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Comments
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.
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.
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein
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.
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
Rift Amplification
Brackley, Northamptonshire
www.riftamps.co.uk
Rift Amplification
Brackley, Northamptonshire
www.riftamps.co.uk