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Old 26th December 2004, 08:12 AM   #1
Semir is offline Semir  Hungary
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Default Transformer problem

Hi everyone!

My first, but perhaps most easily solvable problem is that: I dont have one )

The second gives the base of the first: how should I know what kind to buy?

I dont know anything about transf. (except that I can calculate every tits-and-bits of them... That was thaught, but I cannot do anything with that in practice )
How should I know what kind of transformer should be bought? How many VA-s needed, and these things, for a 2x25w of a class a amp? And what else should be considered?

The regulation can be done, I know that. (Except: should the trasformer filtered by BIG-big capacitors and R, or it is enough to use a good regulator circ, with a BR bridge?

Hope!

Thanks a lot!
Yours,
Semir


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First, I thought I knew something. But the more I read and ask, the duller I feel myself )
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Old 28th December 2004, 07:42 PM   #2
AndrewT is offline AndrewT  Scotland
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Hi,
Nelson Pass recommends a VA at 7.5 times the the classA output power. Others suggest 6 times These are for push pull topology, it might be different for single ended.
I presume you know your voltage requirement.
For capacitor filtering try WCR = 100 where W =315 or 370 (50/60Hz), C= required cap, R = load impedance. You could vary this by half to double.
Have you considered a capacitance multiplier?
regards Andrew T.
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Old 28th December 2004, 07:47 PM   #3
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Try this:

Everything you wanted to know about PSU


http://www.tnt-audio.com/clinica/ssps1_e.html

http://zero-distortion.com/start.htm


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Old 3rd January 2005, 12:36 AM   #4
PRR is offline PRR  United States
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> How many VA-s needed, and these things, for a 2x25w of a class a amp?

2X25W is 50 Watts. Perfect Class-A operation needs DC supply power twice the output power. The AC VA rating of a transformer must be almost twice the DC power needed. So you are looking at 200VA, 4X the audio power. In fact, few Class-A amps reach 50% efficiency, trannies run HOT at rated VA, and transformer price does not rise as fast as VA, and in fancy-audio we should not skimp on the power transformer. Pass's reported "7.5X rule" seems a little big but should be very happy. So 300VA-400VA.

> The regulation can be done, I know that. (Except: should the trasformer filtered by BIG-big capacitors and R, or it is enough to use a good regulator circ, with a BR bridge?

Linear regulation, allowing for reasonable utility variation and losses, will increase power demand (and heatsinking!) about 50%, a LOT. And a power output stage is very much like a regulator, only with audio as a reference. And big caps are necessary for bass power (at least in Class B) and not all that expensive. Therefore we almost never regulate audio power amp main supply power.
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