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Old 9th April 2004, 02:15 AM   #1
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Default transformer power question

I have an older tranny that I pulled out of a once working amp. It measures 31-0-31 VAC with no load attached, so I am guessing it is actually a 30-0-30 V transformer. On the back of the amp chassis, it is stated input: 120V 2A. This would give 240 watts of power (and the amp was a 125W amp), so that makes some sense. However, the transformer is VERY large physically (about 4.5"W x 4.25"D x 3.5"H) and weighs 7 pounds. About what amp rating is the transformer? I am not sure how you would figure it... I suppose it would be either 4 amps or 8 amps. Which one? Is there a way I can test it to find out?

EDIT: Could this be suitable for a Pass Labs project? If so, which ones?
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Old 9th April 2004, 03:50 AM   #2
sam9 is offline sam9  United States
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Does "2A" refere to the promaries or secodaries? Figuring out the VA rating of a mystery transformer is very ambiguous. First I think the 125W rating is a bit optimistic. If you measured 31V with no load my guess is that the measurement under load will turn out aboy 28V, but this varies by manufacturer, so who knows. To really get 125W out I would be looking for something a little closer to 35-0-35. Anyway, suppose you actually got 30-0-30, then I would gues you are looking at 90W.

Supossedly manufacturers typically "derate" their transformer and filter cap requirements by 70% so maybe the 125W figure is within commercial reality.

Take a look at the two App notes on the page: http://www.signaltransfer.freeuk.com/

They will walk you through some calculations. Even if you don't know all the input parameters, you can put in some trial figures and maybe get a range of possabilities.


I've tried using mystery transformers a couple of times and not been too happy with the results - my hopes seem to exceed reality in this realm.
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Old 9th April 2004, 11:13 AM   #3
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Can you accurately measure the diameter of the wire on the low voltage side? That would give a clue. Judging by the size, I would expect the rating to be about 300-400VA. That translates to about 180-240 watts DC continuous available at the filter caps for class A amp. Enough for 300-400 watts of normal class B amplifier.
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