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Old 30th November 2010, 10:03 PM   #11
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I think that in most big PA installations, they would rather have a large number of (almost redundant) amplifiers. (RAIA) Redundant Array of Inexpensive Amplifiers (I made that up). If one fails the show goes on. It's lots easier to quickly replace smaller amplifiers.
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Old 30th November 2010, 10:44 PM   #12
PoweRex is offline PoweRex  France
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Quote:
Originally Posted by AndrewT View Post
what is that special transformer? Why is it needed, but only when a single phase rectifier is used.
Why three single phase transformers and then only if we use a 3phase rectifier?
I think there is a lot contained in your message that I don't understand. I need more help.
That so called special transformer converts an available 3-phase mains to a single-phase one in order to feed your universal input amplifier which is designed to work with single-phase mains thus originally using a single-phase rectifier : this solution will let you use your amplifier where you only have access to 3-phase supply without modifications, but you'll not notice much sonic improvements.

If you want to profit entirely of the 3-phase supply benefits, you'd better install a 3-phase rectifier instead of the original single-phase one in your amplifier. I suggested the use of isolation transformers in this case as they will provide galvanic isolation from the raw mains with reduced capacitive coupling : this allows bi-directional smooth filtering of high-frequency junks and improves the signal-to-noise ratio of your amplifier allowing you to remove the LC mains filter from your amplifier as such filters degrade dynamic performances. If you're lucky enough to find only one 3-phase isolation transformer (3-phase in/3-phase out), then it's perfect but the 3 phases must be perfectly balanced (120 phase lag between each output, when loaded).

With a 3-phase power supply, you'll triple the ripple frequency accross the filtering capacitor : this allows you to divide the capacitor's value by 3 for the same ripple amount. For example, if your mains frequency is 50 Hz in single-phase conditions, the ripple frequency accross the capacitor is equal to 100 Hz : with a 3-phase mains, it will be 300 Hz. By the way, efficiency is higher with a 3-phase configuration (99%) where a single-phase configuration gives only 80% : these values are valid when dual-alternance rectifiers are used.

The advantage of smaller supply filtering caps is faster current delivery to the amplifier's output stage, in dynamic conditions, to feed the loudspeakers' low impedance rapidly. For my personal system, I've designed a 400 Hz single-phase mains generator where the rectified voltage of the amplifier's power supply is even faster than a 3-phase 50 Hz (800 Hz vs 300 Hz ripple) thus dividing the filtering capacitors' value by 8 for great sonic improvements ! Can't go back to 50 Hz anymore...
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Old 1st December 2010, 12:49 AM   #13
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In the age of cheap silicon making a dual input amplifier is not that difficult either use a 3 phase full wave rectifier feeding a PFC circuit which will operate between 300 and 750V or use 3 separate switch mode power supplies feeding the common DC rails so the power supplies can be used ganged on single phase power and balanced on 3 phase supplies, though I still think multiple single phase amplifiers keeps the speaker voltages on the sane side as well as reducing the chance of wiring errors. For HiFi series connecting speakers is a bad idea.
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Old 1st December 2010, 01:46 AM   #14
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Quote:
Originally Posted by PoweRex View Post
That so called special transformer converts an available 3-phase mains to a single-phase one
Perhaps I am misunderstanding you, but there is no need for a special transformer to supply a single phase load from a three phase supply. All you need is a single phase transformer. Two primary leads of the xfmr are taken either line-line or line-neutral from the three phase system. Secondary leads feed the amp (either directly to the rectifier or simply providing 120VAC to the amp's plug). As long as the xfmr voltage ratings are suitable for the voltages used and desired, you're good to go.
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The advantage of smaller supply filtering caps is faster current delivery to the amplifier's output stage
Hmmmm. Current travels at the speed of light, regardless of capacitor size and ripple frequency, but I understand your point. However, if we consider a 20 kHz signal, it really makes no difference if we are dealing with 100Hz or 300Hz recharge on the power supply capacitors. I would argue the same for the rising edge attack of a kick drum; by the time the dynamic attack has started and is over, a 300Hz recharge is actually quite slooooow, and cannot provide recharge fast enough.

The real advantage of higher ripple frequency is a reduced requirement on the LC filter size; this results in a lower source impedance as seen by the output stage. Apples to apples, three phase rectification results in lower peak-to-peak ripple.

Side note
To really get deluxe in the European DIY community, where three phase is available, you could incorporate delta-wye and delta-delta transformers, which will result in a 12 pulse rectifier. Now we're talking increasing our rectifier ripple to 600 Hz. Heck, take it another step with zigzag windings, and you can get out to 1200 Hz. As nice as this seems, the cost and weight of the iron just isn't worth the cost of a capacitor.
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Old 1st December 2010, 03:44 AM   #15
dmills is offline dmills  United Kingdom
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Common practise is to bring three phase to the amp racks, and split it there to supply single phase to the individual amps, at typical per module power levels (less then 10 KVA) it just is not worthwhile messing with three phase PFC inputs.

For example one of the racks at work has 6 * Camco Vortex 6 amplifiers (36kW nominal), it is fed from a 32A 5 pin three phase connector at 400V phase - phase, 230V phase-neutral (Nominally good for about 18KW continuous), of course program audio is ~1/8th average to peak power so and the heating in the connectors, fuses and breakers is proportional to the square of RMS Current, so in practise that 32A feed is just not a problem driving amps with a theoretical rating twice that of the supply connector.

Yea a three phase input is quite possible but given the switch mode PFC input topology normally used in modern lightweight power amps the caps are not that major a factor anyway.

Besides single phase amps have a huge advantage when it comes to needing to use one to run a toy 'speaker on a stick' rig down the rat and carrot for my mates sons band..... Also, easier to fix, and easier to bodge an emergency power supply for.

In the PA game, standard is IMPORTANT, I don't want something I cannot cross rent to the guy down the road because his distro wont power it, and if I need to get out of trouble, being able to plug into a domestic socket is very, very, useful. The advantages of having individual amplifiers on single phase massively override the smaller caps in an operational environment.

Besides, the elephant in the room when it comes to gig power consumption is lighting (at typically 1kW PER LAMP) not audio which is always penny ante in comparison.

The considerations when buying an amp for PA go something like (exact order to taste, but note that audio quality is seldom top of the list):

Reliability,
Weight,
Size,
Ability to drive very low Z loads,
Built in DSP?
Remote monitoring?
Airflow direction,
Sound quality.

The DSP and remote monitoring may or may not matter (application dependent), but for a road guy almost all the others will matter more then minor sound quality issues.

After all this is combat audio we are talking about and nine times out of ten the guitars will be stupid loud even with the PA shutdown, every patch on the keys will be a different level, and the drummer will insist on 57 china cymbals.... The amp rack not being a 6 man lift matters more then a little ripple on the DC rail.

Regards, Dan.
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Old 1st December 2010, 04:18 AM   #16
imix500 is offline imix500  United States
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Indeed. These are what we use at work, except for the single space versions- we build our own.
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Old 1st December 2010, 04:26 AM   #17
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Log time ago I worked on a printer that used 3 phase 50 amp y power. One of the motors used all 3 phases. The rest of the machine power supplies motors etc used 2 of the 3 phases (220volts) or one of the 3 phases ( 110volts). None of the 220 or 110 parts were special, a lot of them were off the shelf parts and were used in other equipment that did not have the special 3 phase motor. Those products ran of off a 220volt 50 amp line. You can use a 110volt or 220 volt system almost anywhere but you can only use a 3phase system in a place that has 3 phase handy.
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Old 1st December 2010, 05:32 AM   #18
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Without getting ultra exotic with inverters or motor-generator sets, the phase converter is a popular, efficient, and cheap way to get three phase from single phase.

Ronk Electrical Phase Converters - Run 3 Phase Equipment On Single Phase Power

These are really designed for motor load, though, and don't provide the best phase balance. Certainly wouldn't be the best choice for audio.

That printer must have needed constant torque in order to require three phase power, or maybe the hp was so large they were only available in three phase.
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Old 1st December 2010, 08:29 AM   #19
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Quote:
Originally Posted by zigzagflux View Post
Without getting ultra exotic with inverters or motor-generator sets, the phase converter is a popular, efficient, and cheap way to get three phase from single phase.
These are not that useful without a step up isolation transformer either on the single phase or the 3 phase supply, only delta connected small motors in Australia use 240V 3 phase everything is 415V Europe is probably similar with 400V power.
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Old 1st December 2010, 11:31 AM   #20
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Aint it funny how all large music festivals these days are 'green' yet they need as much power as a small town! All that lighting and those speaker arrays....
How bout a festival using horns and candle light?

I had a dude show up on a tube fest with a 300B SE build that had a 3phase supply. I happen to have a couple of 3phase outlets in my house so it was a cool setup. The <10watt amp had seperate supplies with seriously oversized custom 3phase toroids and all tube rectification. It weighed a ton and sounded ok, but really, once an amp is up to a certain level I doubt the effort is worth it...
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