Transformer VA Rating vs. Current Rating - diyAudio
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Old 18th May 2004, 08:36 AM   #1
JhonDoe is offline JhonDoe  Philippines
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Default Transformer VA Rating vs. Current Rating

Hello..

Sorry for this stupid question.. I'm just a newbie in electronics my questions are:

In my country when i buy a transformer i can only get an EI laminated tranformer and the only rating i know about the trafo is their current rating.

Can someone explain to me the equivalent rating between Current and the VA value of a transformer?

I have plans to build a gainclone, a Bridge parallel of OPA541/549(2 chips per sides) for 1 channel. What sould be the VA/Current rating of the trafo?
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Old 18th May 2004, 08:56 AM   #2
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VA rating is (V*V)/R at the secondary side.

The current is V/R this is valid for RESISTIVE load.

If you rectify and add smoothing caps the VA rating will drop. If you have a huge cap battery the VA will drop to half but as an average value the VA or DC watt out will be 0.9-0.7 of the VA of the transformer.

100VA => 90-70 watts DC continuously

100 VA + 100 000 uF => 50 watts or so out

The VA rating is also dependent how you plan to use the amp. It's only about the temperature inside the transformer. When you have reached 105 deg C inside a toroid transformer you have reached the max VA out take.

In your case I'll believe 150-300 VA is normal if you are going to use the amp at home.
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Old 18th May 2004, 09:11 AM   #3
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hi,
i know what you mean, the current rating as stated on the traffo has no meaning to me....

for gainclones, you can use 12-0-12 or 24-0-24 traffos, like those off deeco in raon...you can choose one that used a 1 1/4 in center leg....

for a bridged type gainclone, you can use two traffos, 12-0-12 and bridge rectify them to have separate plus and minus 35vdc rails....


http://www.diyaudio.com/forums/showthread.php?s=&threadid=30760&perpage=15&pagenu mber=5

tony
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Old 18th May 2004, 09:20 AM   #4
JhonDoe is offline JhonDoe  Philippines
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In my case i got 10 ampere dual secondary (2x25-0-25) trafo, is this enough for a sigle channel?
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Old 18th May 2004, 11:48 AM   #5
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Quote:
Originally posted by JhonDoe
In my case i got 10 ampere dual secondary (2x25-0-25) trafo, is this enough for a sigle channel?
yes, it should work...
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Old 18th May 2004, 04:28 PM   #6
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Quote:
Originally posted by peranders
In your case I'll believe 150-300 VA is normal if you are going to use the amp at home.
Why does everyone state such huge ratings for the transformers for use with gainclones? My subwoofer amplifier (gainclone) takes about 500mA at full volume, including the cooling fans, so I am using a 60VA transformer (25v dual secondary) which should give me +/-30v @ 1A, which is ample. For the circuit I am using (bridge + parrallel I) was advised to use a 300-500VA transformer!
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Old 19th May 2004, 02:31 AM   #7
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Quote:
Originally posted by Matttcattt


Why does everyone state such huge ratings for the transformers for use with gainclones? My subwoofer amplifier (gainclone) takes about 500mA at full volume, including the cooling fans, so I am using a 60VA transformer (25v dual secondary) which should give me +/-30v @ 1A, which is ample. For the circuit I am using (bridge + parrallel I) was advised to use a 300-500VA transformer!


please note that in my country, traffos are overrated, i do not know what standards are being followed if at all....

therefore, even if they state 10 amps secondary, it could very well be just the same as your 60va traffo...

of course you can always get away with lower rated traffos, that's your call...
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Old 19th May 2004, 04:45 AM   #8
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JohnDoe,

from angeles city? i see your interested in building gainclones now JohnDoe. Good luck.
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Old 19th May 2004, 05:23 AM   #9
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Quote:
Why does everyone state such huge ratings for the transformers for use with gainclones? My subwoofer amplifier (gainclone) takes about 500mA at full volume, including the cooling fans, so I am using a 60VA transformer (25v dual secondary) which should give me +/-30v @ 1A, which is ample. For the circuit I am using (bridge + parrallel I) was advised to use a 300-500VA transformer!
500ma into even an 8 ohm load is only 2 watts peak how can you run a sub with only 2 watts? I*R=V which is 4v V*I=P which 2 watts. Also 25v dual secondary gives a static (no load) voltage of 35v per rail. rms math, 25v*1.41 =35v

but if that's all you GC need's then 60va is plenty.
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Old 19th May 2004, 05:29 AM   #10
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Default Yes, they probably are oversized.....

But larger transformers sound better..........as they operate further from saturation.

Jocko
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