Power transformer size -- step down to 100v?

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noviygera

noviygera

Member
Joined 2009
Hello.

I need advice on the VA rating for the step down 120v to 100v transformer.

Will use a Japanese region Sony TA-N7B that takes 100v. My thought is to get a balanced transformer (50v secondaries). Rating on the back panel is 215 watts.

What VA for the step down?
500VA is enough?

thanks,
Herman
 
Ketje

Ketje

Member
Joined 2012
noviygera said:
Hello.

I need advice on the VA rating for the step down 120v to 100v transformer.

Will use a Japanese region Sony TA-N7B that takes 100v. My thought is to get a balanced transformer (50v secondaries). Rating on the back panel is 215 watts.

What VA for the step down?
500VA is enough?

thanks,
Herman
Click to expand...
You only need an 120V to 20V transformer with 215W/100V=2,15A capability.That is 20Vx2,15A=43VA.With a 50VA 20V transformer opposite phase in series with the 120V you get 100V 2,5A to use.
Mona
 
PRR

PRR

Member
Joined 2003
Paid Member
noviygera said:
..Rating on the back panel is 215 watts....
Click to expand...

Name-plate ratings are often "nominal", reflecting local power assumptions, especially on class-B amplifiers.

Look at the service manual. Partial attached.

This is a 100W+100W amp. In common class B we would estimate >320W from the wall. This happens to be a lower-efficiency design and I estimate 525W at FULL rated output (test-bench).

In UN-clipped speech/music service the average (heat in transformer) will be much less. Maybe 160VA average loading.

Now look at the several Region labels. AEP and UK come close to my full-power estimate. Canada is lower, and US is lower yet, even though it is the SAME output power, just different primary winding. Apparently AEP/UK want the "most" it can suck, Can and US want a "typical", and defined differently in each country.

If you are going to Test-Bench this, you want much more than 500VA to be sure your transformer does not limit the results much.

If you are going to play music at home, the step-up won't see big power long enough to get warm. I would be comfortable with 250VA. (Being _me_ I would have a meter on the "100V" side and see if it dipped in the loud parts; also lay a hand on the step-up every so often to check for heat.; but I'm just that kind of nerd.)

It seems to me that for not-LOUD use, a 120V:24V 40VA transformer as used in furnace and A/C applications could be re-wired as AutoTransformer. However this is dangerous if not done exactly right.

I see no reason for "balanced power". Certainly it wasn't balanced in Japan, and it was good enough for the Japanese home market.
 

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TonyTecson

TonyTecson

Member
Joined 2003
noviygera said:
Hello.

I need advice on the VA rating for the step down 120v to 100v transformer.

Will use a Japanese region Sony TA-N7B that takes 100v. My thought is to get a balanced transformer (50v secondaries). Rating on the back panel is 215 watts.

What VA for the step down?
500VA is enough?

thanks,
Herman
Click to expand...

why even use a step down traffo? if i were you i will convert that unit to 120 volts....most japanese gears have traffos where in the bobbins for the primary and the secondary are separable, then it is just a matter of adding a few turns to the primary...
 
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