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 How to measure output power
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 2nd June 2012, 03:23 PM #11 diyAudio Member     Join Date: Sep 2003 Location: Midland, Michigan I have an idea for you. Please send me a private message and include your email address so that I can reply. __________________ Frank
 2nd June 2012, 05:27 PM #12 diyAudio Member     Join Date: Nov 2005 Location: San Antonio Connect your voltmeter across the speaker. Play your test audio. Measure the RMS volts on the speaker. Compute the power using P = E^2/R where R is the speaker impedance. A 400Hz tone would be much better to use. If you can't obtain one, maybe an audio editor like Goldwave could be used to "stretch" your 1kHz sinewave to 400Hz. This isn't precision, but should give a useful rough measurement. __________________ It is error only, and not truth, that shrinks from enquiry. - Thomas Paine
 2nd June 2012, 06:03 PM #13 diyAudio Member     Join Date: Jun 2011 Location: North of Boston @ Frank Thanks for the mp3 test tone. I'm going to use that and some others at various frequencies and use sofaspud's method to get a rough approximation. @sofaspud thanks for reminding me of the math. I always forget the E^2/R versions of the power formula.
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Join Date: Aug 2009
Location: Brazil
Quote:
 Originally Posted by Original Burnedfingers How do you figure this? Bias x B+ x Output Transformer=output Watts
The Bias & B+ are informed by the amp datasheet or service manual.

Example in a big Triode amp:
0.80mA x 900V x Transformer= output Watts
The transformer value is a Primary/Secondary ratio I unknow.

Some expert would help??
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 5th June 2012, 10:37 AM #15 diyAudio Member   Join Date: May 2007 For a perfect single-ended output stage, maximum output power is half of (quiescent current x supply voltage). For a real SE stage it is a bit less than this. The transformer ratio is not relevant to this calculation, as all it does is transform impedance.
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Join Date: Feb 2009
Location: Calgary
Quote:
 Originally Posted by EddieRich Thank you for the answers. Unfortunately, I don't have distortion analyzers, or even a scope. I was hoping it could be done with a multimeter. I'm not looking for exact, precise numbers. I just to want to see if I'm in the ballpark.
You obviously have a computer... That computer has a sound card. It may not be the best quality equipment, but it's generally good enough. Use that along with the free or demo spectrum analyzer and o'scope software around and you have a distortion analyzer and an oscilloscope.

Be careful with the sound card, though. They tend to have max input voltages on the order of 5 V, so you may have to build a 10:1 voltage divider so you don't fry the sound card when measuring the amp output.

~Tom
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diyAudio Member

Join Date: Aug 2009
Location: Brazil
Quote:
 Originally Posted by DF96 For a perfect single-ended output stage, maximum output power is half of (quiescent current x supply voltage). For a real SE stage it is a bit less than this. The transformer ratio is not relevant to this calculation, as all it does is transform impedance.
Best Regards
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 7th June 2012, 01:32 PM #18 diyAudio Member   Join Date: Nov 2010 Don't use the speaker for the load as you will have no idea as to what impedance it is. Here's the simple answer:- Use an 8.2R, 5W resitor as the load. Get your speaker and add a 330R resitor in series with it. Now put the speaker/330R resistor combination in parallel with the 8.2R resistor. The amp will now see an 8R load. You can then put your meter across the output the amp and wind up the sine wave until you can hear it starting to distort (it will be fairly quiet), back off the level so it no longer distorts and read the meter (should be RMS volts). You now have the RMS Volts and the correct load impedance so you can work out the power. Regards Henry
 7th June 2012, 02:10 PM #19 diyAudio Member     Join Date: Nov 2005 Location: San Antonio I believe the speaker impedance is 8 ohms. The question is, at what frequency? I'm also assuming 400Hz is the frequency used by the speaker manufacturer. A suitable multimeter can give a "ballpark" distortion indication: Take a peak reading of the sine wave. Compare that with an RMS reading of the same sine wave. The latter should be 0.707 of the former. If it isn't, that's a good sign that the signal is clipping. __________________ It is error only, and not truth, that shrinks from enquiry. - Thomas Paine

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