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Fuling 2nd October 2005 11:07 PM

Oh happy day!
I have this big pile of PSU parts and heatsinks that would be nice for an Aleph project, the problem is that the mains transformer is suitable for an A5 and the heatsinks for a Mini-A or at best A3.

BUT: minutes ago I found a big 1,5kVA autoformer with 230V primary (is it still called primary in an autoformer??) and secondary taps at 150, 170 and 190V.
This allows me to get roughly 15, 18 or 20V out of my 2x24V 750VA mains toroid.
Sweet, now I can build this thing and then choose the highest voltage that the heatsinks can handle!

Maybe I even can raise the bias a bit if I use the lowest voltage...


Nelson Pass 2nd October 2005 11:39 PM

I usually choose bias over voltage.


Fuling 3rd October 2005 05:50 AM


I usually choose bias over voltage.
Thats what I thought!;)

jleaman 3rd October 2005 05:58 AM

Try the mini a's.. I bet yu wont need much more power of them.. if you do they are stock 10 watts @ 15V rails if you need more you up the rails to 20V like mine and get more like 15Watts.. BUt make sure you got heat sinks.. They do get hot.. :hot:

Fuling 3rd October 2005 06:54 AM

Hehe, I know about the heat issue!
Ive built a couple of Mini-As before, this time Im aiming a little higher.
I will probably use 4x IRFP260 per channel, simply because I have some ~40 pieces to to get matched pairs from.
IRFP260 wouldnt have been my first choice if I were about to buy mosfets, but Ive used them in Mini-As before and they sound fine.
This is after all a bit of a junkbox project:)

grataku 3rd October 2005 04:37 PM

The best you can do is probably bring down the voltage to 190 on the primary, the 750 will turn in to a 500VA, below that the flux on the nucleus will be too low to provide the kind of current you need without excessive sag of the secondary voltage.

Fuling 4th October 2005 02:49 PM


Thanks, I didnt think of that!

jeapel 7th October 2005 11:55 AM


I m not sure it s more to start a discussion but my
first impression is :

750VA => volt x amp

if you change the primary volt for -30%

maybe you get now a 750va -30% = just a 525va in your hands

but i like option with autotransformer and choose output volt

maybe the solution would be oversize to 1000va the toroid

for aleph 3 the primary ac amp is 2 A for me a

24ct vac 10 A (250 VA) wired like a autotransformer

would be enough power to step down main 120vac (Canada)

to option 1=120
option 2=108
option 3=96vac


Nelson Pass 7th October 2005 06:36 PM

Using different taps on the transformers does not necessarily
change the VA all that much. A lot of that rating comes down
to core size.


jeapel 7th October 2005 09:20 PM

hi Mr. Pass

just for the fun here is a extreme example :-)

a transfo 120vac to 24vac 10 A 240 VA

now with 60 vac on primary i get 12 vac

if the VA rating is almost constant to 240 VA

now i get 12 vac @ 20 A (240 VA) ????

there is something wrong there maybe more



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