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Title: Grounding line level shields to rack
Post by: Jerry Turnbow on April 07, 2014, 08:11:36 pm
Hey, folks - I'm bidding on some work where the A/E has specified that "Shields for line-level audio circuits shall be grounded to rack sheet metal at each termination and and at each end of the cable".

I suppose if it's not grounded at either the input or the output, the net effect that of just having a balanced pair between the units, with the shield acting as a Faraday shield, but we typically connect the shield at one end only.

Has anyone heard of and/or had any experience with this practice?  I've been in this business in some form or another since 1976, and this is the first time I've seen such a request.

Thanks!
Title: Re: Grounding line level shields to rack
Post by: Ivan Beaver on April 07, 2014, 08:45:26 pm
I have never seen that.

Over the past five hundred or so installs I always followed the following-and it worked great with no buzz issues.

Connect the OUTPUT shield to connector (XLR or euroblock) and LIFT THE INPUT shield.

Sometimes hooking both up will induce ground currents and therefore buzz.

Of course this only assume balanced connections.  Unbalanced is a totally different story.
Title: Re: Grounding line level shields to rack
Post by: duane massey on April 07, 2014, 11:14:26 pm
Ivan, we used to debate the question of lifting the input or the output shield. I had one situation that made me go with the output lift. We did an install with a mixer location approx 100' from the equipment rack, and used a multipin connector as a disconnect. When the disconnect was taken apart we had a noise issue with the main feeds, and it was traced back to the fact that the input shield was lifted at the rack, thus there was no shield connected at either end when the console was not connected. Of course, this was a number of years ago, and today's gear may not be as sensitive, but lifting the shield on one end od a balanced line seems to be the proper approach (IF the electrical supplies are properly grounded).
I did see MANY goofy wiring specs back in the day, and the one described by Jerry did appear a few times.
Title: Re: Grounding line level shields to rack
Post by: Kevin Graf on April 07, 2014, 11:27:08 pm
Tony Waldron has several papers on wiring large systems.

"Tony Waldron's Audio & EMC ranting and ravings"

http://www.fragrantsword.com/twaudio/

The same or similar papers:
http://www.nutwooduk.co.uk/archive/Old_Archive/020514.htm
http://www.nutwooduk.co.uk/archive/Old_Archive/020722.htm
http://www.compliance-club.com/archive/old_archive/021122.htm
http://www.compliance-club.com/archive/old_archive/020918.htm
Title: Re: Grounding line level shields to rack
Post by: John Roberts {JR} on April 08, 2014, 12:01:59 am
Shields should be grounded at both ends, and the middle if possible. Audio (+) and audio (-) do not get grounded .

JR
Title: Re: Grounding line level shields to rack
Post by: Ivan Beaver on April 08, 2014, 07:30:13 am
Ivan, we used to debate the question of lifting the input or the output shield. I had one situation that made me go with the output lift. We did an install with a mixer location approx 100' from the equipment rack, and used a multipin connector as a disconnect. When the disconnect was taken apart we had a noise issue with the main feeds, and it was traced back to the fact that the input shield was lifted at the rack, thus there was no shield connected at either end when the console was not connected. Of course, this was a number of years ago, and today's gear may not be as sensitive, but lifting the shield on one end od a balanced line seems to be the proper approach (IF the electrical supplies are properly grounded).
I did see MANY goofy wiring specs back in the day, and the one described by Jerry did appear a few times.
I got the idea from Neil Muncy-who is considered the "guru" of grounding/shielding.

It has always worked for me.
Title: Re: Grounding line level shields to rack
Post by: Brad Weber on April 08, 2014, 11:19:13 am
Hey, folks - I'm bidding on some work where the A/E has specified that "Shields for line-level audio circuits shall be grounded to rack sheet metal at each termination and and at each end of the cable".
Well, if the rack chassis is grounded and all audio shields are connected then since all of the grounds go back to a single point all of the audio shields should be tied to the rack(s), albeit rather indirectly.
 
I agree with JR that you ideally connect the audio shield at both ends.  If you have to deal with a ground loop then I agree with Ivan and drop the shield at the device input (receive) end.  Then there is the hybrid approach of connecting shield at the source device and connecting the shield via a 0.1 microFarad capacitor at the destination device input.
 
Some great resources on grounding, power distribution, thermal management and cable management are available at http://www.middleatlantic.com/resources/white-papers.aspx (http://www.middleatlantic.com/resources/white-papers.aspx).
Title: Re: Grounding line level shields to rack
Post by: duane massey on April 08, 2014, 06:52:34 pm
This is for Ivan, Brad, Jr, and everybody smarter than me:
If you have a long run of cable (say 100' or more) from rack to mixer, and you lift the shield on the input side at the rack, then wouldn't you have an issue if the output of the console is unplugged? You would have 100' of cable with neither end grounded, which is problematic. Am I missing something here?
Title: Re: Grounding line level shields to rack
Post by: John Roberts {JR} on April 08, 2014, 09:50:33 pm
JR does not say to ever lift a shield ground. If you have gear that can not tolerate having both shields grounded, lift  that gear as high as you can, then drop it preferably on a concrete floor.

JR

PS: OK here's an old trick for the new dogs. IF you encounter flawed gear that is happier with a shield ground lifted, solder a 0.1uF disc cap in place of the lifted ground. This will look like a short at RF but not for mains frequency. 

Title: Re: Grounding line level shields to rack
Post by: Ivan Beaver on April 09, 2014, 07:30:48 am
This is for Ivan, Brad, Jr, and everybody smarter than me:
If you have a long run of cable (say 100' or more) from rack to mixer, and you lift the shield on the input side at the rack, then wouldn't you have an issue if the output of the console is unplugged? You would have 100' of cable with neither end grounded, which is problematic. Am I missing something here?
I would say the BIGGER problem is that the signal will not be able to get from the console to the rack if it is UNPLUGGED.

When something is unplugged and the circuit is not complete-it is not a normal condition-therefore who cares?

What good would the system be?  And the noise should not be a concern.

Or maybe I am missing something in the question---------------
Title: Re: Grounding line level shields to rack
Post by: duane massey on April 09, 2014, 09:47:20 am
Ivan,  the venue had a portable console that was only used for certain events, and was disconnected when not in use. The system was still used for DJ's and background music/paging, so it was active at all times.
Title: Re: Grounding line level shields to rack
Post by: Ivan Beaver on April 09, 2014, 12:47:16 pm
Ivan,  the venue had a portable console that was only used for certain events, and was disconnected when not in use. The system was still used for DJ's and background music/paging, so it was active at all times.
Then the unused inputs should be muted.
Title: Re: Grounding line level shields to rack
Post by: duane massey on April 10, 2014, 12:45:33 am
There was no way to mute the inputs. The rack was in an upstairs area that was inaccessible to the employees, and this was pre-digital era. We floated the ground on the output side and tied on the input side (at the rack) and the problem went away. Soundcraft board, Rane analog rack gear, the old days......
Title: Re: Grounding line level shields to rack
Post by: Ivan Beaver on April 10, 2014, 07:29:44 am
There was no way to mute the inputs. The rack was in an upstairs area that was inaccessible to the employees, and this was pre-digital era. We floated the ground on the output side and tied on the input side (at the rack) and the problem went away. Soundcraft board, Rane analog rack gear, the old days......
So what you are describing is a "non normal" situation.

When I was talking about how I do grounding-it was referring to normal situations.

Anything out of the "norm" often requires a different approach.

Also a lot of the analog gear "back in the day" had bad pin 1 problems.  Just to compound things.
Title: Re: Grounding line level shields to rack
Post by: duane massey on April 11, 2014, 10:22:06 am
I'm just grateful that a lot of the challenges we faced "in the day" are no longer an issue. I do kinda miss the look of awe on client's faces when you could walk in, clip one or two shields on a couple of cables, and make the "noise" vanish, although (in my life as an onsite service guy) I still find problems created by the previous "installer" that can be fixed this easily.
Title: Re: Grounding line level shields to rack
Post by: John Roberts {JR} on April 11, 2014, 11:09:28 am
The irony surrounding lifting shield grounds to reduce noise, is that every day the amount of RF we encounter increases. At some point you may have to re-connect that clipped shield to reduce RF interference.

JR
Title: Re: Grounding line level shields to rack
Post by: Ivan Beaver on April 11, 2014, 01:06:17 pm
I'm just grateful that a lot of the challenges we faced "in the day" are no longer an issue. I do kinda miss the look of awe on client's faces when you could walk in, clip one or two shields on a couple of cables, and make the "noise" vanish, although (in my life as an onsite service guy) I still find problems created by the previous "installer" that can be fixed this easily.
I used to carry around clip leads for exactly that purpose-to make or change the grounding path of a particular piece of gear.

And many pieces of top of the line professional gear had XLRs-but was NOT balanced (on the outputs and some inputs).

Add to that all the gear that had pin 1 problems and you would spend most of the time setting up for a gig trying to get rid of noise.
Title: Re: Grounding line level shields to rack
Post by: Steve M Smith on April 11, 2014, 01:46:48 pm
Add to that all the gear that had pin 1 problems and you would spend most of the time setting up for a gig trying to get rid of noise.

I keep reading about 'pin 1 problems' in old equipment.  What does that refer to?


Steve.
Title: Re: Grounding line level shields to rack
Post by: John Roberts {JR} on April 11, 2014, 02:14:01 pm
I keep reading about 'pin 1 problems' in old equipment.  What does that refer to?


Steve.

Maybe you don't have google where you live?  8)

"Pin one problem" refers to balanced/differential (3 wire interface) gear, where noise current dumped into pin 1 (like from a shield ground, or ground loop) contaminates the audio on pins 2 and 3. In hindsight this seems obvious but many product designers struggled with this, decades ago, even moi.  :-[

JR

 
Title: Re: Grounding line level shields to rack
Post by: Mac Kerr on April 11, 2014, 03:35:41 pm
Maybe you don't have google where you live?  8)

Google links (https://www.google.com/search?client=safari&rls=en&q=pin+1+problem&ie=UTF-8&oe=UTF-8), and to be more specific, Pin1Problem.com (http://pin1problem.com).

Mac
Title: Re: Grounding line level shields to rack
Post by: Steve M Smith on April 11, 2014, 08:47:38 pm
Maybe you don't have google where you live?

I thought I would ask real experts.


Steve.
Title: Re: Grounding line level shields to rack
Post by: John Roberts {JR} on April 11, 2014, 09:00:08 pm
I thought I would ask real experts.


Steve.

"Real experts" have written white papers on the subject, easily found using your favorite search engine. This is kind of old news now so hard to repeat over and over.

JR
Title: Re: Grounding line level shields to rack
Post by: Ivan Beaver on April 11, 2014, 09:19:50 pm
I thought I would ask real experts.


Steve.
And it appears as if the "real experts" agree with the people who"discovered" the issue-found solutions and told the world.

"If it appears as if I can see further-it is because I am standing on the shoulders of GIANTS".

The "geeks" have made life much easier for us :).

It was much harder in years past.  Now almost anybody can get a sound system working.

This is not necessarily a good thing :(
Title: Re: Grounding line level shields to rack
Post by: Stephen Swaffer on April 11, 2014, 10:24:19 pm

It was much harder in years past.  Now almost anybody can get a sound system working.


That depends on you definition of "working".

Just curious-but in the absence of access to "mute" an input, wouldn't a dummy "shorting" plug or jack work almost as well?  Essentially giving zero volt DC input?
Title: Re: Grounding line level shields to rack
Post by: Ivan Beaver on April 12, 2014, 09:39:38 am
That depends on you definition of "working".

Just curious-but in the absence of access to "mute" an input, wouldn't a dummy "shorting" plug or jack work almost as well?  Essentially giving zero volt DC input?
It is truly sad that these days it seems as if anybody who can make sound come out of a PA is considered an "expert". :(

Just look at all the bad installs out there and all the photos people post in which they are "proud" of what they have done-----------

Yes just shorting the input to a line level device (pins 2 and 3 only) would "mute" it.  Or it coudl be done with a simple switch.

Mic level inputs (especially with phantom power) are a slightly different story-but the same concept applies-
Title: Re: Grounding line level shields to rack
Post by: duane massey on April 13, 2014, 03:11:59 am
It was a multi-pin connector, so a shorting jack would not have worked. The problem was solved by lifting the shield at the outputs and connecting it at the inputs. Simple solution with no action needed by the employees and nothing extra added to the signal chain.
Title: Re: Grounding line level shields to rack
Post by: Kevin Graf on April 19, 2014, 10:13:42 am
After 25 posts, no-one  has pointed out that there is no 'ground' connection in a balanced analog interconnect system.

The first paper by Neil Muncy

"Pin 1 Revisited"
 Neil Muncy called our attention to the Pin 1 problem (the improper termination of the shield of audio wiring to the circuit board rather than to the shielding enclosure) in his classic 1994 paper, reprinted in the June 1995 Journal of the AES. When he wrote his paper, most commercially available audio gear had pin 1 problems. It was, indeed, difficult to find equipment without it -- even the most prestigeous consoles had serious pin 1 problems! Over the next decade, the better manufacturers redesigned their products to correct their mistake, but sadly, many have not done so. This is the first installment of a two-part article published in the SynAudCon Newsletter.
http://www.audiosystemsgroup.com/Pin_1_Revisited.pdf

"Pin 1 Revisited -- Part 2"
 This is the second installment of a two-part article
http://www.audiosystemsgroup.com/Pin_1_Revisited_Part_2.pdf

Later it became Audio Engineering Standard 48

AES standard on interconnections
-Grounding and EMC practices-
Shields of connectors in audio equipment containing active circuitry