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Old 4th May 2012, 10:24 AM   #431
alkasar is offline alkasar  France
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Quote:
Originally Posted by DarpMalone View Post
Wouldn't the VA rating on a 100Vct / 5.6A be 560? Pardon my ignorance
What is "Vct" ?
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Old 4th May 2012, 10:53 AM   #432
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100 Volt center tapped ? ( 50 - 0 - 50 Volts )
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Old 4th May 2012, 11:19 AM   #433
alkasar is offline alkasar  France
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Quote:
Originally Posted by ashok View Post
100 Volt center tapped ? ( 50 - 0 - 50 Volts )
ah yes of course
For the use in this context, a dual secondary transformer (each secondary independant) or center tapped (center wire of both secondaries is common) are equivalent. For audio, the usual is to use toroidals (or R-core) where the secondaries are separate.

2X50Vac 560VA is the same rating as 100Vct 560VA.

to summarize,
to get 250Wrms (mono) in 8 ohm, a 100Vct 280VA transformer is ok
to get 2x250Wrms (stereo) in 8ohm, a 100Vct 560VA is needed (or better, 600VA to take into account the losses here and there in the amp)


note: It's quite rare to need 2x250Wrms to listen music, unless you do public adress or this kind of things
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Old 4th May 2012, 01:07 PM   #434
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Originally Posted by alkasar View Post
+-70Vdc translates back into about 2x50Vac transformer. Itt should be rated 50*5.6=280VA or more.
If one board (mono) requires +-72vDC * 5.6A to produce approx. 250W, does this 100VCT transformer @ 5.6A not only cover one channel? Would I not then require a second identical transformer to supply 250W to the second channel?

I fear 6mths from now I'm going to look back at this post with shame, wondering "what the hell was I thinking?"

Anyway, I know that 250W/ch is lots of juice. It's for 2, 12" subs... I'd like to be able to work them out every now and then.

Thanks for your patience.
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Old 4th May 2012, 03:32 PM   #435
alkasar is offline alkasar  France
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Quote:
Originally Posted by DarpMalone View Post
If one board (mono) requires +-72vDC * 5.6A to produce approx. 250W, does this 100VCT transformer @ 5.6A not only cover one channel? Would I not then require a second identical transformer to supply 250W to the second channel?

I fear 6mths from now I'm going to look back at this post with shame, wondering "what the hell was I thinking?"
You can either use one 100Vct 560VA or two 100Vct 280VA transformers to achieve 2x250Wrms in 8ohms. Now that I know it's for two subs, it's better to use two transformers, one in each sub if the amp goes into the sub

Note that the calculations above were considering 100% amp efficiency.
Also, many speakers rated 8ohms do actually go down to 5 or 6 ohms. This requires more current to achieve same power.
If you think you'll really use the 250Wrms, I would recomend to chose 350VA transformers to have some margin.
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Old 4th May 2012, 03:34 PM   #436
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These are the transformers that I bought at a surplus store in Toronto. I bought one for each L25D board. (I wound back the secondaries until I got 55-0-55 on each transformer)
Click the image to open in full size.

As you can see the label states 575VA. I figure it's a misprint because, 1) it's much smaller than the 420VA toroids that I have. 2) The label then says "62-0-62 (2.5A)"

My thinking is that it should be approx 310VA... After winding the secondaries back to 55v-0-55v, I figured it's about 275VA... Am I way off?
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Old 4th May 2012, 03:45 PM   #437
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Quote:
Originally Posted by alkasar View Post
You can either use one 100Vct 560VA or two 100Vct 280VA transformers to achieve 2x250Wrms in 8ohms.
Wouldn't one 100Vct 280VA transformers be only 2.8A?
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Old 4th May 2012, 04:07 PM   #438
alkasar is offline alkasar  France
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Quote:
Originally Posted by DarpMalone View Post
Wouldn't one 100Vct 280VA transformers be only 2.8A?
Yes if you use the transformer as a single 100Vac supply, No if you use it as a dual rail +- supply served by 50Vac each.
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Old 4th May 2012, 04:23 PM   #439
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Quote:
Originally Posted by alkasar View Post
No if you use it as a dual rail +- supply served by 50Vac each.
So, using the 100VCT transformer in a dual rail +- configuration I'll be getting the required 5.6A? & to apply this to my modified transformers above, I'll get +-55v and 5A (Which at the end of the day is what I wanted anyway)
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Old 4th May 2012, 04:36 PM   #440
alkasar is offline alkasar  France
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Quote:
Originally Posted by DarpMalone View Post
So, using the 100VCT transformer in a dual rail +- configuration I'll be getting the required 5.6A? & to apply this to my modified transformers above, I'll get +-55v and 5A (Which at the end of the day is what I wanted anyway)
it's beyond my skills to know what VA are available once unwired from 62 to 55V. It may be more than just prorating. Your assumption seems reasonnable though.
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