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Using DI Ground Lift to Eliminate Buzz

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slejhamer
(@slejhamer)
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Please tell me if I'm right or wrong about this.

Background on the situation: we got a nice new electronic drum kit for our new church building, which has a new PA system. One of the drummers and one of the guitarists installed the drums yesterday after services.

At 4:30 in the afternoon, I get a phone call: why is everything buzzing like crazy? I say I don't know, call the sound man. Oops - forgot, we don't have one. So I grab spare instrument and mic cables, adapters, and, after a last-second thought, my el-cheapo Behringer DI box.

Get there, swap out cables, change power strips, no luck. Plug in the DI box between the drums and system, turn on phantom power at the mixer and then flip the DI's ground lift switch - ah! the lovely sound of silence. E-drums come through loud and clear. Problem solved.

But then the questions came up: is this safe? Are the drums no longer grounded?

My thinking is: both the mixer and drums are independently grounded via their 3-prong power cords; the problem developed when connecting the drums to the system, which created a ground loop (basically using two different paths to ground the drums) and thereby introducing the hum. By lifting the ground, I broke the loop - but both mixer and drums remain grounded. Thus, it should be safe to use the drums with the DI box permanently connected and ground-lift enabled. And I need to buy a new DI box.

Yes? No? Maybe so?

"Everybody got to elevate from the norm."


   
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leear
(@leear)
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What you are saying as far as I can tell is correct. A real easy fix would to probably get the drums and the PA on the same circuit. If it is already well then I'm stumped.

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slejhamer
(@slejhamer)
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Topic starter  

Thanks leear. Good idea about getting them on the same circuit, but not physically possible in this case - they are too far apart, at opposite ends diagonally.

I came across what must be the holy bible of ground loop information; 86 pages on the topic!
http://blueguitar.org/new/articles/other/ground_loop.pdf

It does seem consistent with my thinking:

If the system is grounded at both ends and ground potentials start flowing, you'll get a ground loop. Ground loops can be identified by a low hum (60Hz in the US and 50 Hz in Europe) through the sound system. First place to check: direct box. Direct box usually has a switch labeled "Ground Lift" for solving this type of problem. The ground lift switch will lift the ground (safely) and the hum should stop. If the switch isn't there, use a special ground lift cable on the XLR cable. Do not, by any means, disconnect the grounding pin of the AC cord.

Also THIS SITE has information and a diagram that explains the ground loop just as I suspected.

When the circuit through the DI box completes the ground loop conduction path, what happens? A current flows around the loop! And that current causes very small voltages to appear in the signal paths as well, sometimes by magnetic induction, sometimes through the poor design of preamp circuits in the PA, or the use of unbalanced signal connections.

That's why you will find a "ground lift" switch on most DI boxes, to break this very ground loop.

Sounds like we're good to go! 8)

"Everybody got to elevate from the norm."


   
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PRNDL
(@prndl)
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I'm not an expert, but I believe the DI unit uses a transformer to couple and isolate the signals.
Lifting the ground lets each unit ground itself. It's safe as long as each unit uses a three prong plug.

The most common cause one of the AC lines has swapped the hot and return lines.
To test this, you use an inexpensive "AC line monitor and polarity tester".
If the polarity is wrong, the outlet needs be rewired.

Another cause could be very equipment that use two prong cords. The temporary solution is to reverse the plug, but those old units are electrically unsafe and should be upgraded to a three prong plug.

It's also unsafe to defeat a three prong plug by clipping the ground pin or using a special adapter. That could cause this problem.

Another thing to try is to use a different AC source (outlet) and see if the problem goes away. It's rare, but some places use different phases of AC as a way to balance the overall load (i.e. one phase to one wall, and another to the opposite wall, and the third to the back wall).

IMHO, the DI box is the easiest and safest method.

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kent_eh
(@kent_eh)
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My thinking is: both the mixer and drums are independently grounded via their 3-prong power cords; the problem developed when connecting the drums to the system, which created a ground loop (basically using two different paths to ground the drums) and thereby introducing the hum. By lifting the ground, I broke the loop - but both mixer and drums remain grounded. Thus, it should be safe to use the drums with the DI box permanently connected and ground-lift enabled. And I need to buy a new DI box.

Yes? No? Maybe so?

Yup, got it first try.
That's what the switch is there for. If it was "probably dangerous" the humble little lift switch would have been lawyerized out of existence long ago :wink:

Most touring PA systems run a power cable along side the snake so the board (and effect rack) can get power from the same source as the stage (ideally from the same panel, and hopefully it's wired right) to prevent ground loops.
Even then, there can be ground loops that require ground lifting.

I wrapped a newspaper ’round my head
So I looked like I was deep


   
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