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Old 11th July 2010, 08:13 PM   #1
sekhar is offline sekhar  India
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Default transformer rating for a 1000W rms amp

hi every body i am in a bit of a confusion ....... wat should be the va rating for a transformer that is gonna be used to power a 1000W rms amp on 4E load.............
therotically i know the following
secondary voltage = 65-0-65V@ 17A ie 2200VA

am i wrong cause never saw such a big transformer inside a 1000W amp not even in a 1500W amp ............ wat do u people suggest ...... what should i do






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sekhar
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Old 11th July 2010, 09:04 PM   #2
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Where did you get 17A from? For 1000W into 4R the current is 15.8A.

And you would need approx 110V rails to drive that load, resulting in a transformer of 79-0-79.

The VA of this transformer would be approx 2000, but smaller can often be used as you are only taking current from one half at a time.
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Old 11th July 2010, 09:15 PM   #3
sekhar is offline sekhar  India
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here is the calculation i thought it is correct >>>>>>>>>>>>>
p=1000W
P = V X V / R
root over of 1000 X 4 = V => 63.45V ~ 65V
current rating of transformer = 65/4 = 16.25 ~ 17A

tell me where i am wrong ...........???????? how do u calculate the rating of a transformer for any given amp .......... kindly help





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sekhar
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Old 11th July 2010, 09:33 PM   #4
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It appears the error comes from your multiple roundings.

P = IxI x R so I=sqrt(P/R)
this is the RMS current in the load, 15.8A

P = VxV / R so V=sqrt(PxR)
this is the RMS voltage across the load, 63.2V
you need the peak to determine the rail voltage
Vrail = sqrt(2) x Vrms = 89.5V
but there is the drive voltage and losses to add also. drive voltage can be approximated to Vgs_th in case of MOSFETs. losses are due to source/emitter resistors and can be calculated using ohms law and peak current.
once you arrive at the total then work back, divide the total by sqrt(2) to find the transformer voltage

edit: the voltages in my first post were incorrect, sorry
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Last edited by richie00boy; 11th July 2010 at 09:41 PM.
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Old 11th July 2010, 10:43 PM   #5
Nisbeth is offline Nisbeth  Denmark
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The transformer in a commercial amp would probably be sized to between 1/8th (consumer surround-amps) and 1/3rd (pro audio) of max. current draw (Max. Pout/efficiency). Let's assume that Pout is 1000W and the amp is 50% efficient at full power, then you get a rough transformer size of between 250VA and 650VA. There might well be audible benefits to using a bigger tranny, but it is pretty far from 650VA to 2kVA+, especially if you aren't going to be running at full power for extended periods of time.


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Old 11th July 2010, 11:25 PM   #6
kevinkr is offline kevinkr  United States
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Commercial practice back in the days when I was involved in power amplifier design was to size the transformer at about 70% of the current required for maximum output power into the load.. Power amplifier testing per FTC directive in the USA requires one hour of preconditioning at 1/3 rated power into the lowest rated impedance and the above was quite sufficient to insure the transformer did not burn up during testing. (Note that there is a similar DIN requirement for audio amplifiers.)

Commercial amps in this power range often use glass G/H output stages and can achieve better than 70% efficiency at full power reducing transformer and heat sink sizing requirements significantly.

1KW rms is really better done these days using class D switching amplifiers, much less heat and significantly lower power consumption. This kind of power is probably required for low frequency drivers only in large venues..

Transformers larger than 1kVa in an amplifier introduce all sorts issues including inrush current management, safety concerns in the event of a fault, not to mention weight, and cost..

What is the application for this amplifier?
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