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Old 18th July 2007, 06:33 PM   #1
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Default How to measure Wattage of an Amp


i have an old 4 channel amp, i used only 2 channels till now
now i have decided to dedicate other 2 channels to a sub

can any 1 tell me a simple method to calulate the Rms wattage of my system. i just need an aproximation, i have 2 multimeters, the amp n few 4 ohm speakers a PC with winISD freq. generator with me

any great Ideas
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Old 18th July 2007, 07:53 PM   #2
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Send a message via Yahoo to darw82
Drive you amp with pure sine wave. If you don't have frequency generator, just use the sound card.
You can download the software at

Load the amp with 4 or 8 Ohm high power resistor, or nichrome wire heater

Set the multimeter to AC, measure the output of your amp (with power resistor connected).

For example, if measured voltage is 15 VAC, (loaded with 4 ohm resistor)
then RMS power @4 ohm is 56.25 Watts

RMS Power = (V*V) / R_Load
= (15*15) / 4
= 56.25
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Old 18th July 2007, 08:22 PM   #3
anatech is offline anatech  Canada
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Default One thing to watch out for!

Hi darw82,
You must use approximately 50 ~120 Hz as a test signal. Unless you are using a good Fluke or new Agilent hand held meter, higher frequencies will be inaccurate.

Most digital voltmeters have very poor high frequency response. 1KHz is already out of range for them.

"Just because you can, doesn't mean you should" my Wife
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Old 18th July 2007, 08:38 PM   #4
AndrewT is offline AndrewT  Scotland
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Location: Scottish Borders
without a scope to check when the onset of clipping is ocurring, you will need an attenuated output fed to a speaker so you can try to monitor the distortion due to the clipping. Not accurate but it will give a clue.

Alternatively, measure the PSU supply voltage and get an estimate from that.
regards Andrew T.
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Old 19th July 2007, 12:09 PM   #5
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Default thanks a lot

hi guys thats a lot
i think method is nice enough, i'll try to see if there is any distortion using a Windows oscilloscope in my PC and my MIC

will do, thanks a lot all of you, nice and simple idea
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