Amplifier power supply voltages

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Is there a 'rule of thumb' chart anywhere, that states how much power should be obtained for a given PSU? (my math is very poor)

I have 2 transformers 18-0-18v AC @ 2.22A per winding.

I'm currently building an amp - and was wondering what sort of output power these would yield into 8 ohms?

After rectification, of course they'll be around 25-0-25v DC.

So I'm guessing with a typical bipolar output stage, I should see about 30W with this PSU?

It's this amp that I'm building.

Is this correct?

I'll make my guess....

Seems to me that a peak current of around 2A through 8 Ohms would yield (P=I*I/R) 32W peak. Power is usually quoted as rms so you'd normally state this as a (32/1.414) 22W amplifier.

I guess if you had big capacitors in the psu you might readily achieve more than 2A peaks (theoretically your 25V dc could drive 3A into 8 Ohms) but that wouldn't be a sensible way to view the power output of the amplifier.

Depending on your speakers and room size I would imagine that a 22W rms amplifier would be more than plenty.

I think my transformers are actually 4.4A in total - it states on the transformer that they're 2.22 amps per winding.

I also have 2x transfomers of 25-0-25v AC 1.7A per winding.

I can't make up my mind which will better!

It would be easier to accomodate the lower voltage transformers because I already have some 35V smoothing caps.

I just realised my speakers are 4 ohm, so I should see almost double that.

Pics of transformers - any comments?



Keep in mind that transformer secondary current ratings are AC. After (solid state) rectification and C filtering the DC voltage will be 1.4x higher. The DC current capability without overloading the transformer will decrease accordingly, energy does not come out of nowhere...

How about this transformer?
TRANSFORMER, 100VA 2X 12V; Secondary Voltages:0-12, 0-12; AC Power:100VA; Bolt Hole Diameter:6mm; Current Rating:4.17A; External Diameter:91mm; External Length / Height:47mm; Max Output 2 Voltage:12V; Output Voltage Fixed:12V; Primary Voltages:0-120, 0-120; Regulation:10%; Secondary Power:50VA; Supply Voltage VAC:120V; Temperature Rise:50°C;

12-0-12V AC 4.17A

Surely this wouldn't yield 100W though?

Would these be any good? (I have two of these as well!)

Or do I just stick with the 25-0-25 ones?
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