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Forum: General Discussion

Topic: Ground Loop Issue When Connecting Second VGA Monitor

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As many of you know I'm venturing in to the world of video and have posted my video starter kit here.

However I have noticed a hum/buzz, but only when I connect anything to the VGA out on the laptop. It's not noticable when the music is loud and when there is an atmosphere in the club, but at the start of the evenng you can hear it.

I'm also now running the sound for the comedy night in the club and am using VDJ to display the Jongleurs logo on the clubs screens and overlaying the comedians name using Titler amongst other things.

Now while noone complained on Saturday, the hum/buzz was clearly audiable when the comedians were doing there stand-ups.

I've been looking at ground loop isolators, but they all appear to be inline audio filters and I'm not sure this is what I need. When I had this issue before (mains hum), turning down the master volume killed the hum. This time, the hum exists even when all the volumes are down. Even with the mixer turned off. So I'm a tad confused.

Has anyone experienced this before???

Cheers,

Roy
 
Inviato Mon 29 Mar 10 @ 4:54 am
I had same problem and solved it as follows

Disconnect the ground wire on the mains of your power supply.
In fact I did not cut it off, just purchased an adapter that I place on my mains outlet cable.


Johans
 
Have already done that on the lappy. That solved the problem then. Just been speaking to someone locally and the suggestion is that the nxt earth in the chain is the screen. I'm gonna modify an IEC (kettle) lead and disconnect the earth and try it out later.

Will report back.

Cheers,

Roy
 
 
Like I said, I don't think this is audio related... If I turn the master volume down, even turn the mixer off, the hum is still there, so it's not "within" my setup. Therefore this won't help.

Didn't get a chance to try it out this evening, but will be in the bar tomorrow helping with some re-wiring so will try unearthing the then.

Cheers,

Roy
 
Could it be the USB powered video converter or the USB charger you are using to power it?
Does it happen when you use composite out of the converter?
Does it happen when you use VGA out of the converter?
What type of gear are you running the video siganl to once it comes out of converter? Splitter, multiplexer?
Do you get the loop hum VGA out of the laptop to a regular monitor?
 
TDBennet,

Good questions. Here's the answers...

TDBennett wrote :
Could it be the USB powered video converter or the USB charger you are using to power it?

I'm certain it's not the converter as I got the hum when I simply connected the laptop to the 32" LCD via VGA cable.

TDBennett wrote :
Does it happen when you use composite out of the converter?

Need to check again, but I'm certain that composite on it's own didn't make a hum. It was only when I plugged in to the VGA out. Will check again tomorrow.

TDBennett wrote :
Does it happen when you use VGA out of the converter?

Still get the hum :(

TDBennett wrote :
What type of gear are you running the video siganl to once it comes out of converter? Splitter, multiplexer?

Currently it's only local. By that I mean a 15" flatpanel for my own use and composite to a single 32" LCD. Not linking back to their VAMS yet.

TDBennett wrote :
Do you get the loop hum VGA out of the laptop to a regular monitor?

Have never heard one, but then I've never had a second output connected with a PA system before. When I use a second monitor at home, all I have on are the powered PC speakers. Never a hum through them.

Thanks for helping me break this down. Will know more after trying various things tomorrow.

I never have time to plug/unplug everything normally as the club is open when I arrive and I don't want to do that sort of an equipment check in front of customers.

Cheers,

Roy
 
you guys are right in using a "proccess of elimination" to track that problem,breakin the ground somewhere could be the fix.sometimes reversing polarity on an AC plug (at the monitor?) will do it..I suspect the Clubs wiring....its a drag no doubt...
 
Right, hum found and partly resolved...

Went down the club this morning and plugged in each piece of equipment one at a time, whilst having the sound system set to "live" levels.

Everything went well until I connected the 2nd screen. In this instance I wasn't using the video converter, just the local flat screen that I took in to the club. Hum was there even if I just touched the VGA plug to the socket on the lappy.

I turned the power off to the screen and tried touching it again - same problem. Unplugged the screens power from the wall socket and the problem stopped!!!!

So I brought a spare home made IEC/kettle lead with me and disconnected the earth and tried again - problem solved. No hum!!!!

Yippee I thought, so I continued to plug things in one at a time and this time plugged in the video converter. Experimented with powering it from the lappy and from the USB mains adaptor I bought - so far so good...

Then I plugged the composite cable in to the nearest 32" screen and then in to the adaptor - boom!!!! big hum... So now I obviously have an earth hum from this screen. However, can't get to the power supply as its well hidden. Plus this is only a temporary solution.

Tomorrow I'm hoping we get the cable run from the DJ Booth to the VAMS matrix when I can try again. Worse case will be that I get an earth hum from the matrix. The it will be a case of trying to remove the earth from it's lead - not sure if its IEC though.

Will report back once I've tried that config.

Cheers,

Roy
 
 
Roy, if you are going to eventually distribute video on a wider scale at this venue, you may find that the distribution equipment will help with the ground loops. Quality gear should provide good sound and image. If you have distribution gear there now try placing it in the mix and see if it helps with the loop caused by the monitor.
Also, do you get the ground loop if you use your video converter and the composite video option with this monitor?
 
@JoeyKJ, thanks for that will lookn in to something like this if necessary...

@TDBennett, that's what I am hoping. However the VAMS is on the opposite side of the club and I need a composite cable run from the booth before I can try this out. Actually I want two run - one in and one out, so I have a local view of what's playing on the screens without having to crick my neck!!!

Like I said, the plan is to get these two cables run tomorrow. Then I can test again.

Cheers,

Roy
 
Sounds like a fun project, post some pics when you get it up and running.
Good luck.
 


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