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Old 11th September 2006, 08:33 PM   #1
prorms is offline prorms  United States
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Default how do you measure wattage of a poweramp????

iam not sure if you measure the voltage and amps going into the power amp form power supply or do you measure the voltage and amp at the speaker line out?
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Old 11th September 2006, 08:49 PM   #2
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Yes, Measure the voltage developed across a known value power resistor.. So make sure your AC mains doesn't sag through the use of a VariAc. Put in 1 kHz (for example) increase the output though a volume control or osc. level control. Monitor the waveform appearing across the load. Adjust output for just under clipping. Check to see that your VariAc is delivering the correct mains voltage. Measure the RMS voltage appearing across the load. P=I2R, E/R =I. or E2/R.
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Old 11th September 2006, 08:58 PM   #3
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Hi seanmacss
I'm not sure if I'm understanding your question.
For a power output measure on audio amp some laboratory devices are required:

one valid function generator,
one 2-4-6-8-Ohms load
one distortiometer
one audio true rms voltmeter with 20 Hz-20 kHz frequency range
one oscilloscope.

This measure you can made also with the soundcard of your computer,an attenuator ,and a 4 and\or 8 Ohm resistor,but this is dangerous for the soundcard.

The load must to be connected to the speaker output.
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Old 12th September 2006, 12:56 AM   #4
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make yourself a calorimeter --
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Old 12th September 2006, 01:19 AM   #5
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Quote:
one 2-4-6-8-Ohms load
Resistive only???
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Old 12th September 2006, 01:25 AM   #6
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Leolabs asked:
Quote:
Resistive only???
In my opinion, Yes. I've always spec'd low inductance or "zero inductance" resistors. It's always a good base-line to use Resitive loads. The voltage and current will be in phase. You'll be comparing apples with apples as it is industry standard to use resistive loads.

Just my opinion.
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Old 12th September 2006, 02:12 AM   #7
AJT is offline AJT  Philippines
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Default Re: how do you measure wattage of a poweramp????

Quote:
Originally posted by seanmacss
iam not sure if you measure the voltage and amps going into the power amp form power supply or do you measure the voltage and amp at the speaker line out?
you need a signal source, a scope or thd meter, a set of dummy loads to test your amp. normally it will be the voltage at the output going into the speakers, but since your speakers can get damaged with just a 1watt input, better use a dummy load, a resistor 8ohms and high wattage, say 100watt ought to do it.

measure the output voltage wthen distortion is about less than 0.1% or just when the amps begin to clip isf you have a scope.

be warned though that doing these tests may fry your amp, so proceed with extreme care.
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Old 12th September 2006, 02:18 AM   #8
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As I said, get yourself a calorimeter -- fwiw the best true RMS meter in the last millenium was the HP3403C which used a thermopile to very acurately measure voltage -- if you have a thermos, some distilled water, a thermometer and a couple of 25 ohm 50 watt resistors just measure the temperature delta over a few seconds -- you can do the math.
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Old 12th September 2006, 05:36 AM   #9
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Jackinnj
Now that sounds like fun.

Is this something you have personally done? I would be most interested in the results.

Right now im stuck here in Kuwait, but when I get home this could come in handy.

For the moment I use the 'method one' with resistors, scope and fancy DMM (HP3456A)

A thermal transfer method might be more fun, and it get's rid of any non linear issues in the wave.

One of the things I fished out of the trash here was a Racal Dana 5002 line level meter. It's a very vancy ACVM with a lot of computing power including measureing audio power.

If I can get that home, it will be a nice thing for the lab.

Thanks in advance.
Jack Crow in Kuwait
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Old 12th September 2006, 07:19 PM   #10
prorms is offline prorms  United States
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Default measure the wattage

use a 2ohm 4ohm 8 ohm 16 ohm resistor as a speaker .
1khz audio singal generator in .multimeter measure the voltage
across the resistor that will give you the rms of the amp
do the same for amperage
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