Amplifier power supply voltages
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 18th December 2010, 03:15 PM #1 audio_tony   diyAudio Member   Join Date: Aug 2004 Location: UK Amplifier power supply voltages 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. __________________ Check out GraphTool and English help for WavGene/Wavespectra and Vintage Technics audio.
 18th December 2010, 03:30 PM #2 Bigun   diyAudio Member     Join Date: Jan 2009 Location: Waterloo, ON or Herefordshire UK 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. __________________ "The test of the machine is the satisfaction it gives you. There isn't any other test. If the machine produces tranquility it's right. If it disturbs you it's wrong until either the machine or your mind is changed." Robert M Pirsig.
Arnulf
diyAudio Member

Join Date: Feb 2009
Quote:
 Originally Posted by Bigun 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
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 18th December 2010, 03:49 PM #4 Bigun   diyAudio Member     Join Date: Jan 2009 Location: Waterloo, ON or Herefordshire UK thanks Arnulf ! so....... it's only 32/2 = 17W rms. __________________ "The test of the machine is the satisfaction it gives you. There isn't any other test. If the machine produces tranquility it's right. If it disturbs you it's wrong until either the machine or your mind is changed." Robert M Pirsig.
 18th December 2010, 03:58 PM #5 audio_tony   diyAudio Member   Join Date: Aug 2004 Location: UK 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? __________________ Check out GraphTool and English help for WavGene/Wavespectra and Vintage Technics audio.
audio_tony
diyAudio Member

Join Date: Aug 2004
Location: UK
Quote:
 Originally Posted by Bigun 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.

 18th December 2010, 04:17 PM #7 AndrewT   R.I.P.   Join Date: Jul 2004 Location: Scottish Borders 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. __________________ regards Andrew T.
 18th December 2010, 04:23 PM #8 bosedtobosed   diyAudio Member   Join Date: May 2008 i think it would be less, the voltage drops on load
 18th December 2010, 04:29 PM #9 Rundmaus   diyAudio Member     Join Date: Aug 2005 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
 18th December 2010, 04:38 PM #10 audio_tony   diyAudio Member   Join Date: Aug 2004 Location: UK 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? __________________ Check out GraphTool and English help for WavGene/Wavespectra and Vintage Technics audio.

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