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-   -   How accurate is RMAA distortion measurement? (http://www.diyaudio.com/forums/tubes-valves/33366-how-accurate-rmaa-distortion-measurement.html)

analog_sa 2nd May 2004 04:10 PM

How accurate is RMAA distortion measurement?
 
I wonder if anyone has experience using RMAA to test amps. Last night i tested my latest IT PP creation and RMAA reported unbelievably low THD (around 0.02%) and absolutely disastrous IMD of around 5%.

I am using the loop-back mode and passing the amp output through a resitor attenuator to compensate for gain. Measurements were taken at around -2db which sounded moderately loud if connected to a speaker.

Similar measurement on a GC type amp yielded imd bellow the noise of my laptop soundcard.

Does my tube amp really suck?

It is very simple. A 5687 (Ea=180v, Ia=11mA) feeds a Lundahl LL1660 18mA connected as SE-PP. The output of the transformer feeds a pair of Sovtek 2A3 with cathode bias running at around 270v A-C and 58mA. And there is an input stage for some extra voltage gain.

This is my first attempt to use a transformer as a phase splitter and although my expectations were low the sound really surprised me with dynamics and speed - so much better than any active splitter i've tried. As a first iteration it's not perfect - it may sound a bit cluttered in complex passages and that's why i'm curious about IMD. Not to mention that the 1660 rings like a bell on square waves.

My longer-term (next week?)plan is to complete the front end following the Olson/Thorsten concept of splitting the phases from the input.

Any ideas re the dismal IMD score?

Frank Berry 2nd May 2004 04:23 PM

Does the amplifier sound good to you?
If your answer is yes, forget the measurements.
Remember ..... It's all about sound.
Many times the measurements don't agree with what we hear.

jan.didden 2nd May 2004 05:44 PM

..what is RMAA???

Jan Didden

analog_sa 2nd May 2004 05:44 PM

Frank

I certainly agree. Still, there is this nagging suspicion that IMD of 5% cannot be good. Nor does it make any sense for an amp to develop a disproportionate amount of IMD compared to harmonic distortion. If the thd was 5% i would have no problem with that and would expect a certain amount of imd as well.

Most likely there is a problem with my measurement setup.

Does the amp sound good? Yes, it sounds spectacular in many respects but not quite 'relaxed'. I guess it's difficult to have 'speed/dynamics' and 'relaxed' at the same time :) And these square waves can give anyone nighmares.

peranders 2nd May 2004 06:52 PM

Quote:

Originally posted by janneman
..what is RMAA???

Jan Didden

RightMark Audio Analyzer

Russian freeware, excellent!

http://audio.rightmark.org/

dimitri 2nd May 2004 06:56 PM

Jan Didden wrote ..what is RMAA???

RightMark Audio Analyzer
http://audio.rightmark.org/
the russian analog of Spectra(+)

jan.didden 2nd May 2004 07:13 PM

Thanks P-A, Dimitri, I was unaware of RMAA

Jan Didden

peranders 2nd May 2004 07:32 PM

Jan, if you don't have tested RMAA, do so, because it's a pleasant experience, easy to use , nice looking reports etc. Also the support is really good! And the most peculiar thing is that these russian dudes don't want any money for it! They have also a support forum.

ChocoHolic 2nd May 2004 09:00 PM

Poor Sound Card???
 
WOW!!!
This soft ware seems to be exactly what I need!!!

BUT??!!! What's this? For first tests, I measured a simple cable
which should have acceptable low influence on the signal.
==> ...did this in order to check the performance of my sound card,
some older sound blaster....
The results do not differ much, no matter
if I choose 44kHz/16Bit or 192kHz/32Bit(float)...
They are much below what one could expect from 44kHz/16Bit
anyway!
The frequency response drops below -3db already slightly above
6kHz!!!!!!!!!!!!!!
Noise and dynamic around 85dB.
THD around 0.5 percent.
IMS around 0.3 percent.
Crosstalk around 82dB...

May be that's the reason, why I never felt lucky
with any music from this machine :whazzat:
...and always stated: For music you can kick that stuff!!!

1. Do you think my sound card can really be that poor?!
2. If yes, which sound card is fine?

Cheers
Markus

ChocoHolic 2nd May 2004 09:03 PM

....will check this with my lap top soon....


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