Measure Amp Power

From ride500–(at)–ide.ri.net Sun Feb 14 22:55:43 CST 1999
From: ride500–(at)–ide.ri.net
Newsgroups: alt.guitar.amps
Subject: Re: measuring output power with a DVM
Date: Sun, 14 Feb 1999 23:40:55 GMT
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In article <36C62F81.A3F986A--(at)--oxinternet.net>,
gallar–(at)–oxinternet.net wrote:
> If I measure the output voltage on the speaker of my “30 watt” amp, how
> would I go about converting that to a power measurement.
> The speaker is 8ohms and I will assume no internal loss from the
> speaker.
> Can I use P=V^2/R ? Or do I need to use AC formulas?

P=V^2/R is always true, AC or DC.

BUT

If you want to get a measure of RMS (root means square) power, which is more
meaningful than PEAK power, you need to do some fiddling around. There’s two
ways to do this.

If you have a voltmeter that measures AC volts in RMS, you’re in luck. If
it’s a TRUE RMS meter, you’re even more lucky. If it is just a peak ACV
measurement you’re a bit less lucky, but not completely out of luck.

IF THE DMM MEASURES ACV IN TRUE RMS:

Don’t use a speaker if you want a meaningful number to come out. If you use a
speaker the voltages will vary widely with frequency due to the reactcance of
the speakers themselves.

Simply plug into a dummy load, crank up the signal input, measure the ACV on
the terminals. Use P=V^2/R. That’s your power output in Watts RMS.

IF THE DMM MEASURES ACV IN RMS, BUT NOT TRUE RMS:

This means that the meter is not actually measuring the “areas under the
votlage curves” as in a true RMS meter, but is making an approximation. As a
result, your numbers will be off UNLESS YOU HAVE A SINE WAVE INPUT. If
you’ve got a signal generator, just use the sine wave output wave shape, and
follow the directions given above. If you use a guitar signal, your measured
output power will be a bit higher than the actual RMS power capability.

IF THE DMM MEASURES ACV IN PEAK:

Here it gets a TINY bit mathematic. There’s two way to do it. One’s easier
than the other.

Do the same thing as above. Use the SINE WAVE input, if you have the ability
to do so. (NOTE you can approximate a sine wave by using the neck pickup,
and plucking the string precisely over the 12th fret (which is the midpoint
between bridge and nut). This will minimize harmonics. Of course, if this
is a guitar amp it would also behoove you to crank the master volume so that
the minimu amount of preamp distortion is present. If you have a direct
input to the power amp (perhaps called power amp in) use this.) Measure the
ACV on the meter.

Now you’ve got a choice. You can divide this ACV by 1.414 (the square root of
two) to get VRMS, and finish the equation as above.

OR

You can simply plug in the peak ACV measurement you found, which results in a
calculation of PEAK power. RMS power is simply 1/2 of PEAK power. If you
manipulate the equations a bit you can see it clearly makes sense.

Hope this helped…

Ken Gilbert
Tube Guitar Amp Design/Repair Technician
The Guitarist’s Choice http://www.tgcguitar.com
http://www.geocities.com/SunsetStrip/Garage/5701

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