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Author Topic: Power from multiple sub panels  (Read 10527 times)

g'bye, Dick Rees

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Re: Power from multiple sub panels
« Reply #10 on: December 07, 2013, 04:16:55 pm »


Having a few DIs with ground lift functionality and a few isolation devices like these: http://www.radialeng.com/r2011/twiniso.php should solve most problems.  If you still have trouble, it's time to look at repairing or replacing equipment with pin-1 problems.

I use the EbTech "Hum Eliminator":

http://www.ebtechaudio.com/hedes.html

Model HE2-XLR
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Mike Sokol

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Re: Power from multiple sub panels
« Reply #11 on: December 19, 2013, 09:33:23 am »

My experiments with ground loop induced hum shows that a typical XLR run (20 to 100 ft) will have something on the order of 1 ampere of shield current per volt of ground loop differential voltage. Since I've measured 2 or 3 volts between different grounds in outlets from separate service panels, that implies perhaps 2 or 3 amperes of current flow in your XLR shield. Note that the only limit to this current is the actual resistance of the interconnecting cable's shield resistance plus any connector resistance.

How your sound equipment responds to this shield current assault depends first on the "pin-1 problem" which is the result of manufacturers not running short/direct safety ground paths to the gear chassis, often intermingling their safety ground path with other signals on a circuit board. I've found that some active floor monitors will hum with as little as 100 mA of ground loop current, which can come from even 1/10th of a volt difference between grounds. While other active floor monitors can take 5 or 6 amperes of ground loop current (corresponding to 5 volts ground loop differential) and still be very quiet. However, note that 5 amps of current in the shield makes for a pretty good heater, and I've often wondered why some of my XLR cables would get hot during festivals with strange power setups. Now I know why.

Since these typical ground loop currents are on the order of hundreds of mA, and only occur in the shield with no return through the twisted pair, you can simply clamp a standard clamp-ammeter around the outside of the XLR cable to look for ground loop currents. No need to split out the twisted pair from the shield since the return current path is external to your clamp meter jaws and thus won't cancel out, such as what happens in a power cable. See attached for a graphic.

How much ground loop current to accept is up to you, but know that ground loop differential voltages are not static and will change depending on loading of remote circuits. Thus you can have a perfectly quiet stage during setup and sound check, but when the big coffee makers kick on in the kitchen you can have your floor monitors begin to hum like crazy. As we all know, that makes artists on stage very angry and makes us look like idiots.

That's why I always run audio isolation transformers between output of my drive rack equalizers and the snake feeding my FOH amp rack and active monitor wedges. I have a very nice ISO-8 from Whirlwind in my main rack, as well as keep a few Ebtech Hum Eliminator boxes in my gig box for special situations like needing to run a long XLR cable to a green room for a powered speaker. I now run audio isolation transformers on the outputs of my drive rack as a matter of SOP. I'm a simple man who has to do complex things, so if audio transformers on the outputs means that one less thing to worry about during a gig, them I'm all for it.

This problem does not go away simply because you have a digital board. On several of the X32 and PreSonus installs I've seen lately, there was so much ground loop hum between the analog outputs on the console and the floor wedges and FOH speakers that they installed Whirlwind or Ebtech isolation boxes in the XLR paths. How much you pay for your audio iso boxes is up to you, but realize that the frequency response of your system can be compromised by a cheap transformer. I tend to use the higher priced Whirlwind ISO boxes in my FOH system, and save the less expensive Ebtech Hum Eliminator boxes for floor wedges and green room sends.

One more thing to remember is that XLR connectors with the shield improperly bonded to the shell will short out the ground lift switch in many DI boxes and Isolation boxes that use metal XLR jacks. I've found a bunch of these cables recently in several installation with new imported XLRs. So the first thing I now check when a ground-lift switch doesn't seem to be opening the ground loop is to open up the XLR connector and look for the extra jumper. A little nip-tuck with a pair of wire cutters is the correct fix. I've included a graphic of an improperly shell-bonded shield in an XLR connector.
« Last Edit: December 19, 2013, 09:59:30 am by Mike Sokol »
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Mike Sokol
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Mike Sokol

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Re: Power from multiple sub panels
« Reply #12 on: December 19, 2013, 10:03:31 am »

+4 on the whole post.

Frank, would that be +4 dB or what?  ::)
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Jordan Wolf

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Re: Power from multiple sub panels
« Reply #13 on: December 19, 2013, 11:14:55 am »

Mike,

One location I worked at recently just bought a bunch of XLR cables from Monoprice (I think it was their "Premier Series").

I opened one up to check out the solder job (not bad, actually) and found that Pin-1 was bonded to the XLR shell.  Someone's going to have fun snipping all of them...

The cable insulation is also very thick - I'm not a fan.

Just an FYI.

I do check for the XLR Shell bond fairly frequently, and am only occasionally surprised by them.
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Kevin Graf

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Re: Power from multiple sub panels
« Reply #14 on: December 19, 2013, 11:37:17 am »

The reason for not connecting portable cable connector shell's to the shield is because with a portable cable the metal shell may come in contact with other metal things that are a  different potential.  Chassis jacks and cable connectors permanently attached to these jacks don't have this problem.  Of-course it's hard to guarantee that the cable will always be used this way.

Tony Waldron has some good papers on big system wiring.

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

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Re: Power from multiple sub panels
« Reply #15 on: December 19, 2013, 11:54:39 am »

The reason for not connecting portable cable connector shell's to the shield is because with a portable cable the metal shell may come in contact with other metal things that are a  different potential.  Chassis jacks and cable connectors permanently attached to these jacks don't have this problem.  Of-course it's hard to guarantee that the cable will always be used this way.

Tony Waldron has some good papers on big system wiring.

http://www.fragrantsword.com/twaudio/
Yup, I can remember in the early 70's that all my XLR cables were wired with "telescoping shields". That is, one of the XLR connectors (female?) had the shield bonded to the shell, while while on the other end of the cable (male?) the shell was left floating. When you connected two XLR cables together, then the female XLR would "telescope" its shield thru to the male XLR shell.

But I can also remember getting ground loops (and even sparks) when one of these "bonded" XLR shells laid on a metal amp rack. I even saw it spark on a metal covered outdoor stage. I think that the AES standard for XLR wiring changed sometime back then (late 70's?) to eliminate all XLR shell bonding to the shield, in order to eliminate this exact problem. Perhaps the workers soldering these imported cables in sweat shops don't have the latest AES papers to guide them.  ;)   
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Steve M Smith

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Re: Power from multiple sub panels
« Reply #16 on: December 19, 2013, 12:57:49 pm »

Reading this and other posts here has made me realise how lucky we are with power in the UK compared with what you have in the US.

We usually just turn up and have a choice of three industrial connectors to plug into with no problrms.  The US appears to have an ever increasing* inventory of outlets and wiring arrangements, grounding methods, etc.

Is it due to different states having different rules and/or various legacy systems still in use or being adapted?

(* ever increasing in my knowledge of them rather than in actuality).



Steve.
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g'bye, Dick Rees

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Re: Power from multiple sub panels
« Reply #17 on: December 19, 2013, 01:04:30 pm »

Reading this and other posts here has made me realise how lucky we are with power in the UK compared with what you have in the US.

We usually just turn up and have a choice of three industrial connectors to plug into with no problrms.  The US appears to have an ever increasing* inventory of outlets and wiring arrangements, grounding methods, etc.

Is it due to different states having different rules and/or various legacy systems still in use or being adapted?

(* ever increasing in my knowledge of them rather than in actuality).



Steve.

 You have shepherds.  We have cowboys.
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Mike Sokol

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Re: Power from multiple sub panels
« Reply #18 on: December 19, 2013, 01:25:02 pm »

We usually just turn up and have a choice of three industrial connectors to plug into with no problrms.  The US appears to have an ever increasing* inventory of outlets and wiring arrangements, grounding methods, etc. Is it due to different states having different rules and/or various legacy systems still in use or being adapted?

You have to understand the "states rights" concept to get your head wrapped around the NEC (National Electrical Code) problem. When a new NEC book is released, it's up to each state to determine when and if it will apply to that state. Same goes for each city and county and even each inspector. They will determine what year code book will apply for their district, and if they'll accept all of the code recommendations. Note that I said "recommendations" since the NEC is NOT a legal document, only a suggested set of rules that each district can decide to follow or not.

A good example is the latest requirements for AFCI being installed in bedrooms for new housing (Arc Fault Circuit Interrupters). While many would argue that AFCIs produce nuisance tripping and don't actually save lives from fires, that's a different discussion. However, at least a few of our states (North Carolina and perhaps Indiana, if memory serves) have declared it too expensive for new housing (approx $300 additional on a $150,000 house) and thus would slow the building of new housing. So it's not accepted as part of the local code in some states and is ignored in new builds.

The same thing applies for state speed limits. During the great gasoline shortage and rationing of the 70's the Federal government mandated 55 MPH speed limits on Interstate Highways to save fuel. However, states like Texas (with 80 MPH speed limits) said it was unconstitutional to force them to a 55 MPH speed limit. The only way the federal government could enforce the 55 MPH mandate in those states was to threaten to stop all Federal road grants for highway maintenance and improvements. Of course, money talks so all states adopted 55 MPH for a few years until the gasoline shortage was deemed to be over.   

The NEC has no such financial leverage, so it's a real patchwork of code compliance.
« Last Edit: December 19, 2013, 01:27:12 pm by Mike Sokol »
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Steve M Smith

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Re: Power from multiple sub panels
« Reply #19 on: December 19, 2013, 01:34:09 pm »

I prefer Dick's explanation!

We also had speed limits imposed during the 1970s fuel shortage.  70 MPH on all motorways and dual carriageways.  This was and still is known as the national speed limit.

Before that, these roads had no limit (other than the maximum speed of your car).  It's hard to believe now.


Steve.
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