you need is a device (or a few) called a Humbucker. These are availble from several manufacturers in single channel or multi channel for RGBHV operation. To save money, a multichannel can be used to handle up to 5 baseband video paths.


Audio isolation transformers do the job for line level signals. They use XLR balanced connections.


The first thing though is to have an electrician check all lines and grounds for proper earthing and potential.


Then you need to work one section of the signal path at a time. If you jump all over the place putting "Iso" boxes everywhere you may well overspend way beyond what is necessary.


Check out Kramer, Extron, Calrad, Humbucker, Jensen Transformers, Shure,

TECNEC, BiTronics, B&H audio video for starters.
 

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Discussion Starter · #3 ·
I have tried a clamp on ferrite common mode choke that I stole from a power cord not in use. It should be effective on a 50 to 60hz hum, but had no effect on the distortion. I've also tried running an extra ground from the casing of the sound board, to no avail.
 

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Discussion Starter · #4 ·
I already know where my disruption is occurring. The worst is between the computer and the video mixer. The second source is between the sound board and video mixer. Leading me to believe that it's a difference in ground potenial given the two seperate power sources. Just wanted to see if anyone had some ideas about correcting the issue outside of iso transformers and the like. Thanks for the replies.
 

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What are you seeing? Usually there are shaded hum bars (darker color) or dark gray bars that drift horizontally or vertically through the image. Usually there is no gross distortion of the image.


The first issue is multiple AC ground points. The best way to install a system like this is for all power to be on the same phase (can usually be on separate legs) and all ground are tied to a central point.


One thing of note there should be NO ground connections between the building framework and any sound or video signal wiring. The ONLY point of grounding should be the electrical safety ground on the equipment.


Metal racks containing AV gear should be physically and electrically from the building ground by insulating all conduits etc. The current practice is to install a PVC coupling between and conduit and the rack.


Aside from a major power rewire and some involved trouble shooting, the isolation transformers are your best bet. These are quality units and will not degrade your signal(s) at all.
 

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Discussion Starter · #6 ·
Yeah, I've been playing with it for the last 6 hours. Checked every connection and ground that I can get to and I get no change. Rewiring is such a hassle and I'm wondering if it could be a combination of things. I'll just have to bite the bullet and isolate (transformers that is) the problem cables. I really appreciate you batting this back and forth with me. It helps having someone on the outside of the problem who can comment on something I may have overlooked. Thanks a lot Gizmologist.


By the way, that's exactly what I see (the hum bars scrolling vertically up the screen). The one from the audio source is very faint, but the one from the computer stands out like a sore thumb.
 

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Discussion Starter · #7 ·
I almost forgot, the power comes into the main buss at a 400A service via one pair of wire/ one transformer. All the same phase. All connections to every panel in the building eventually ground to that main buss box.


Any RF or EM interference picked up by any cables shielding should be able to be eliminated/reduced by use of the common mode isolators/reducers......


Did I mention that I didn't build this system, I'm just the guy that people call when things go wrong and things have definitely gone wrong. This building has been through numerous remodels and, from what I understand, the A/V system was not installed by professionals or people that really knew what they were doing. Just a little disclaimer for ya there, wouldn't want to confuse people. Might hurt my street cred or worse, my business!!! LOL!


Anyway thanks again and if you know of anywhere that makes quality iso transformers for decent prices let me know.
 
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