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Sound Reinforcement - Forums for Live Sound Professionals - Your Displayed Name Must Be Your Real Full Name To Post In The Live Sound Forums => AC Power and Grounding => Topic started by: frank kayser on February 18, 2020, 05:36:22 pm

Title: Ground Loop What to do? Should I bother?
Post by: frank kayser on February 18, 2020, 05:36:22 pm
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



Title: Re: Ground Loop What to do? Should I bother?
Post by: Chris Hindle on February 18, 2020, 06:15:56 pm
Start unplugging shit until the noise goes away?
A leaky MOV in a power strip *could* be causing this...
Chris.
Title: Re: Ground Loop What to do? Should I bother?
Post by: Jeff Lelko on February 18, 2020, 07:09:45 pm
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!
Title: Re: Ground Loop What to do? Should I bother?
Post by: Brian Jojade on February 18, 2020, 07:31:04 pm
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.
Title: Re: Ground Loop What to do? Should I bother?
Post by: Jean-Pierre Coetzee on February 19, 2020, 05:09:10 am
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.
Title: Re: Ground Loop What to do? Should I bother?
Post by: Kevin Graf on February 19, 2020, 08:55:35 am
Check for a mis-wired power strip that swaps the Neutral & Safety Ground.
a 3 LED tester won't find this.
Title: Re: Ground Loop What to do? Should I bother?
Post by: Taylor Hall on February 19, 2020, 09:31:24 am
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.
Title: Re: Ground Loop What to do? Should I bother?
Post by: Tim McCulloch on February 19, 2020, 10:50:54 am
If the audio has to come from the projector - Radial Stage Bug 5 or Rapco's similar model.
Title: Re: Ground Loop What to do? Should I bother?
Post by: frank kayser on February 19, 2020, 11:57:08 am
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



Title: Re: Ground Loop What to do? Should I bother?
Post by: Keith Broughton on February 19, 2020, 02:29:59 pm



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.
Title: Re: Ground Loop What to do? Should I bother?
Post by: frank kayser on February 19, 2020, 05:02:17 pm
S/N ratio.  Gain staging.
I measured neutral to ground on the stage and Ah Ha! 0.4v. Found it!
Went to the other outlet, and also found 0.4v.  Grrr...
Measured ground from one to the ground on the other.  0.00v  So much for a ground loop.


So I went to check the video feed and audio.
The youtube sound was coming from the projector.  What?


Went through the menus to turn off internal speaker and look at the other settings.  I found volume at 1.
That COULDN'T affect the "line out" could it? It turns out more like a "headphone out" than a line out.
The volume control DOES affect that out.


So after >4 hours beating my head against the wall and endlessly climbing ladders, turns out to be very low signal combined with excessive gain that produced the noise.  D'Oh!  I can't lock out the volume buttons on the remote control. I can lock out the entire remote.
Nothing can be made foolproof as fools are just so ingenious!


Thanks all for great leads and suggestions.  Yeah, although it would not have changed anything that I found, an ISOLATION device will be a very good addition.


frank

Title: Re: Ground Loop What to do? Should I bother?
Post by: Kevin Graf on February 19, 2020, 06:03:00 pm
But 0.4V between N & SG is a typical value for a circuit that is using current.
While 0.0V on a loaded circuit indicates a problem.
Title: Re: Ground Loop What to do? Should I bother?
Post by: Dave Garoutte on February 19, 2020, 07:37:16 pm

Nothing can be made foolproof as fools are just so ingenious!

frank

Congrats.  Successful troubleshooting is so gratifying, even when it's a D'oh! moment.

Please see my tag line.
Title: Re: Ground Loop What to do? Should I bother?
Post by: frank kayser on February 19, 2020, 08:07:40 pm
Congrats.  Successful troubleshooting is so gratifying, even when it's a D'oh! moment.

Please see my tag line.
Nothing can be made idiot-proof; only idiot resistant.


Yes, there is a certain gratification on a successful post-mortem.
As far as "finding it", even a blind hog finds an acorn once in a while!
As for the tag line, I've used that line more than once.


But 0.4V between N & SG is a typical value for a circuit that is using current.
While 0.0V on a loaded circuit indicates a problem.


Good to know!  Thanks!
Title: Re: Ground Loop What to do? Should I bother?
Post by: Brian Jojade on February 20, 2020, 01:06:08 pm
Ah, yep.  If the signal level is so low and you have to suddenly crank the gain up to get what you wanted, that's a different indicator.

Your initial test - that you put the projector next to the mixer and were able to get a suitably clean signal - indicates that there is a definite noise floor on the line that you probably want to deal with.  While it may only be a slight buzz, that probably can be heard during quiet moments during the video.  Proper isolation and balanced lines should be able to eliminate that.

And - if you want to disable the volume controls on the remote, you can usually disassemble the remote and varnish over the pads for volume down. (or tape if you want a less permanent fix).
Title: Re: Ground Loop What to do? Should I bother?
Post by: Mike Caldwell on February 23, 2020, 05:53:45 pm
For noisy audio from a video projector audio in addition to the projector volume being turned way down (had that a couple times) if the source to the projector is VGA and the cable from the computer is the kind with the audio line as part of the VGA cable some of those
cables audio lines pick up all kind of sync buzz from the video lines.

It the above is the issue no amount of isolation will fix the problem, the audio is already contaminated by the time it hits the projector.

I cut one of those open and there was no shielding at all on the audio lines.
Some work and I don't think some were ever intended to really work!
Title: Re: Ground Loop What to do? Should I bother?
Post by: frank kayser on February 24, 2020, 12:03:30 pm
For noisy audio from a video projector audio in addition to the projector volume being turned way down (had that a couple times) if the source to the projector is VGA and the cable from the computer is the kind with the audio line as part of the VGA cable some of those
cables audio lines pick up all kind of sync buzz from the video lines.

It the above is the issue no amount of isolation will fix the problem, the audio is already contaminated by the time it hits the projector.

I cut one of those open and there was no shielding at all on the audio lines.
Some work and I don't think some were ever intended to really work!


Hi Mike,
The projector is on the ceiling, and we have 1 HDMI and 1 VGA cable run to an accessible location.  The "installer" did not run a separate audio cable from the projector to the VGA cable location.  Why? No answer. If (when) I rerun the cables into a conduit or in the wall, I'll be sure to run at least a shielded cable for the audio.  For some devices, it may require something more robust.



The HDMI cable is quiet, though regularly abused.  Again, just hanging from the ceiling, and users just pull on the cable to get that extra little bit of length.  So far, they've pulled an extra four feet out of the ceiling.  I have no idea how much cable is up there.  The user community did pull the last cable until it failed.  I've been loathe to make changes because that would imply that I thought the installation was a 5hit job (which it was), orange extension cord and all.  Politics. 


Ah, yep.  If the signal level is so low and you have to suddenly crank the gain up to get what you wanted, that's a different indicator.

Your initial test - that you put the projector next to the mixer and were able to get a suitably clean signal - indicates that there is a definite noise floor on the line that you probably want to deal with.  While it may only be a slight buzz, that probably can be heard during quiet moments during the video.  Proper isolation and balanced lines should be able to eliminate that.

And - if you want to disable the volume controls on the remote, you can usually disassemble the remote and varnish over the pads for volume down. (or tape if you want a less permanent fix).


Thanks for the advice, Brian.  No doubt you're correct.  It definitely should get a balanced line, and suitable ISO device.
Good advice on the remote.  I'll take that under consideration.


Again, thanks everyone!
frank




Title: Re: Ground Loop What to do? Should I bother?
Post by: Jonathan Johnson on February 24, 2020, 02:55:25 pm
Please see my tag line.

"Idiot proof" only means that it will serve to prove that people are idiots.
Title: Re: Ground Loop What to do? Should I bother?
Post by: Rob Spence on February 24, 2020, 07:31:22 pm
Another thing is to put an HDMI audio extractor ahead of e projector. Then the remote wonít affect the audio level and you donít have to deal with what ever quality audio path is the projector.
Title: Re: Ground Loop What to do? Should I bother?
Post by: Steve-White on February 24, 2020, 09:31:23 pm
Start unplugging shit until the noise goes away?
A leaky MOV in a power strip *could* be causing this...
Chris.

Yep.  Dismantle and swap until itís gone.
Title: Re: Ground Loop What to do? Should I bother?
Post by: frank kayser on February 25, 2020, 10:53:33 am
Another thing is to put an HDMI audio extractor ahead of e projector. Then the remote wonít affect the audio level and you donít have to deal with what ever quality audio path is the projector.


Ding! Ding! Ding!  We have a winnah! I didn't even think along those lines, and of course, did not know they existed.
I LOVE that idea!  I'm looking for one RIGHT NOW!
Thanks much, Rob!  A beer is waiting for you when you're in the area next!


frank



Title: Re: Ground Loop What to do? Should I bother?
Post by: Mike Caldwell on February 25, 2020, 02:12:21 pm

Ding! Ding! Ding!  We have a winnah! I didn't even think along those lines, and of course, did not know they existed.
I LOVE that idea!  I'm looking for one RIGHT NOW!
Thanks much, Rob!  A beer is waiting for you when you're in the area next!


frank

The audio extractor prior to the projector will work however if VGA connections are still used from time to time and audio is needed you will still need a way to get audio from the computer at that time.
Title: Re: Ground Loop What to do? Should I bother?
Post by: frank kayser on February 25, 2020, 03:05:19 pm
The audio extractor prior to the projector will work however if VGA connections are still used from time to time and audio is needed you will still need a way to get audio from the computer at that time.
Yeah.  I've been thinking about that too.  Getting crowded up there by the projector.
- Audio split from HDMI box
- Radial SB5w to go balanced
- Stereo combiner (or switch) (or homerun to mixer)


Many of the Audio split from HDMI boxes offer TOSLINK - that would eliminate the Radial and the balanced line.
TOSLINK a good or bad idea in the ceiling?  (by time you count corners, maybe 40') TOSLINK to analog audio by the mixer...

Title: Re: Ground Loop What to do? Should I bother?
Post by: Mike Caldwell on February 25, 2020, 06:15:17 pm
For the VGA needs you could get a VGA to HDMI converter, some of those have an audio to HDMI insert function.
That's getting to be a lot of adapters and converters though.

A home run audio line to the computer location with the proper interface to connect to the computer would work for any video format. The issue there would be that most computers
when connected to HDMI will automatically route the audio out to the HDMI, the requires someone being around who knows how to redirect the audio to the headphone jack or USB if your going that way.
Title: Re: Ground Loop What to do? Should I bother?
Post by: Brian Jojade on February 25, 2020, 08:45:20 pm
Another thing is to put an HDMI audio extractor ahead of e projector. Then the remote wonít affect the audio level and you donít have to deal with what ever quality audio path is the projector.

The problem you can run into with these though, is that some devices add latency with HDMI signals.  Usually the device is designed so that the delay in the audio and the delay in the video is the same.  When you start mixing and matching the decoders on devices, you can sometimes end up with undesirable results.

If you want to see a sample of this, go to any big box TV display that has multiple brands of TVs set up and fed with an HDMI splitter.  Watch for scene cuts, and you'll see how different each TV might be.  It can be quite a bit sometimes!
Title: Re: Ground Loop What to do? Should I bother?
Post by: Caleb Dueck on February 26, 2020, 01:37:10 pm
For the VGA needs you could get a VGA to HDMI converter, some of those have an audio to HDMI insert function.
That's getting to be a lot of adapters and converters though.

Typically, assume a projector has an HDMI input.  If not, add up the cost of lamps for the next 5 years, and ask if replacing it with a laser model wouldn't make sense. 

I just had a project this week where running input(s) directly to the project, with audio back to the DSP, sort of (barely) made sense. The client asked to add one additional input, which tipped the scales to running all inputs to the video rack to a switcher, and running a single HDbaseT Cat6 to the projector.  Most projectors now also have HDbaseT, so that eliminates two failure points (separate Tx and Rx).  Audio de-embedded at the switcher and run to the mixer/DSP. 

The only times where using the projector as a switcher and audio de-embedder makes sense - classrooms, where there is a baby amplifier up there as well (in a recessed ceiling box typically) powering the speakers, no rack at all. 
Title: Re: Ground Loop What to do? Should I bother?
Post by: frank kayser on February 26, 2020, 05:17:09 pm
Typically, assume a projector has an HDMI input.  If not, add up the cost of lamps for the next 5 years, and ask if replacing it with a laser model wouldn't make sense. 

I just had a project this week where running input(s) directly to the project, with audio back to the DSP, sort of (barely) made sense. The client asked to add one additional input, which tipped the scales to running all inputs to the video rack to a switcher, and running a single HDbaseT Cat6 to the projector.  Most projectors now also have HDbaseT, so that eliminates two failure points (separate Tx and Rx).  Audio de-embedded at the switcher and run to the mixer/DSP. 

The only times where using the projector as a switcher and audio de-embedder makes sense - classrooms, where there is a baby amplifier up there as well (in a recessed ceiling box typically) powering the speakers, no rack at all.


Good food for thought, Caleb, especially in the light of some folks complaining about image quality (especially corner sharpness and overall brightness).  I'd like to get one with a short focus lens so I can hang some stage lights in an area off limits due to the current projector beam. 
Laser, huh?  God, I love this place - always learning something!




The problem you can run into with these though, is that some devices add latency with HDMI signals.  Usually the device is designed so that the delay in the audio and the delay in the video is the same.  When you start mixing and matching the decoders on devices, you can sometimes end up with undesirable results.

If you want to see a sample of this, go to any big box TV display that has multiple brands of TVs set up and fed with an HDMI splitter.  Watch for scene cuts, and you'll see how different each TV might be.  It can be quite a bit sometimes!


I've seen many times loss of sync between the audio and visual.  I have to look away.
frank
Title: Re: Ground Loop What to do? Should I bother?
Post by: Rob Spence on February 29, 2020, 01:10:07 pm
I discovered the hdmi extractor when I also had several inputs to the projector. My projector was about 100 cable feet from the mix position and one input was 130í from mix.
My solution was an hdmi splitter, hdcp remover, & hdmi to cat6 at the long source, back to hdmi at mix, to audio extractor, then to switcher. Moved the laptop to near mix and hdmi to switcher. Out of switcher via SDI toward projector. At projector, convert to hdmi, then to dvi (projector only had dvi & vga), then Geffen DVI detective and to projector.
This allowed me to solo the audio from the long source (a cable tv box), while seeing a preview so I could switch to it at the right time. This was a private party and the video was the Kentucky Derby. The rest of the time was background music and video from the laptop.
Title: Re: Ground Loop What to do? Should I bother?
Post by: Mike Caldwell on February 29, 2020, 09:19:02 pm

Good food for thought, Caleb, especially in the light of some folks complaining about image quality (especially corner sharpness and overall brightness).  I'd like to get one with a short focus lens so I can hang some stage lights in an area off limits due to the current projector beam. 
Laser, huh?  God, I love this place - always learning something!


The laser beam is not what actually projects the video, the laser heats up phosphor disc white hot replacing the light bulb.