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Old 30th December 2011, 05:36 PM   #1
GloBug is offline GloBug  Canada
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Default Question about Transformer VA

Hey guys I am picking out a transformer from my junk pile for a LM3875 gainclone build.

Let's say I have a 220VA transformer that has multiple secondary voltages. (22-0-22, 13.8)

Can I draw the 220VA from the 22-0-22 winding if I don't use the remaining 13.8v winding?, or do I have to subtract the unknown VA of the 13.8v winging from the total VA?

I have it narrowed down to two transformers with a workable voltage.
One has slightly more VA then the other, but it also has more secondary windings. So I'm having a tough time choosing the best one for my build.

Here are the specs:

PT#1 -removed from 20wpc HarmonKardon hk330i
220VA 22-0-22, 13.8 volt secondaries.

PT#2 -removed from 30wpc Technics SA-222
260VA 27-0-27, 36, 5.8 volt secondaries.

Which do you think would deliver the most power using only the main center-tapped secondaries?

The voltage might be a secondary decision after I sort out this part.

Thanks for your time.
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Old 30th December 2011, 05:51 PM   #2
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It is not just the VA of the transformer, but also the wire gauge of the windings that need to be considered.

But, considering they were pulled from amplifiers, it is fairly safe to assume the main power windings are the 22-0-22 / 27-0-27 sets.
You should be able to pull full VA rating from those windings.

Since 27-0-27 may exceed the 84V limit of the chip during no-load and mains fluctuations, I would go with the 22-0-22 unit.
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Old 30th December 2011, 06:07 PM   #3
AndrewT is offline AndrewT  Scotland
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Do you have 220VA or 220Vac transformers?

I cannot see how HK could afford to use a 220VA transformer to power a 20W+20W amplifier.

I would expect the 22-0-22 transformer to be rated from 30VA to 80VA.
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Old 30th December 2011, 06:27 PM   #4
GloBug is offline GloBug  Canada
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Thanks for your response guys, don't mind the double post.

I suspected the wiring size played a role, but yes they are the biggest gauge of wire.

These are 120v transformers.

I got the VA ratings from an online calculator that converts from watts. I tried the old fashioned way too.

The HK330i stated 155w 120volt on the back. 1.5KG
The SA-222 stated 185w 120volt on the back. 1.63KG

Someone had mention around 20VA per pound as a conservative guess which would fall inline with Andrews numbers.

I guess I did the wrong math? That's a pretty big difference.
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Old 30th December 2011, 07:00 PM   #5
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Quote:
Originally Posted by AndrewT View Post
Do you have 220VA or 220Vac transformers?

I cannot see how HK could afford to use a 220VA transformer to power a 20W+20W amplifier.

I would expect the 22-0-22 transformer to be rated from 30VA to 80VA.

It was sold as a 'high current' amp (whatever that means), and it is a 4-channel unit.

So maybe 80-160 VA?


EDIT: Correction, 2 channels with 2-way speaker selection, not 4-channel.

Last edited by discrete; 30th December 2011 at 07:14 PM.
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Old 30th December 2011, 07:32 PM   #6
GloBug is offline GloBug  Canada
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When I compare the dimensions to some transformers on Digikey it seems that it might be a lower VA then I am hoping.

This is very confusing because the other research I did shows a transformers VA rating to be a HIGHER number then the wattage ratting, there are even formulas to figure it out.

Not I'm not disagreeing, just wondering what gives? Why such a discrepancy?

Anyways here is a picture of them.
The one on the left is the Harmon @1.5kg. 155w
The one on the right is the Technics @1.6kg 185w
Attached Images
File Type: jpg DSC06665.JPG (155.7 KB, 198 views)
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Old 30th December 2011, 09:00 PM   #7
GloBug is offline GloBug  Canada
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"just wondering what gives? Why such a discrepancy?"

Oh well, no one could explain that last time I asked, very odd.

Nonetheless, should I walk from these PT's or give them a try? It would be nice to use some of these parts.
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Old 30th December 2011, 09:50 PM   #8
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From the physical size I think Andrew is correct.
My estimate is 50 VA.

Antek is good value for money if you want to buy new, e.g.:

Antek - AS-2222

Shipping to Canada is $28, but you may fit 2 to 3 transformers in one 20 lb shipment.
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Old 30th December 2011, 10:19 PM   #9
GloBug is offline GloBug  Canada
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Look's like I may have to go that route. Not a big deal, was just trying to make my "heap" a little smaller!
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Old 30th December 2011, 10:49 PM   #10
AndrewT is offline AndrewT  Scotland
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the voltage and size would suit opamp or discrete opamp circuits.

22Vac could be regulated down to 20Vdc and that suits some opamps.

Keep them for preamp and active crossover and small PSU etc.
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