### 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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