Film-Tech Forum ARCHIVE: Processor to amplifier wiring

This is topic Processor to amplifier wiring in forum Film Handlers' Forum at Film-Tech Forum ARCHIVE.

To visit this topic, use this URL:
https://ft-forum.com/ft/cgi-bin/ubb/ultimatebb.cgi?ubb=get_topic;f=1;t=006950

Posted by John Westlund (Member # 913) on 02-13-2005, 04:30 PM:

I recently began work at a new theatre. We have a CP-200 processor and the amps are all QSC, 2 1400's, an MX1500 and an MX1500a. The amps are hooked up using the terminal strip and there is nothing hooked up to the ground terminal. I am wondering if it needs to be hooked to something. Also, is the shield for the wiring supposed to be hooked up to anything? Thanks, John

Posted by Gordon McLeod (Member # 33) on 02-13-2005, 04:40 PM:

The shields should be terminated at the amplifier ends only
and anything going into the cp200 should have its shields terminated on the e terminals of the 200
the norm for cinemas is shields terminate at the recieving end only

Posted by Thomas King (Member # 2745) on 02-14-2005, 02:25 AM:

How is that a good thing? Surely the point of having a ground/screen is to establish a common zero potential point between appliances as well as protecting the signal.

Posted by Samuel Hynds (Member # 2610) on 02-14-2005, 09:25 AM:

Its a way of stopping ground loops. If both shield ends are connected a ground loop is formed. Also the output of the CP200 is unbalanced - while the input of the amplifier is balanced. It took Dolby to get to the CP650 to get balanced outputs. [Roll Eyes]

Posted by Thomas King (Member # 2745) on 02-16-2005, 08:01 AM:

Confusing! Ours are grounded. Then again, they're all on the same supply, so they ought to be sharing an earth through the mains ground anyway. It's been a long time since I did my sound engineer training.

Posted by Gordon McLeod (Member # 33) on 02-16-2005, 08:56 AM:

Well maybe you better take a refresher course

Posted by John Westlund (Member # 913) on 02-16-2005, 03:58 PM:

Thanks for the help Gordon. So just make sure the E terminals are hooked to the - on the amp and the L,C,R etc. are hooked to the + of the amp for that channel and hook the shield to the ground on the amp? I've already hooked up the non sync cd player and hooked the shield to the E terminal on the cp200. Thanks.

Posted by Phil Hill (Member # 371) on 02-16-2005, 04:44 PM:

While the mechanical rack mounts and/or AC line "earth" connection provides a common ground between equipment, it should only tie all the equipment "cases/enclosures" together and not provide a reference for the LL signals.

Connecting both ends the shields/screens of the LL signal lines will form another ground path and thus form a ground loop. Which of course leads to hum.

Think of connecting the shield only at the input end of any device as "extending" the chassis shielding of that device.

Rane Audio used to have a application guide as to the whys and hows of properly connecting balanced and unbalanced audio devices. It may even be online by now. Google time.

Posted by Steve Guttag (Member # 268) on 02-16-2005, 07:28 PM:

Phil,

Be careful of what you say...RANE emphatically states that BOTH ends of the shielded cable should be terminated for the most noise/hum free lines. In fact, if you check RANE equipment, you will find that pin-1 of their XLRs are also bonded to chassis ground!

http://www.rane.com/note151.html

RANE is correct in their theories BUT it presumes Ground is Ground and that all grounded pieces of equipment are indeed at the exact same potential to ground and thus will not allow any current flow over the shield lead and thus no hum. This will also provide the best RF shielding too.

However, in the real world...gound is not ground and ground-loops form. As such, people have developed the tie one end of the shield to ground theory. It is certainly the safest bet. For those that need RF shielding one can tie one end to ground and the other to ground via a small capacitor so from a DC standpoint, there is no connection but higher frequencies my be properly earthed.

Posted by Phil Hill (Member # 371) on 02-17-2005, 12:54 AM:

Steve, you're absolutely right. It was not Rane that I was thinking of, I think I meant that other company whose name I cannot think of right now. The one headed up by that red-headed audio guru named Gene. (DAMN! And they say memory is the 2nd thing to go...)

It's the one that makes the POD series of EQs.

Posted by Frank Angel (Member # 248) on 02-18-2005, 05:24 AM:

quote: Steve Guttag
it presumes Ground is Ground and that all grounded pieces of equipment are indeed at the exact same potential to ground

Hahahahaha!

Posted by Richard Hamilton (Member # 321) on 02-18-2005, 07:55 AM:

John,
In regards to your original question. You say you just started at a new theater. Is the theater new, or your employment there new? If it is an older theater, are they having problems with the sound, or are you just curious about the wiring practices? You didnt state any problems in the sound is why I ask. Or is it new and you are doing the install?

Rick

Posted by John Westlund (Member # 913) on 02-18-2005, 09:25 AM:

The theatre is not "new" but is undergoing a total remodel. The sound system is being totally redone with new speakers, processor, and a DTS unit. I was just curious on what the norm was wiring the rack so that I make sure its done right.

Posted by Frank Angel (Member # 248) on 02-20-2005, 05:46 AM:

You have a much better chance of it being done right, if it is being built from scratch. Assuming the electrical contractor knows what he's doing and you can get the contractor who is installing the booth equipment to TALK to one another, you will be ahead of the game.

It's usually when you are dealing with booths that have had stuff added and subtracted and added again over the years that you find bizarre problems. One balanced legs get unbalanced by loads being hooked up in disregard of distributing the amperage loads evenly across the legs (many times the only regard for balanced load distribution is an electrician opening up a breaker box and looking for a spare breaker, and THAT's where he adds another 50 amp load). If everything is referenced properly back to the ground point, which is easy when you are doing a new construction, there will be no ground loops, even if shields are not properly connected. After all, in homes people routinely hook up unbalanced equipment willy-nilly and those are almost always without benefit of input or output transformers. Why don't they see nasty ground loops with the frequency that they are found in commercial setups? Because usually EVERYTHING is pluged into one or two sockets in the same room all run to the same leg of the mains and usually to single phase.

In old commercial buildings and theatres, there are mulitple legs feeding multiple parts of the building and pieces of equipment in desparate configurations, many of which are not in close physical proximity yet still need to be connected together by their audio connection -- a condition that is ripe for ground loops.

In the studio that I work in, when I first to there, everything was so hot that you could cause a hum just by placing your had on a piece of equipment. If you leaned on any metal part of the sound console, it would start to hum. I took a meter and measured the potental between the ground point on an AC outlet on one side to the ground point on the AC outlet on the other side of the room, literally only 20 feet away and there was a voltage of +17 volts. With +1V = 100% audio modulation, it is any wonder what +17V hanging around everything will do to audio gear not properly grounded. Even "electronically balanced" inputs can't reject ground potentials that far off the mark. We ran copper from every outlet back to the best ground we could find. And that helped, but it certainly made everyone aware that zero Earth is not an equal opportunity potential.

People think that all you have to do is mount everything in the same rack and there will never be any ground differential between pieces of equipment in the same rack. Oh contrair, paint and different types of anodized screws can cause the absolutely uniform grounding that one might expect to see by virtue of screwing a piece of equipment into a rack, to be less reliable than one might think, especially those rack mounts that include side wings which are added so as to make units "rack mountable."

Then there's the old RCA Technical School rule for eliminating ground loops. They called it the RCA Snip & Solder test. It consisted of the technican turning up the volume until hum could be heard. Then he would find the shield on the input of that device and he would cut it. If the hum got louder he would resolder it....if it got lower, he would leave it cut. He would also tie it to the chassis to see if that made any change. Not exactly the most comprehensive approach, but it worked often enough for them to try this method first before biting off bigger fish to fry.