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Sound Reinforcement - Forums for Live Sound Professionals - Your Displayed Name Must Be Your Real Full Name To Post In The Live Sound Forums => AC Power and Grounding => Topic started by: Mike Sokol on November 23, 2013, 08:58:24 PM
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Anybody here using balanced power in a studio? For those of you new to the concept, it uses a center-tapped, 120-volt isolation power transformer that provides 60 volts on the hot contacts of the receptacles with an inverted phase 60-volts on the neutral contacts. So measuring from H-N gives you 120-volts, but measuring H-G and N-G gives you just 60-volts. This is supposed to "balance" out any hum fields and provide better sounding recordings. Perhaps it also provides a few other advantages I haven't thought about.
I've read about balanced power for years and only occasionally ran into it in a studio here and there, but the University I'm now teaching at weekly uses it in their SSL studio. Freaked me out the first time I measured the receptacle power for the students, but I quickly figured out the wiring once I saw the Furman balanced power units in the machine room.
So is this balanced power stuff snake oil or the real deal? I could actually test this for hum by dragging a guitar amp between a standard power studio and the balanced power studio while checking for speaker hum level using a calibrated mic.
Any thoughts?
Mike Sokol
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Back in the day I remember loading in a couple tours that had giant rolling isolation transformers that I was told made balanced power. I also knew a local company that had one. That's said that is the extent of my knowledge on the subject. I too would be interested to hear more about it and applications where it was beneficial.
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Mike, land-based balanced power was being touted 20-25 years ago, conveniently coinciding with the Epic Rise of the Pin One Issue. Causal? I'm not taking sides here ;)
The first time I read anything about b.p. it was kind of snake-oily without a whole lot of initial explanation about where the "power" Gremlins entered but somehow "balancing" the AC would rid the studio grid of them by not feeding them after midnight. 8) Or somehow the AC got 6dB quieter. Or there is a volcano, a virgin and a full moon involved...
At any rate it looked expensive and inconvenient to use in live audio so I didn't give it much more thought.
Was the install in the Uni's studio pro-active or reactive to a specific problem? SSL recommendation? What does the studio staff think about it?
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I mentioned this on another thread but will repeat it here:
In the UK, building site tools are powered by 110 volts, stepped down and isolated from our 240 volt supply by a transformer. The centre point of the transformer is connected to ground. This is for safety so that if there is a fault condition, the maximum potential to ground is only 55 volts.
I'm not sure if it would have any benefit for audio.
Steve.
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Was the install in the Uni's studio pro-active or reactive to a specific problem? SSL recommendation? What does the studio staff think about it?
I believe it was part of the original studio design which included the SSL console. However they did a few things with the install that was not so great. For instance, the SSL's power supply draws about 15 amps steady-state, and its machine room was fed from a sub-panel with around 250-ft of 12-gauge wire on a 20-amp receptacle. So there was a 12-volt drop from 118-volts in the sub-panel down to 106-volts at the power supply receptacle due to the 250-ft of run. I figured this out while troubleshooting the SSL for occasionally not booting up correctly (yup, lots of digital stuff) and when I told their tech support that the AC voltage was around 106 volts they kinda flipped out.
As luck would have it, I found an unused 30-amp twist-lock outlet in the machine room with a shorter run of 10-gauge from the same panel. So we changed out the cable feeding the rack for a 30-amp twist-lock (yup, we up-sized all the proper cables), and ended up with a solid 115-volts at the SSL supply, which made the console much more happy. Oh yes, there's a pair of really big Furman balanced power transformers, one for the console and one for the studio power. Because this was balanced power the studio tech assumed the console would run on the magical lower voltage, but that's not the case.
I'll do some more snooping around for grins, but I'm still thinking balanced power is an expensive gizmo that confuses technicians with no real benefits. But maybe I'm wrong.
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There are some useful noise cancellation effects, but it also introduces a number of possible safety problems.
This reply to an article singing the praises of balanced power is useful reading...
http://www.soundonsound.com/sos/nov06/articles/crosstalk_1106.htm
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There are some useful noise cancellation effects, but it also introduces a number of possible safety problems.
This reply to an article singing the praises of balanced power is useful reading...
http://www.soundonsound.com/sos/nov06/articles/crosstalk_1106.htm
The reply talks about the safety aspects only. Nothing really about WHY it's supposed to work. Any of the reading I've done on the subject stipulated that double pole breakers and GFIs MUST be used in a balanced power distribution system, which should render it reasonably safe.
Although I have no practical experience with balanced power, the reading I've done on it's advantages seems to zero in on the line filters commonly found in guitar amplifiers. The article made the claim that these filters were essentially balanced (inductor in series with line and neutral, and bypass caps to ground), and wouldn't pay nice with normal 120-0V power. The claim was that a formerly noisy tube guitar amp, was so quiet with balanced power, that people wondered if it was actually on.
I look after a couple SSL installations, and SSL certainly doesn't require balanced power (or make any mention of it, that I can think of). The only problem we had was that the two main PSUs for the console shipped with main circuit breakers that were sized for 240V operation, not 120. They were right on the edge, and would trip after an hour or so of operation. Once that was spotted, and larger (20A?) breakers were installed, everything worked fine. We did a proof of performance, and the installed console met or exceeded spec. The PSUs don't have any special filtering on the line side, just the circuit breaker, fuse, and thermal switches on the heat sinks.
GTD
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Balanced differential power is one of those good ideas on paper. Instead of 120V humming into every high impedance guitar coil, or high impedance signal lead, we have 60V and the symmetrical mirror image opposite polarity 60V so capacitive coupling to nodes, symmetrical to both legs will cancel (in theory). Half voltage with the potential to cancel to zip, is a good thing to people chasing hum problems.
While a fuse on the primary should protect against over current, conventional GFCI my have trouble. Perhaps 2x GFCI? one for each polarity? Or just forget this silly stuff.
JR
PS: Not to get all pedantic but in terms of electrical wiring verbiage , the two 60v legs are the same phase, but opposite polarity.
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what about 2 pole 220v to elimimate hum ? 2 hot legs and a ground to an equipment transformer.
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Just out of curiosity I looked up one of the Furman units mentioned... $2500, 3 rack units, and 90 lbs for 20 amps of balanced power. On the bright side there's a convenient front mounted USB charger so you can charger your phone with it. ;D
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Just out of curiosity I looked up one of the Furman units mentioned... $2500, 3 rack units, and 90 lbs for 20 amps of balanced power. On the bright side there's a convenient front mounted USB charger so you can charger your phone with it. ;D
PHONE ?! I DONT NEED NO STINKING FONE ! OR BADGE ! :o
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One of the problems associated with balanced power is that the standard on and off switch in most gear is one pole.
It is designed for the normal standard power configuration.
The Navy with the balanced power on the ships requires power switches to be two pole and to open and close on the hot and the neutral/common. Leaving the ground connected.
The one pole switch will leave a neutral of 60 volts running to ground if the transformer has a center ground tap.
Switching power supplies that have no tranny???
So Go for the further review and test on the switching and see what it puts out.
Same for the tranny with the center tap . ??
I would like to see where this goes because I have not heard from anyone testing on this side with balanced power.
When I was on the ship I had the my electronics modified to include the two pole switch. along with fuses on hot and neutral sides.
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Just about everything I have seen built since about 1980 has a double pole mains switch.
Steve.
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When looking at the simple computer power supply there appears to be only a single poles switch.
http://www.pavouk.org/hw/en_atxps.html sample schematic of power supply.
With the digital boards and amplifiers stating to use this technology it does not look good for use on balanced power.
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One of the problems associated with balanced power is that the standard on and off switch in most gear is one pole.
While the power switch will function fine, the larger issue is may be that the mains fuse will generally just disconnect one leg. Arguably this is only a safety issue if there is a fault in the transformer primary winding. It is not acceptable to fuse the neutral mains lead, for normal power.
It is designed for the normal standard power configuration.
The Navy with the balanced power on the ships requires power switches to be two pole and to open and close on the hot and the neutral/common. Leaving the ground connected.
The one pole switch will leave a neutral of 60 volts running to ground if the transformer has a center ground tap.
Switching power supplies that have no tranny???
nope, switching supplies have small HF transformers, while there may be some circuitry on the hot side of that transformer.
So Go for the further review and test on the switching and see what it puts out.
Same for the tranny with the center tap . ??
I would like to see where this goes because I have not heard from anyone testing on this side with balanced power.
When I was on the ship I had the my electronics modified to include the two pole switch. along with fuses on hot and neutral sides.
And hopefully you bypass those neutral fuses when on shore power. It seems like an optimal re-settable circuit breaker would have a common bi-metallic actuator heated by both neutral and hot currents, so both contacts always open synchronously. Not a trivial or cheap mechanical design to tool up.
JR
PS Balanced power seems like too much hassle for too little benefit.
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While the power switch will function fine, the larger issue is may be that the mains fuse will generally just disconnect one leg. Arguably this is only a safety issue if there is a fault in the transformer primary winding. It is not acceptable to fuse the neutral mains lead, for normal power.
nope, switching supplies have small HF transformers, while there may be some circuitry on the hot side of that transformer. And hopefully you bypass those neutral fuses when on shore power. It seems like an optimal re-settable circuit breaker would have a common bi-metallic actuator heated by both neutral and hot currents, so both contacts always open synchronously. Not a trivial or cheap mechanical design to tool up.
JR
PS Balanced power seems like too much hassle for too little benefit.
Those units are not in service. One was a 1964 Scott tube amp that had a filter capacitor shorting to the pin 1 ground.
I have not completed the tear down and repair on the power supply and filter capacitors. multi section 75 uf at 600 v.
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Jump to page 26 on this PDF to get Bill Whitlock's take on balanced power schemes. He wrote the paper for Middle Atlantic.
http://www.middleatlantic.com/pdf/PowerPaper.pdf
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That would be Bill Whitlock, now AES committee chair on EMI/RFI.
From his recent PowerPoint:
An Overview of Audio System Grounding & Interfacing
9/4/2012 page 201
Bill Whitlock
So-Called “Balanced Power”
• Properly called SYMMETRICAL power
• Has very seductive intuitive appeal
• NOT similar to balanced audio lines in any way!
• Uses transformer having 120 V center-tapped secondary
• Both line and neutral output blades are energized at 60 V
• Although advertising often implies endorsement, NEC seriously restricts
its use – because it’s potentially dangerous!
• ONLY FOR PROFESSIONAL USE
• NOT to be used with lighting equipment, especially screw-base bulbs
• MUST have GFCI at outputs
• Only technical function is to reduce leakage currents
• Leakage currents are trivial system noise sources
• Reported noise reduction generally less than 10 dB
• Any real benefit likely due to its clustered outlets
This is an example of “marketing gone wild” if ever there was one!
http://centralindianaaes.files.wordpress.com/2012/09/indy-aes-2012-seminar-w-notes-v1-0.pdf
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Just about everything I have seen built since about 1980 has a double pole mains switch.
Steve.
all my QSC amps have a single pole switch.
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(Almost) Everything I've ever done was single pole. A few odd ball switched secondary using 2 pole.
JR
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Just out of curiosity I looked up one of the Furman units mentioned... $2500, 3 rack units, and 90 lbs for 20 amps of balanced power. On the bright side there's a convenient front mounted USB charger so you can charger your phone with it. ;D
Yup, we've got two of them in the machine closet. One for the SSL and the other for studio outlet power. Looks impressive, but I'm not buying into the cost/benefits factor.
JMS
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So basicly we are to a decision that balanced power offers no advantage.
An isolation transformer can provide just as much protection from noise.
If we go on Large Ships like Navy or Cruise ships we might want to verify if we are on balanced power to ensure we take appropriate protections.
Ocean going vessels may have balanced power by design. Turning off the power switch will not shut off the power on the second leg and leave 60 volts running.
http://www.norfolkwire.com/default.aspx?page=item%20detail&itemcode=BROPOW+Z6-62P
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So basically we are to a decision that balanced power offers no advantage.
Yes, I would say that's a fair statement according to the wisdom of the swarm/collective/gaggle.
An isolation transformer can provide just as much protection from noise.
That's a secondary question that's not really been discussed on this thread. But yes, from my own experience a properly placed and grounded power isolation transformer (with it's own G-N-E bond) will reduce a lot of noise from power lines. Now my experience was while measuring military grade component during manufacturing of nuclear missile guidance systems, so it may not translate to audio systems. But the theory and physics should be the same for both systems.
So have any of you used AC power isolation transformers with a secondary grounding system to solve a noise problem? If so, what were the details?
Mike Sokol
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So have any of you used AC power isolation transformers with a secondary grounding system to solve a noise problem? If so, what were the details?
AFAIK every AC isolation transformer I've used passes through the ground. They have all taken 3Ø 4W (G-X-Y-Z)) on the line side and put out 3Ø 5W (G-N-X-Y-Z) on the load side. I find that with newer equipment AC isolation is more work than worth, but they keep showing up on the truck.
Mac
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Just to be accurate there are multiple vectors for mains noise signal corruption. This symmetrical power while IMO impractical (for safety/compatibility reasons), really does have a scientific basis for potentially reducing some types of noise. Instead of 120V signals all around, the dual 60v is 1/2 the peak voltage to begin with. The outside world is not looking at the two mains leads differentially. To the outside world it does look like less voltage. In theory this means less noise picked up by nearby signal conductors. Besides 6 dB smaller mains voltage waveforms the two symmetrical waveforms will possibly cancel out. Visualize one leg pushing noise while the opposite leg is pulling an exactly opposite version of that noise.
6dB+ less noise is not inconsequential if hum is the dominant noise in your studio, while good practice using well shielded (balanced) wiring will generally make hum a non-issue (with modern well designed gear).
So IMO not a pure marketing invention. More like a well intentioned, if not slightly desperate, attempt to deliver a quieter working environment in simpler times.
JR
PS: WRT isolation transformers, all transformers are bandpass filters so very HF noise on mains can get filtered out by an isolation transformer. FWIW the products normal transformer will act like a filter too.