Ground Loop (?) Question

Dr Bob · 25th March 2007, 12:50 AM

This may be a Steve Remote question--I am not well versed in AC power problems. Here is the situation and the way I solved it, but now I am trying to figure out exactly what was wrong and why the solution worked. Today I was recording the dress rehearsal of a Mozart Magic Flute production here at the university that employs me. I recorded it from a central control room in a different building than the hall in which it was performed. The set up is that we go from the built-in house patch panel to a 32 channel Whirlwind splitter. I take the direct split to the control room and our Sony DMX board. This has been working fine. On occassion I have taken the isolated split for FOH. No problem with that either. I always leave the splitter grounded and it has always been quiet before today.

For today's rehearsal the split went to a TV remote truck from the local cable provider's public access channel. Their shore power is couple of 110 lines to outlets in the hall (I presume on different circuits). When we powered up the mics both my control room and the truck were getting 60 Hz hum in four of the five active mic lines. I checked just to be certain--the truck wasn't trying to provide phantom. I never encountered this problem before, but I solved it by lifting the grounds of each line at the splitter.

So the questions: What was the probable cause of the problem? And why did lifting the grounds fix it? Or is there even enough information here to venture a guess? I fixed it but I am really curious.

David Rick · 25th March 2007, 05:44 PM

Two different buildings?

I'm suprised that this ever worked. And that may be the hint. Since these tie lines are permanently installed, I'd guess that someone went to a lot of trouble (and used a lot of ground braid) to make sure there was a low impedance ground path between your control room and the stage termination panel (other than your mic cables!). Probably the on-stage outlet grounds are home-runned to the same star termination.

Then along comes a truck and plugs into some random outlets that weren't as carefully wired, and suddenly you've got mic cables involved in some new ground loop that noone expected.

Was interrupting the shields in the snake to your control room the only thing you tried? What about the run to the truck? Also do you know for certain the folks in the truck were actually using earth ground with the power they pulled, instead of lifting it and endangering everyone?

David L. Rick
Seventh String Recording

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25th March 2007, 12:50 AM	#1
Dr Bob Gear Head Join Date: Jan 2004 Location: Storrs. CT Posts: 53	Ground Loop (?) Question This may be a Steve Remote question--I am not well versed in AC power problems. Here is the situation and the way I solved it, but now I am trying to figure out exactly what was wrong and why the solution worked. Today I was recording the dress rehearsal of a Mozart Magic Flute production here at the university that employs me. I recorded it from a central control room in a different building than the hall in which it was performed. The set up is that we go from the built-in house patch panel to a 32 channel Whirlwind splitter. I take the direct split to the control room and our Sony DMX board. This has been working fine. On occassion I have taken the isolated split for FOH. No problem with that either. I always leave the splitter grounded and it has always been quiet before today. For today's rehearsal the split went to a TV remote truck from the local cable provider's public access channel. Their shore power is couple of 110 lines to outlets in the hall (I presume on different circuits). When we powered up the mics both my control room and the truck were getting 60 Hz hum in four of the five active mic lines. I checked just to be certain--the truck wasn't trying to provide phantom. I never encountered this problem before, but I solved it by lifting the grounds of each line at the splitter. So the questions: What was the probable cause of the problem? And why did lifting the grounds fix it? Or is there even enough information here to venture a guess? I fixed it but I am really curious.

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25th March 2007, 05:44 PM	#2
David Rick Gear addict Join Date: Mar 2004 Posts: 367	Two different buildings? I'm suprised that this ever worked. And that may be the hint. Since these tie lines are permanently installed, I'd guess that someone went to a lot of trouble (and used a lot of ground braid) to make sure there was a low impedance ground path between your control room and the stage termination panel (other than your mic cables!). Probably the on-stage outlet grounds are home-runned to the same star termination. Then along comes a truck and plugs into some random outlets that weren't as carefully wired, and suddenly you've got mic cables involved in some new ground loop that noone expected. Was interrupting the shields in the snake to your control room the only thing you tried? What about the run to the truck? Also do you know for certain the folks in the truck were actually using earth ground with the power they pulled, instead of lifting it and endangering everyone? David L. Rick Seventh String Recording