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Bob Cole
ground loop toolkit
on May 4, 2009 at 3:46:16 am

Just ran into some ground loop problems at a press event, and would appreciate advice on the best little toolkit to deal with them.

First, the sound reinforcement company techs used an XLR-to-XLR ground lifter box on the main mic feed to the mult box. There was a second problem on the AC to the tripod location; I was recording to tape for safety and also to a laptop for a quick edit/uplink, and despite a little "ground lifter" consisting of a three-prong adapter with the ground cut off, the plugged-in laptop caused another hum (ran the laptop off battery to solve that one).

What do you have in your bag to deal with common ground loop problems on location?

Bob C


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Bill Davis
Re: ground loop toolkit
on May 4, 2009 at 10:14:59 pm

Bob,

I'm sorry, but removing the ground pin from ANY AC plug is boneheadedly STUPID. And should NEVER, EVER be recommended by anyone.

Hums are inconvenient. Shocking/Injuring/Killing people on a set is VASTLY more inconvenient.

Safe ground lift adaptors are readily available. Yeah, they cost money. But noone doing sound or sound for video should practice without them.

Sorry to be harsh. But one mistake in this area could destroy your reputation and possibly bankrupt you both professionally and personally.

Ground pins have a FUNCTION. To keep people using electricity safe and healthy.



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Bob Cole
Re: ground loop toolkit
on May 5, 2009 at 1:45:38 am

[Bill Davis] "Safe ground lift adaptor"

Could you name some devices that you could recommend? For both AC and XLR cables?

Thanks!

Bob C


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Bill Davis
Re: ground loop toolkit
on May 5, 2009 at 6:34:40 am

Hosa makes a cheap one that works just fine and is under $6.
Search: Hosa GLT-255 XLR Female to XLR Male Ground Lift Hum Stopper

Or you can just go to any Guitar Center and drop $20 on one.

The cost is insignificant compared to the danger of just cutting the third pin from a grounded plug.

Oh, and the point is that THIS outlet or THAT outlet and the state of it's wiring may or may not put you in danger at any one point. But that's NO guarantee that in six months you won't find yourself in a hotel where they shaved a few thousand off the building costs by hiring idiots to wire things.

Then that extension cord, or equipment IBEC power cord, or whatever that you've happily been using for so long that you've totally forgotten that you snipped the ground prong off with your dykes - that's the cable you'll find yourself using - while standing on a piece of carpet where a recent champagne bucket spill has you a single sodden piece of shoe leather away from a world of hurt.

Sure, 120 probably won't kill you. Unless you're unlucky enough to grab something live and find that your arm and hand muscles have contracted under current and you simply CAN'T let it go.

I'm not so good with that concept.

Accidents have a name because they're a universal human experience.

Cutting ground prongs ups the odds a LOT. Plus it puts a dangerous cable out into the world where someone who's got NO clue is free to use it to power their home built kenetic metal sculpture.

Just say no to cutting ground pins and the world is a slightly better and safer place.







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Bob Cole
Re: ground loop toolkit
on May 5, 2009 at 3:27:40 am

[Bill Davis] "I'm sorry, but removing the ground pin from ANY AC plug is boneheadedly STUPID. And should NEVER, EVER be recommended by anyone. "

I accept your view, but since I'm already publicly stupid, I thought I'd ask a dumb question.

In the service panel, I thought I saw that the neutral wire was connected to the ground. So why is there even a necessity for a separate ground wire?

Wincing as I anticipate the lash again,

Bob C (and sincerely, I do accept your judgment that removing a ground pin is stupid)


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Brian Reynolds
Re: ground loop toolkit
on May 5, 2009 at 8:14:16 am

I always work on the assumption that on EVERY job with different powering points you will give trouble.
But NEVER, NEVER, NEVER, NEVER remove the earth on the powering side.
Have a go at this.
1. make your power feed from the same point as the rest of the AV equiptment.And run ALL of your gear from this common power.
2.Get yourself an XLR-XLR cable about 100mmm long with only pins 2 & 3 connected between the connectors.(2 needed for stereo)
3.Get a small audio transformer 1:1 ratio in a box and insert it in the line for isolation.(2 needed for stereo)
4.Make sure that any audio cables are away from power cords.

I have never known of a hum problem that cant be fixed with one or all of these steps.



The difference between Knowledge and Wisdom is... Knowledge is the knowing of facts.... Wisdom is the sensible application of good quality knowledge...


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Bob Cole
Re: ground loop toolkit
on May 5, 2009 at 12:58:39 pm

[Brian Reynolds] "an XLR-XLR cable about 100mmm long with only pins 2 & 3 connected between the connectors.(2 needed for stereo)
3.Get a small audio transformer 1:1 ratio in a box and insert it in the line for isolation"


Thanks for the advice -- more please!

When you write, "only pins 2 & 3 connected between the connectors," do you mean simply don't use pin 1? You don't mean to short out 2 & 3, right?

What is a "small audio transformer 1:1 ratio?"

In your Item 1, the word "your" was linked to a post that has been deactivated. Is there something there that I should know about?

Also in Item 1, I assume you mean audio devices only from a common power point. We often need (for the lights) every circuit that is available within a 100'.

Finally, in response to Bill too, there are many practical sources in houses which have only two prongs. I assume that these are wired as I stated earlier, with the "cold" also serving as ground.

Thanks again for your help.

Bob C


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Brian Reynolds
Re: ground loop toolkit
on May 6, 2009 at 9:03:00 am

Yes your right the XLR-XLR has no connection on pin 1,in one of the connectors.
The transformer I use is a 600 ohm - 600 ohm that is often found in electronics stores for telco isolation.
I tend to make most of my adaptors, some makers use switches for earth lifting.

The difference between Knowledge and Wisdom is... Knowledge is the knowing of facts.... Wisdom is the sensible application of good quality knowledge...


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Bob Cole
Re: ground loop toolkit
on May 6, 2009 at 4:30:43 am

I've checked out some of the rec's here. But I thought I saw something on location once that I haven't seen online. It was an XLR hum eliminator which had a switch, so that you could either pass through all three XLR pins, or eliminate one, to get rid of the hum.

Does that sound right, and familiar? It seems better than the Ebtech HumXLR, just because if the XLR cable isn't really the source of the hum, you don't have to disconnect and reconnect to track it down.

Bob C


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Jordan Wolf
Re: ground loop toolkit
on May 8, 2009 at 1:59:48 am

The ground is meant to conduct electricity away from the devices we use so that, if there is a fault in the wiring (internally or externally), the parts of the gear that we touch will not become electrically charged and electrocute us. Eliminating that safety is, at best unsafe and at worst fatal.

Just say this to yourself if you think about snipping that ground prong: "What if this kills someone?"

I think that there needs to be more of a distinction between Pin-1 Lifts and Ground Lifts. One applies to audio signal, the other to potentially fatal electrical distribution.

Pin-1 Lifts can work fine - I prefer an isolation transformer myself. ProCo has some, as does Whirldwind and Jensen.

Stay safe!

Wolf
<><


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Bob Cole
Re: ground loop toolkit
on May 8, 2009 at 11:54:50 am

[Jordan Wolf] "Pin-1 Lifts can work fine - I prefer an isolation transformer"

I'm just curious - why does the Pin-1 Lift work? Why do they call a hum-inducing ground a "ground loop?"

Bob C


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Brian Reynolds
Re: ground loop toolkit
on May 8, 2009 at 12:41:56 pm

Analogue audio signals are infact a varying ac voltage from just under 1 volt (line level) to almost nothing at (microphone level).
If when 2 or more pieces of equipment are connected together and running on 110-240v they will have an earth connection which is there for safety reasons and must not be removed this may not be connected to the same point in the main switch board, but this is fine for safety.
When the audio cables are connected to the equipment another earthing connection is created directly between the units and will often be a shorter distance than the "electricial earth connection" this gives a very slight volt difference between the gear and often shows up as hum.
If that earth loop is broken by removing the connection on pin 1 the hum then stops.
If an isolation transformer is used it is infact breaking the physical connection of pins 2&3, from the input of the transformer to the output, and if then you disconect pin 1 the units will be TOTALLY audio isolated from each other.
This often refered to as a "Floating" signal (NAGRA audio gear often use this)

I hope this explains a tricky subject to get your head around...




The difference between Knowledge and Wisdom is... Knowledge is the knowing of facts.... Wisdom is the sensible application of good quality knowledge...


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John Livings
Re: ground loop toolkit
on May 9, 2009 at 6:46:38 am

Hi All,

I found this a while back;

http://www.blueguitar.org/new/articles/other/ground_loop.pdf

I have heard of cutting the wire connection from the #1 pin and the "shell" leaving the shielding connected to the #1 pin ? ( If you look closely at your XLR connection, You will see that the #1 pin is soldered to both the cable shielding and a small wire that connects to the connector shell).

If you remove the #1 pin which is connected to both the shielding and the shell, And you were using a Condenser Mic (Phantom Power) Are you not removing the Ground to that Mic?

John


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Brian Reynolds
Re: ground loop toolkit
on May 9, 2009 at 7:00:07 am

Hi John, ground loops happen between pieces of gear and never between the mic and mixer, so the signals you are dealing with most times are at line level.
If you remove the connection to pin 1 in the connector the chances that some day you will need that cable for a phantom powered mic and the cable wont work.
Thats why i like a very short (100mm) XLR-XLR pin 1 lift cable in my kit... easy to insert in a line and easy to remove... problem solved

The difference between Knowledge and Wisdom is... Knowledge is the knowing of facts.... Wisdom is the sensible application of good quality knowledge...


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Ty Ford
Re: ground loop toolkit
on May 9, 2009 at 10:27:53 am

Good work everyone.

Thanks for helping Bob with this. He's a good friend of mine and I'd hate to have to prematurely attend the viewing after his electrocution.

Regards,

Ty Ford

Want better production audio?: Ty Ford's Audio Bootcamp Field Guide






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