Measuring power output of amps

Rocker

I can follow the use of a voltmeter and a grip type current measuring device to calculate output power but how do you measure the distortion value?  For example, 40 watts at 1% distortion or 50 watts at 5% distortion. Is there a standard input signal for power measurement? Always wondered about this, thanks in advance for all responses.

jpfamps

You need a distortion meter.

A modern digital 'scope with FFT function can also be used.

There is no standard input signal level - it needs to be enough to drive the power stage into clipping - although 0 dBu is often used for PA amps.

1 kHz sine wave is the usual waveform employed.

ICBM

It's actually not that straightforward to measure without a spectrum analyser or a specialist distortion meter.

Using an oscilloscope and looking for the onset of clipping with a 1KHz sinewave test signal at a high enough resolution to see clearly when it's just starting is the easiest way to give you a 'clean power' measurement which won't be too far away from the 1% THD value.

ecc83

http://www.epanorama.net/sff/Test_equipment/Meters/Spot Frequency Distortion Meter.pdf

You could knock that up very easily with an NE5532  and you don't need the precision for guitar amps!

BTW do not rely on digital voltmeters for measuring wide band audio, even quite expensive models can read 6dB or more low at 2kHz ref 50/60Hz then fall off a cliff.

I think 1% is a bit 'hi fi' for guitar amps? I have never had the opportunity to test a rake of different amps but I doubt many if any would reach their published spec at 1% thd?

Often forgotten in critical power testing? The load can shift value at high power and mains voltage needs watching, ideally set with a variac  and meter.  I do recall many ago, as huge power amps arrived, DC300A,  BGW etc, getting stable loads was an issue and the main would droop as power reached 500W and more.

Dave.

RiftAmps

Standard input is 1kHz sine wave @ 150mV RMS.

I measure all my amplifiers at 2% THD, but some guys prefer 5% or even 10%. As stated above, you’ll need a distortion meter

DJH83004

I use 1kHz sinewave and 120mV RMS input, digital scope and a harmonic distortion meter which is actually an old Heathkit model which my dad built!, no good for modern HiFi amps but ideal for guitar amps down to 1% THD. 

RiftAmps

DJH83004 said:

I use 1kHz sinewave and 120mV RMS input, digital scope and a harmonic distortion meter which is actually an old Heathkit model which my dad built!, no good for modern HiFi amps but ideal for guitar amps down to 1% THD. 

Heathkit HD-1? I have that one and it’s excellent