Using RMAA v6 for power amplifier distortion tests

[Henry8]

I set up RMAA for mono mode, and looped the soundcard output to its input. I got figures of THD=0.004%, IMD=0.018%

If i now test an amp etc, can i just subtract the above figures to give me the amps true values?

 

[Voegelchen]

No ! This numbers told you only the resolution of your hardware. You cannot get better, even a straight wire (as you have already done) get's no better result. This distortion of your soundcard covers the distortion from your DUT and you won't be able to know (or even extrapolate) how much your DUT is below this values......

 

[wakibaki]

Yes. But...

If you measure a THD of 0.4% (2 orders of magnitude above the floor) then subtracting 0.004% doesn't make a significant difference.

If you measure a THD of 0.04% (1 order of magnitude above the floor) then subtracting 0.004% stilll doesn't make a very significant difference.

If you measure a THD of 0.008% (twice the floor) then subtracting 0.004% makes a significant difference, but you should have less confidence in the result. You are approaching the measurement limit of your equipment. You can swap backwards and forwards between the DUT and the plain wire, and if the results are consistent, this will increase your confidence in the result, but your confidence would be improved with new equipment with a THD floor of 0.0004%, n'est ce pas?

If you measure a THD of 0.004% (at the floor) then you can only say that the DUTs performance exceeds that of the measuring equipment..

w

 

[macboy]

The interesting thing about distortion is that each harmonic could be at any phase wrt the fundamental. The phase of each distortion product could be at any phase in the soundcard and in the DUT. You can't be sure whether they will constructively add or destructively add (subtract). So if you measure 0.009% on the DUT and 0.004% with soundcard only, the best you can say is that the THD of the DUT is 0.009% +/- 0.004%. It could be as low as 0.005% if the soundcard distortion is completely in-phase with the DUT distortion, so they add completely. Or the DUT THD could be as high as 0.013%, with the soundcard distortion completely out of phase so that it actually cancels out some of it during the measurement.

 

[wakibaki]

Good point macboy...

w

 

[Henry8]

So if you measure 0.009% on the DUT and 0.004% with soundcard only, the best you can say is that the THD of the DUT is 0.009% +/- 0.004%.

If this is the case, for most people this will be ok to tweak their designs etc.

Also i am now using RMAA v5 (use left channel in and out), as this is giving more consistant results than v6 in my setup, and i suggest allowing the PC(soundcard) to warmup by 15mins and also the DUT.

 

[Voegelchen]

To get an better oscillator, see here:
http://www.ac-vogel.de/AudioMSys/APN 002-Damping oscillator distortion.pdf
To get a better Input search for (analog) "Notch" filter, also search Google for
"differential bridged notch".
If intersted, I can tell you how to tune it (by formula).

 

[EUVL]

Thanks for the link to your article.

I wish to draw your attention that Bob Cordell has published in 1981 already his design of a THD Analyser where he also used a State Variable Filter.

http://www.cordellaudio.com/papers/thd_analyzer.pdf

Gruß,
Patrick

 

[Voegelchen]

But Bob has never the chance to use LM4562/LMExxx Opamps ! Nor has he measured the performance possible.....

 

[Henry8]

Also i am now using RMAA v5 (use left channel in and out), as this is giving more consistant results than v6 in my setup

Sorry, i change my view. In my setup neither v5 or v6 gives consistant results....i was only testing for THD at the time, and were consistant to +/- 0.001%, but when i also started to do IMD onetime i got 0.154 and redoing the test got 0.077.

As a last test i changed values in my amp which should alter THD but i kept measuring the same THD???

RMAA displays 4 test results at a time, maybe this is not showing the true results but showing the last one???

 

[Pano]

If you are using RMAA then you have the hardware and are clever enough to use a better approach. Why bother with %THD & %IMD when you can actually see the harmonics and noise?

A program like ARTA/STEPS or Audio Analyzer will allow yo to do FFT of your signal and really see what's going on. Then you'll have a much better idea of the noise floor and what the distortion really looks like.

Try it, you'll like it!