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Author Topic: Not Ground Loop Hum  (Read 5167 times)

Mike Sokol

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Not Ground Loop Hum
« on: May 02, 2016, 11:30:28 am »

So this one threw me for a "loop" on Saturday night. We set up our active JBL PA system and ran XLR cables plus Edison power from a central distro point. But I had this really bad hum on the house left speaker stack, and not the house right stack. I put in a audio isolation transformer on the XRL feed, but then the hum turned into a buzz. Didn't make sense until one of the guys opened up the XLR cable we made in-house and found this mis-wiring condition on a jumper. If you look closely, you can see that the shield is connected from pin-2 female to pin-2 male. Of course, shield is supposed to be on pin-1, since pin-2 is hot. This created a condition where the cable would pass a check using a SWIZZ Army Tester (or any other tester, for that matter) since there was indeed continuity between pins 1-1, 2-2, and 3-3. But now the shield currents were being injected into the pin-2 hot of the speaker input. So the lesson is don't rely on just using a cable tester to check your cables.  Open up the ends and inspect them to make sure that pin-1 is the shield and pins 2/3 are the twisted pair. 
« Last Edit: May 02, 2016, 11:41:07 am by Mike Sokol »
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Chris Hindle

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Re: Not Ground Loop Hum
« Reply #1 on: May 02, 2016, 12:34:16 pm »

At least they were "good" enough to NOT jumper to the shell.
"The gift that keeps on giving" that lovely little swap.
Chris.
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Mike Sokol

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Re: Not Ground Loop Hum
« Reply #2 on: May 02, 2016, 12:47:15 pm »

At least they were "good" enough to NOT jumper to the shell.
"The gift that keeps on giving" that lovely little swap.
Chris.

I've seen a bunch of imported XLR cables as of late with the pin-1/shell jumper. So now that's the first thing I check for when I'm working on a church sound system. Usually just takes a little "nip-tuck" with a pair of cutters to correct.

But I've also seen at least one worship site that bought 2 dozen new mic cables with the shield improperly wired on pin 2. I personally unpacked these cables and checked for miswiring after getting all sorts of hum from the mics. Yes, every single XLR cable was miswired like that from the (Chinese?) factory. I wonder just how common that sort of goof is?
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John Roberts {JR}

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Re: Not Ground Loop Hum
« Reply #3 on: May 02, 2016, 01:12:48 pm »

Hand wiring cables is labor intensive but relatively low skilled work.

It is entirely believable that an entire production run could be wired wrong.

Advise who ever those cables were purchased from, so they can check their inventory.

JR 
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Mike Sokol

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Re: Not Ground Loop Hum
« Reply #4 on: May 02, 2016, 02:08:44 pm »

Advise who ever those cables were purchased from, so they can check their inventory.

Uhmmmmmmm These came from Peavey about 10 years ago. But you asked....  ;D
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John Roberts {JR}

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Re: Not Ground Loop Hum
« Reply #5 on: May 02, 2016, 03:01:25 pm »

Uhmmmmmmm These came from Peavey about 10 years ago. But you asked....  ;D
I quit peavey >15 years ago, so that explains that. I should have told somebody that was important before I left.  8)

Peavey used to make cables at a dedicated operation in Quitman, MS but i think they shut that down even before I left.

I suspect Peavey buys cables from overseas just like everybody else now.

JR
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David Buckley

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Re: Not Ground Loop Hum
« Reply #6 on: May 03, 2016, 12:16:59 am »

A lighting company that servers our area hires out a Tecpro party line intercom system, which the last time I had it, the level of buzz exceeded the voice level.  They had wired their (bright orange!) cables wrong, with the shield connected to whatever pin it is carries the unbalanced audio...
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Ike Zimbel

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Re: Not Ground Loop Hum
« Reply #7 on: May 03, 2016, 09:15:52 am »

I've seen a bunch of imported XLR cables as of late with the pin-1/shell jumper. So now that's the first thing I check for when I'm working on a church sound system. Usually just takes a little "nip-tuck" with a pair of cutters to correct.

But I've also seen at least one worship site that bought 2 dozen new mic cables with the shield improperly wired on pin 2. I personally unpacked these cables and checked for miswiring after getting all sorts of hum from the mics. Yes, every single XLR cable was miswired like that from the (Chinese?) factory. I wonder just how common that sort of goof is?
I encountered this a few years back with a mic cable that was included with an Audio Toolbox I had purchased. I discovered it when the cable became part of my bench test set-up, and yes, it was made in China.
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John Rutirasiri

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Re: Not Ground Loop Hum
« Reply #8 on: May 03, 2016, 09:39:52 am »

Would be nice if someone integrates an LCR meter in an audio cable tester.  That way we can know if shield and hot/cold are swapped or solder connection is poor without taking cable apart.

John R.

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Mike Sokol

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Re: Not Ground Loop Hum
« Reply #9 on: May 03, 2016, 09:54:25 am »

Would be nice if someone integrates an LCR meter in an audio cable tester.  That way we can know if shield and hot/cold are swapped or solder connection is poor without taking cable apart.

John R.

Such a tester would need to check for CMMR on the far end of the cable. So it would need to provide a common-mode test signal (1 kHz?) plus a differential test signal (100 Hz?) and listen for cross-talk, I think.
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Scott Helmke

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Re: Not Ground Loop Hum
« Reply #10 on: May 03, 2016, 10:13:56 am »

Would be easy enough to make a little switch/adapter dongle to use with a mic input. Change switch positions for lowest noise, then see if the switch position is in the "all wires normal" position.
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Mike Sokol

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Re: Not Ground Loop Hum
« Reply #11 on: May 03, 2016, 10:38:09 am »

Would be easy enough to make a little switch/adapter dongle to use with a mic input. Change switch positions for lowest noise, then see if the switch position is in the "all wires normal" position.

I think it's easier to just open up to inspect the ends. However, your version could be used to test stage floor boxes for miswiring. I wonder just how many patch panels and floor boxes have been miswired like this?
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Ike Zimbel

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Re: Not Ground Loop Hum
« Reply #12 on: May 03, 2016, 10:55:50 am »

I wonder just how many patch panels and floor boxes have been miswired like this?
Well...I know of one situation where an entire splitter / snake system had this problem. Early '90's, so details are hazy now as to whether it was the whole system or just one Mass-con snake. Discovered on a gig, of course. I also worked on a small, independent TV truck that had some tie-lines to the over-the-console rack miswired the same way. That one was painfully obvious to see how it had gone astray: The rack end was a pair of 12 channel stage-boxes bolted together, with one box being the sends (male XLR's) and the other the returns (female XLR's). Whoever wired it (and I'm sure it was a "late at night with the first gig the next day" kind of thing) had laid the boxes face down on the bench and just gone "the shields all go on the right hand pin", which was half right! The really sad thing was that this truck had been in use for a few years when I discovered this.
iz
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Steve M Smith

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Re: Not Ground Loop Hum
« Reply #13 on: May 03, 2016, 01:19:36 pm »

I recently bought some cables which had pin 3 as the ground.  I rewired mine then let the seller know.


Steve.
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David Buckley

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Re: Not Ground Loop Hum
« Reply #14 on: May 03, 2016, 05:04:11 pm »

Would be nice if someone integrates an LCR meter in an audio cable tester.  That way we can know if shield and hot/cold are swapped or solder connection is poor without taking cable apart.
Doesn't seem hard.  Here's a video demoing a cheap, open source component tester, based on an Arduino, that would be easy to convert into a cable tester.

I think it's easier to just open up to inspect the ends.
Or.... stick a pin through the cable jacket. 

I can't believe I just said that.


I wonder just how many patch panels and floor boxes have been miswired like this?
Last year, was the first serious user of a redone venue.  Had lots of newly-installed waylines, and every single XLR wayline was crosswired.  My theory is that they mis-wired the first wayline, then used that as the "send" line, and used a XLR cable at the far end to patch it to another wayline for testing.  Because the first one was wrong, to get the tester to show "OK" they had to miswire every other line.  Ticked the big box that said "all tested".

We complained to the venue of course, heck, I had a list of things done wrong.  I'm told the venue employed an independent contractor to watch over the installer as they re-terminated and retested each line.
« Last Edit: May 03, 2016, 05:06:56 pm by David Buckley »
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Mike Sokol

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Re: Not Ground Loop Hum
« Reply #15 on: May 03, 2016, 06:25:11 pm »

Last year, was the first serious user of a redone venue.  Had lots of newly-installed waylines, and every single XLR wayline was crosswired.  My theory is that they mis-wired the first wayline, then used that as the "send" line, and used a XLR cable at the far end to patch it to another wayline for testing.  Because the first one was wrong, to get the tester to show "OK" they had to miswire every other line.  Ticked the big box that said "all tested".
As you note, the problem is that two mis-wired connectors can fool a continuity tester into passing a cable run with the shield wired pin 2 or 3. Since testing UTP CAT-5 for proper twists is pretty common and inexpensive, I wonder if there's some way to utilize that type of testing technology for XLR cables.
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Ike Zimbel

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Re: Not Ground Loop Hum
« Reply #16 on: May 03, 2016, 08:55:31 pm »

As you note, the problem is that two mis-wired connectors can fool a continuity tester into passing a cable run with the shield wired pin 2 or 3. Since testing UTP CAT-5 for proper twists is pretty common and inexpensive, I wonder if there's some way to utilize that type of testing technology for XLR cables.

Hmmm...it might be as simple as a pair of XLR to RJ-45 adapters wired as one pair of the 4...but I don't have one of those testers to try it with so I'm just speculating  :-\
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Stephen Swaffer

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Re: Not Ground Loop Hum
« Reply #17 on: May 03, 2016, 08:56:13 pm »

The reason that mis-wiring is an issue is because noise is capacitively (I think-JR can correct me maybe inductively?) coupled to the shield.  A tester that intentionally couples a noise source to the shield should be able to identify the pin the shield is connected to-then all that is needed is a simple pin to pin continuity test.
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Mike Sokol

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Re: Not Ground Loop Hum
« Reply #18 on: May 03, 2016, 09:02:05 pm »

The reason that mis-wiring is an issue is because noise is capacitively (I think-JR can correct me maybe inductively?) coupled to the shield.  A tester that intentionally couples a noise source to the shield should be able to identify the pin the shield is connected to-then all that is needed is a simple pin to pin continuity test.
It's a little more complicated than that, since the twisted pair must be close enough together so they have the same outside noise coupling so it can be cancelled by the CMRR (Common Mode Rejection Ratio) of the balanced input circuit. 
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John Roberts {JR}

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Re: Not Ground Loop Hum
« Reply #19 on: May 04, 2016, 09:34:22 am »

The reason that mis-wiring is an issue is because noise is capacitively (I think-JR can correct me maybe inductively?) coupled to the shield.  A tester that intentionally couples a noise source to the shield should be able to identify the pin the shield is connected to-then all that is needed is a simple pin to pin continuity test.
It seems electrostatic (capacitive) coupling would be the dominant mechanism.

It "might" be as easy as measuring the capacitance between pairs of leads. I would expect the C between 2-3 to be different than 2-1 or 3-1, but this is pure speculation.

My cheap rat shack VOM has a C meter built in.

JR 
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Re: Not Ground Loop Hum
« Reply #19 on: May 04, 2016, 09:34:22 am »


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