# Amplifier power supply voltages

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#### audio_tony

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?

TIA.

#### Bigun

Paid Member
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.

#### Arnulf

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.

1: P = U * I, U = I * R, therefore P = I * R * I = I^2 * R

2: P(RMS) = U(RMS) * I(RMS) = U(peak)/sqrt(2) * I(peak)/sqrt(2) = P(peak) / 2

#### Bigun

Paid Member
thanks Arnulf !

so....... it's only 32/2 = 17W rms.

#### audio_tony

Thanks.

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?

#### audio_tony

thanks Arnulf !

so....... it's only 32/2 = 17W rms.

Thanks both of you.

So it looks as though the higher voltage is a better option then.

#### AndrewT

Hi,
the 18Vac transformers are 80VA, the 25Vac transformers are 85VA.
These can be used to power amplifiers of 40W to 80W.
The 18Vac would do for a 50W into 4ohm amplifier.
The 25Vac would do for a 60W into 8ohm amplifier.

#### bosedtobosed

i think it would be less, the voltage drops on load

#### Rundmaus

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...

Greetings,
Andreas

#### audio_tony

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?

#### AndrewT

Hi,
a 12Vac transformer is a bit low for a power amplifier.
It will still work, but it will limit the maximum voltage that can be delivered.
It might do as a 2ohm speaker amplifier.

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