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[frank kayser]

Hi folks,
We've had a ceiling mount projector tied into our Touchmix16 for years now without issue.
Last night I get a panic call - can't get any volume out of the DVD player.
I get in, loud buzz, little signal.  I swap out HDMI cable, audio cable.  Still a loud buzz overwhelming any signal.  30 minutes to showtime, I abandon that setup. An organizer brings in a replacement projector.  Lots of adapters later, all running 15 minutes before meeting.


So last night I firmly believed there was an issue with the audio section of the projector.
I pulled the projector down today, plugged it in next to the mixer, and no noise.  What?
I plugged it into the original socket and voila! Noise is back.  Oops... Ground loop. Where did that come from? 


Other than moving the projector to be on the same circuit as the sound system, the only thing I can think of would be to use a "cheater" plug, disconnecting the ground.  That's a non starter.  Lift the ground on the audio cable?


While writing this, I had a D'Oh! moment.  I have been assuming up to this very point that the change was somewhere else in the cafe. It just occurred to me that it may not be in the rest of the cafe, but the stage itself where something is leaking to ground.  A number of "new" powerstrips have appeared over the last little while.  I suspected a power strip originally hanging off the projector outlet, but disconnected, no change.


How would I test (measure)?
NCVT should tell me if any voltage is on ground, no?
Measure voltage between ground at sound system and the ground where the projector was plugged in.  That would identify the existence of the ground loop, but not which outlet/circuit is at fault.
Voltage measured between neutral and ground?
Between hot and ground?
Any type of load required to test?


The last question (that I have now) is how much effort should I (non electrician) put into tracking it down?


frank

[Chris Hindle]

Start unplugging shit until the noise goes away?
A leaky MOV in a power strip *could* be causing this...
Chris.

[Jeff Lelko]

Start unplugging shit until the noise goes away?

Yep.  Pesky issues like this are often traced to the most unsuspecting components.  Just recently it took me over an hour to troubleshoot a ground loop on a medium-sized outdoor system - the whole problem boiled down to a single TS cable that was supposed to be TRS...  Good luck!

[Brian Jojade]

Are you taking the audio feed out of the projector??  If so, are you converting the audio feed to a balanced connection and using isolation transformers?  Unbalanced connections tend to be much more finicky for buzz, and not running through isolation can make it even worse.

[Jean-Pierre Coetzee]

Are you taking the audio feed out of the projector??  If so, are you converting the audio feed to a balanced connection and using isolation transformers?  Unbalanced connections tend to be much more finicky for buzz, and not running through isolation can make it even worse.

This man knows what's up. This is your solution.

You need to put a DI or isolation transformer in between the projector and the desk. It is entirely likely that the projector's caps have started leaking and there is stray voltage going to the ground from the projector itself.

There is a also a likelihood that the ground connection on the projector's plug has went faulty and now the only ground connection is the audio cable, therefore causing your buzz. The ground connection's impedance might have just went up significantly enough the the lowest impedance is now the audio cable's shield and therefore more current is going down there than to the earth on the power plug.

[Kevin Graf]

Check for a mis-wired power strip that swaps the Neutral & Safety Ground.
a 3 LED tester won't find this.

[Taylor Hall]

Are you taking the audio feed out of the projector??  If so, are you converting the audio feed to a balanced connection and using isolation transformers?  Unbalanced connections tend to be much more finicky for buzz, and not running through isolation can make it even worse.

This. We use a DI or TIGLBOX whenever there's a 3.5mm connection in play, worth their weight in gold and solve 95% of hum issues with the flick of a switch.

[Tim McCulloch]

If the audio has to come from the projector - Radial Stage Bug 5 or Rapco's similar model.

[frank kayser]

Again, thanks to everyone for their input.  Lots of good ideas.


I'll go over the stage with a fine tooth comb.  As I mentioned, some power strips have "appeared" over the last month or so.  I had bought some unfiltered direct power strips, but found others liked them more than we did, if you get my drift.


And yes, a Radial SB5 or the like would be good insurance for the future, whether I find the fault or not.


Thanks to everyone.


frank

[Keith Broughton]

And yes, a Radial SB5 or the like would be good insurance for the future, whether I find the fault or not.





frank

Start with this , get clean audio, then look for a potential fault.
There doesn't necessarily need to be a fault to get a ground loop to a projector.

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