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Topic: Sound on one leg or three?

Brad Miller
Administrator

Posts: 17775
From: Plano, TX (36.2 miles NW of Rockwall)
Registered: May 99

posted 07-15-2000 04:32 PM

What is the consensus on sound rack AC power between everyone here? Should everything related to the sound rack be put on one dedicated leg of the power (to minimize the possibility of hum and buzz) or should anything and everything be split up equally between the 3 legs of power (to balance the line better)?

I've heard both arguments and am interested in hearing what everyone here has to say about it.

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Gordon McLeod
Film God

Posts: 9532
From: Toronto Ontario Canada
Registered: Jun 99

posted 07-15-2000 04:55 PM

Depending on the hydro inspector it goes both ways
I preferre bring say a single 60 amp leg over to the rack and then putting individual breakers on each outlet

But in many locations the inspectors will insist on balanced loads
Ideally get a seperate transformer just for audio with its own dedicated ground

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Stefan Scholz
Expert Film Handler

Posts: 223
From: Schoenberg, Germany
Registered: Sep 1999

posted 07-15-2000 05:50 PM

One leg with breakers/ fuses for each individual outlet. Not more than 2 amps per 20 Amp breaker.
And solid grounding structures. Do not rely on rack frames. I have used No. 12 leads from a starpoint copper rail to each individual piece of equipment.
Some inspectors may insist on balanced loading - try to balance by connecting each individual screen to a different leg.

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David Johnson
Film Handler

Posts: 54
From: Melbourne Vict Australia
Registered: Oct 1999

posted 07-15-2000 09:21 PM

Hum does not seem to be a real problem nowadays, not like it once was in the High impedence cell day, we bring in two actives and split the load between them. This prevents the breakers triping out with the switch on surge currents, this must be quite a problem in the US with 115 Volt supply, double the surge curent than with the 240V supply.
David

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Francis Casey
Film Handler

Posts: 44
From: Saint John, NB, Canada
Registered: Jun 2000

posted 07-15-2000 09:31 PM

What about a "ballanced power" system for sound ; i have read something about it but do not know of any one using it .In this system a seperate special transformer is installed for the sound systems . It provides 60 volts either side of ground ( giving 120volt ac )with the ground being 0 volts .This is claimed to provide cleaner power and very much less hum and noise from ground loops etc.
Has anyone had any experience with this in a theatre sound system ?
Francis

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Gordon McLeod
Film God

Posts: 9532
From: Toronto Ontario Canada
Registered: Jun 99

posted 07-15-2000 09:46 PM

There is a radio station in British Columbia that has used it but getting electrical approval would be very difficult

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Russ Kress
Expert Film Handler

Posts: 202
From: Charleston, WV, USA
Registered: May 2000

posted 07-15-2000 09:56 PM

I "try" to isolate the sound on a seperate leg when possible, but I usually just end up settling for keeping the dimmers and motors off of the sound leg.

The worst interference I ever encountered was at the Towers Cinemas in Roanoke, VA.

How'd the theatre get its name? You guessed it! It was directly across the street from an A.M. radio broadcast tower array!

There was one booth monitor that, when switched off, would play the radio station! The booth had a solid steel door that had to be closed during performances. Lots of RF.

Does anyone know if that stuff can hurt you? I tried to spend as little time there as possible.

Russ

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Rick Long
Jedi Master Film Handler

Posts: 759
From: Toronto, Ontario, Canada
Registered: Nov 1999

posted 07-15-2000 09:59 PM

I have often had problems with hum with an unbalanced neutral, ie: where a neutral leg is carrying considerable current. Ideally, of course, the neutral should be carrying zero (or as near as possible) current.

We have, in our latest installations, split the load as much as possible to acheive this.
So far, no hum problems.

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Aaron Sisemore
Flaming Ribs beat Reeses Peanut Butter Cups any day!

Posts: 3061
From: Rockwall TX USA
Registered: Sep 1999

posted 07-15-2000 10:15 PM

Russ Said:
"There was one booth monitor that, when switched off, would play the radio station!"

Switched OFF?!? WOW!

I remember a CP50 that was installed in two different places, in one location, if you cranked the volume all the way up, you could get the local AM station loud and clear. After the unit was moved to another location, it began picking up police radios at listenable levels with the fader at a standard setting! That '50 didnt remain there for long, natch...

A few of our theatres have cell phone repeater stations on the roofs (as they are about the tallest buildings in town, and i guess the rent money from the cellular ppl is good)... I was very fearful of the sound systems playing back cell phone calls, but so far we have been lucky...

As for power, I try to keep all the sound systems on a common power leg, especially if I have to 'daisy-chain' the non-sync wiring...

The real scourge with power-line transmitted hum and interference is the damned slide projectors, hell, they are so noisy you can hear the fade in/out hummmm in the telephones in the office! I have tried just about everything to make the noise go away with practically no success

Aaron
http://www.cinema-west.com

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Rick Long
Jedi Master Film Handler

Posts: 759
From: Toronto, Ontario, Canada
Registered: Nov 1999

posted 07-15-2000 10:24 PM

A am assuming, we all twist our lines going to the stage speakers, right? This was an old hard-and-fast rule emminating from the tube days.

One installation (in North Bay, Ontario) where this wasn't done, ended up with the patrons being treated to taxi calls from the local mall.

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David Johnson
Film Handler

Posts: 54
From: Melbourne Vict Australia
Registered: Oct 1999

posted 07-16-2000 01:53 AM

The Balanced supply sounds like a good idea but I cant see it getting approval in Australia, with the MEN system we use the neutral must be tied to the ground.
David

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John Walsh
Film God

Posts: 2490
From: Connecticut, USA, Earth, Milky Way
Registered: Oct 1999

posted 07-16-2000 10:28 AM

We have basic 3-phase AC wiring, with processor and amps connected to different phases, and never really had a problem with audio. There's always the install that has some problem, but we've solved them by employing regular "best practices" like grounding in one place, etc.

We have had problems with electronic dimmers connected to the same phase; when one is dimming, there's a certain point were it willl cause others to flicker. So we put them on different phases.

We have more problems with other equipment generating noise in the sound- like rewind tables, roof-top fans and xenon ignition noise. I keep a bunch of 0.01uF 1000V caps around.

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Josh Jones
Redhat

Posts: 1207
From: Plano, TX
Registered: Apr 2000

posted 07-16-2000 05:40 PM

Russ,

RF power from say an AM radio station wont hurt you even if you spent a week next to a live tower. Damage to the body doesn't become noticeable until Microwave frequency is reached. Just Don't touch anything live with RF! I have friends in radio that have had RF burns. Very painfull, because they penetrate deep into the skin, taking a long time to heal.

As for power we have a 3-Phase balanced transformer just for audio, nothing else.
It "floats" the ground. A Distribution box(breaker box) then splits the audio load over the three phases

------------------
"where to they teach you to talk like this, in some Panema City wana hump-hump-bar? sell crazy someplace else. we're all stocked up here" As good as it gets

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Mark Gulbrandsen
Resident Trollmaster

Posts: 16657
From: Music City
Registered: Jun 99

posted 07-16-2000 08:04 PM

I have done it both ways, but for really large systems split between phases may be the only way out. I really prefer to have it all on one leg if possible. This will prevent many problems from creeping in. Also, if one keeps the light dimmer on the other legs then any scr/triac noise will be less likely to creep into the sound system.
Mark

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