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Author Topic: Ground Loop What to do? Should I bother?  (Read 1928 times)

frank kayser

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Re: Ground Loop What to do? Should I bother?
« Reply #10 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

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Kevin Graf

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Re: Ground Loop What to do? Should I bother?
« Reply #11 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.
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Dave Garoutte

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Re: Ground Loop What to do? Should I bother?
« Reply #12 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.
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Nothing can be made idiot-proof; only idiot resistant.

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frank kayser

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Re: Ground Loop What to do? Should I bother?
« Reply #13 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!
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Brian Jojade

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Re: Ground Loop What to do? Should I bother?
« Reply #14 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).
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Brian Jojade

Mike Caldwell

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Re: Ground Loop What to do? Should I bother?
« Reply #15 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!
« Last Edit: February 23, 2020, 06:01:27 pm by Mike Caldwell »
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frank kayser

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Re: Ground Loop What to do? Should I bother?
« Reply #16 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




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Jonathan Johnson

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Re: Ground Loop What to do? Should I bother?
« Reply #17 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.
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Stop confusing the issue with facts and logic!

Rob Spence

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Re: Ground Loop What to do? Should I bother?
« Reply #18 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.
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Steve-White

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Re: Ground Loop What to do? Should I bother?
« Reply #19 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.
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Re: Ground Loop What to do? Should I bother?
¬ę Reply #19 on: February 24, 2020, 09:31:23 pm ¬Ľ


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