Question about Transformer VA

[GloBug]

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.

 

[discrete]

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.

 

[AndrewT]

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.

 

[GloBug]

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.

 

[discrete]

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.

 

[GloBug]

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

 

[GloBug]

"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.

 

[discrete]

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.

 

[GloBug]

Look's like I may have to go that route. Not a big deal, was just trying to make my "heap" a little smaller!

 

[AndrewT]

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.

 

[!]

"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.

Here is the question: Do you feel the amps they came out of would have met your (use) power requirements? For many uses 20W+ per channel is enough. I wouldn't built a big, fancy cased amp with expensive hardware using one but to knock out a usable beater amp on the cheap, the 22-0-22 one would do the job.

 

[gootee]

Maybe you should go find the service manuals for the amps they came from and see exactly how they were used and how much power they were able to provide. They should be online somewhere.

 

[TonyTecson]

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

transformer VA ratings refer to primary capacity in terms to volt amperes, the secondary load is in watts.....a power factor rating of 70% is typical...

not all energy stored in the primary of the traffo can be transferred to the secondary load, there are losses....

 

[JOHN BALI]

Hi Globug
I was have same confusing about transformer VA rating
Just don't believe what is stated, they must be wrong
better answer if you can measure the wire gauge use inside of that PT
or try read this http://www.diyaudio.com/forums/power-supplies/200849-iron-core-transformer-formula.html

 

[CBS240]

If your unsure of VA rating, why don't you measure open circuit voltage, then load the secondary with a power resistor and determine at what current the voltage drops below what you would expect full load voltage to be? IOW, load the secondary until the voltage begins to significantly drop.

 

[GloBug]

I just might do that. (load it down and measure it.)

Thanks for the responses.

I did end up breadboarding the amp with the 22-0-22, I just had to have a listen.

I split the center tap to give me true dual secondaries.

The truth is this transformer seems to have plenty of snap for my application, I am running efficient speakers with it.

I decided I will get the Antek AS-2222(200VA) or AS-3222(300VA) for the final build, that way I will have the juice in case I need it, like for other less efficient speakers, 4ohm operation etc.