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[Jerry Turnbow]

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!

[Ivan Beaver]

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.

[duane massey]

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.

[Kevin Graf]

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

[John Roberts {JR}]

Shields should be grounded at both ends, and the middle if possible. Audio (+) and audio (-) do not get grounded .

JR

[Ivan Beaver]

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.

[Brad Weber]

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.

[duane massey]

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?

[John Roberts {JR}]

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. 

[Ivan Beaver]

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---------------