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Current filtering for audio lines in theaters

01/18/2008 7:29 AM

What type of transformer should be used for filtering 25 KW monophase audio ampli lines for a small performance hall installation, and is it OK to set it in a separate compartment in an existing switchboard, since multiple distribution lines extend after the filter?

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#1

Re: Current filtering for audio lines in theaters

01/18/2008 10:20 AM

Help me, I am slow. What does 25 KW monophase mean?

What are you trying to filter? That is, what lines are you filtering, the actual audio lines, power lines, what?

Are these inputs to a mixer, outputs from a mixer, the speaker lines?

Can you describe your setup in more detail?

Where did you buy your equipment? Can they be of help? Generally, better stores have people that are trained in sound reinforcement.

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#2
In reply to #1

Re: Current filtering for audio lines in theaters

01/18/2008 10:32 AM

25KW is the maximum power to use for power amplification. All audio loads shall be on one single phase, of course.

The filtering is for the power lines before amplification.

These inputs are before mixers, power amplifiers and speakers, of course.

These are the power lines from the switchboard to the audio electronics around.

It's for preparing the installation. No equipment has been bought. Others will supply it when needed.

The endpoints shall be plug inlets for now.

Did I answer your questions?

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#3
In reply to #2

Re: Current filtering for audio lines in theaters

01/19/2008 4:48 AM

It sounds like what you are looking for is an isolation transformer? I'm not all that familiar with high end audio, but that's the only thing I could see using a transformer on your input line for. For that, basically you just need a single phase 1:1 isolation transformer rated for the voltage and current that you are going to be running.

Also, about placing the transformer in a separate panel. There shouldn't be a problem with it, but check with your local codes and make sure that this is permitted.

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#7
In reply to #2

Re: Current filtering for audio lines in theaters

01/19/2008 6:04 PM

Yes. The more I think about it I would probably call the manufacture of the amplifiers. Their technical support should be the best source of recommendations.

Good luck!

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#4

Re: Current filtering for audio lines in theaters

01/19/2008 11:03 AM

In a office building fed via a 3 phase service entrance the loads supposed to be balanced accross the 3 phases. Measuring the current on the ground lead is the imbalance, and should be relatively small, well below 10%. When the load consist mostly computers, their switching power supplies distort the waveform brutally, resulting in large harmonic currents fed back toward the power source and anybody in the neigbourhood. They have to mitigate it, but that is no help for you.

In the meantime you are stuck with a very dirty power (and its ground lead) of high harmonics right thru audible frequencies and beyond. It is not good, and can cause all sort of chirping and buzzing in your audio system. As you cannot ask your neighbours to turn of their noisemakers, when you need quiet, you have to protect yourself.

An isolation transformer for your sound system ONLY, will separate the power supplies from the power net just fine. But it will do its best ONLY, if your sound grounds are separate from every other user of power. They have to be tied ONLY to each other, then a single run to the designated ground of the building or the floor. This practice ensures, that there is no noise voltage DIFFERENCE showing up anywhere in your system, ready to be amplified. The proper expression is: avoiding ground loops.

Murphy's law dictate, exactly that will happen, given half a chance.

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#5
In reply to #4

Re: Current filtering for audio lines in theaters

01/19/2008 11:20 AM

Hi Leveles.

Thank you for your very clear answer.

Skipper_A

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#6

Re: Current filtering for audio lines in theaters

01/19/2008 3:16 PM

Hello ,

You might try that question over at www.filmtech.com

As an owner of a drive in movie theatre , I don't really know the answer to your specific question , but wiser minds over there - have most likely done just what you are asking.

Best Regards,

Joe Woodall, Managing Partner

Georgia Adobe & Renewable Energy

2395 Bowman Hwy. NW.

Dewy Rose GA 30634

706-283-3938/7693

sales@georgiaadobe.com

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#8

Re: Current filtering for audio lines in theaters

01/21/2008 3:56 PM

What you want is called a power conditioner and can be purchased at or through any high-end AV store. Check out http://www.furmansound.com/

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#9

Re: Current filtering for audio lines in theaters

01/22/2008 7:48 AM

Finding a 25 kW power conditioner maybe pretty difficult. I would start here. I use Sola equipment frequently and have had good results with their products.

Good luck with your project.

Mike

HTTP://www.solaheviduty.com/products/powerconditioning/index.htm

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#10

Re: Current filtering for audio lines in theaters

01/22/2008 9:01 AM

Well that is true. I'm sure front end filtering is more difficult to find. Putting power conditioners on your amplifiers and other equipment will help though.

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Guru

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#11

Re: Current filtering for audio lines in theaters

01/22/2008 10:32 AM

Well, The Sola line of power conditioning transformers are an excellent line of product. They are different from a simple isolation transformer. They do more by regulating the line voltage for you too, and improve waveform (reduce harmonics, etc) by driving one leg of the magnetic core into saturation. The price for it is, that they are much bigger, throw of much more heat, and are mechanically noisier. That, the biggest is just 15kWatt for your 25kW requirement does not matter, as an audience won't hear the difference. Or you could feed all the low power audio equipment from one 1 - 2kW(?) one, and the two power amplifiers from a simple isolation transformers, as those brutes are past of any place where noise can creep in. There is no problem with this kind of mixing of the power sources, but the separation of the whole sound ground from the rest still strictly applies.

By the way, your average, competent electrician - not having installed any such sound system before - will not believe in separate grounds. You will have great fun enforcing it.

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#14
In reply to #11

Re: Current filtering for audio lines in theaters

01/24/2008 3:47 PM

Thank you for your commentary. You just don't know how right you were about the electicians' skills. They always own the whole wisdom of the world!

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#12

Re: Current filtering for audio lines in theaters

01/24/2008 10:05 AM

You can parallel the Sola units to get more capacity. 3-10KW units will give you 30 KW of capacity. But the previous comment is right, they buzz like the devil, especially in damp locations. You'll need to isolate them physically to prevent noise transfer. Those units are NOT isolation transformers however so you have to make sure you don't have floating grounds or reversed phases. You may want to consider installing an isolation transformer too for floating ground issues.


On the amplifier output side, make sure you use twisted pairs for the speaker outputs and pass them repeatedly through large ferrite cores at either end. the twisted pairs will prevent the speaker runs from picking up common mode interference (cell phone GSM "galloping" and AC hum from nearby power line runs, etc.) each conductor of the pair will pick up the interfering signal 180 degrees opposite phase of the other and the interference cancels itself out. The ferrite will act as a low-pass filter to damp out high frequency noise that could damage piezo tweeter horns (their impedance is frequency dependent and the higher the frequency, the more like a short circuit they look like. Induced current from a lightning strike nearby could fry them if they aren't protected.). Make sure ALL the equipment is tied back to a common ground point. flat copper foil ribbon (or mashed copper tubing) will handle high frequency transients (static electricity discharges, lightning strikes etc.) much better than the equivalent cross section of round cable will due to skin effects. This will prevent floating grounds and ground loops as well.

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#13

Re: Current filtering for audio lines in theaters

01/24/2008 10:13 AM

One other thing. A LOT of BS is circulating in the marketplace about special magic speaker cables that somehow or another do all sorts of crazy things.

Don't believe it. It is all bovine excrement!

Make sure the wires are twisted and are of proper size to handle the peak current and you will save thousands of dollars and sound just as good. Skin effect is not even measurable until you get into the mid-high ultrasonic frequency range or extremely high power levels (more than we are discussing here for sure), so anyone that talks about skin effect or litz wire or any of that is talking out of his or her nether regions and should be promptly beaten within an inch of his or her life.

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#15
In reply to #13

Re: Current filtering for audio lines in theaters

01/24/2008 3:54 PM

Thank you for your very enlighting answers.

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#16
In reply to #15

Re: Current filtering for audio lines in theaters

01/24/2008 3:57 PM

You are more than welcome. If there is anything else I can do to help, just give a shout.

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