Hirata
Hi Bobby,
Have a look at: http://www.diyaudio.com/forums/showthread.php?postid=1383839#post1383839
and: http://www.audiomisc.co.uk/distortion/page1.html
Cheers, Edmond.
abzug said:
Hi Bobby,
Have a look at: http://www.diyaudio.com/forums/showthread.php?postid=1383839#post1383839
and: http://www.audiomisc.co.uk/distortion/page1.html
Cheers, Edmond.
Thanks. But those links don't really explain why such effects won't affect a THD measurement.
Hirata distortion measures something different than THD. You can actually lower THD and INCREASE Hirata distortion. I don't even understand the mechanism completely, BUT I have measured it.
As long as "THD measurement" refers to a steady state measurement in system equilibrium (which is the classical way using analog gear for which you may need several seconds to read the meter value), transient (mainly thermal) distortion mechanisms may get masked. But if you were to measure a small signal spectrum sequence after a power burst with small FFT-lengths (say, less than 0.25 seconds) you would see those transient changes. Same happens when you measure small signal THD vs. a temporary load bias current condition, or with similar transient test methods which test the system in a non-equilibrium state.abzug said:
- Klaus
john curl said:
Hi John,
The Hirata distortion measurement technique has always fascinated me, but I have never implemented it. How did you implement your Hirata distortion measurements?
Thanks,
Bob
This sounds like a technique somewhat related to SYSTEM MEASUREMENT AND IDENTIFICATION USING PSEUDORANDOM FILTERED NOISE AND MUSIC SEQUENCES, Hawksford, M.O.J. JAES, vol. 52, no. 4, pp. 275-296, April 2005
By the way, I'm interested in doing some simulations of possible non-static distortions (mainly because my measurement capabilities are crap). But how to create accurate models of say transistor dependence on thermal modulation? Any SPICE guru here?
thermal modeling is limited in Spice - the device temperature is a fixed parameter and not a "live" simulation variable
some people have attempted "patches" - notably Fairchild for switching MOSFETs where a device dissapation-thermal model drives some dependent sources to give Vth and R_on tempcos that vary with a simulated device temperature
my "hidden distortion" sim showed that it is theroretically possible for "coupled" distortion mechanisms can escape single tone THD measurement
http://www.diyaudio.com/forums/showthread.php?postid=492499#post492499
but i had to re-explain it a few pages later
anyone interested really must get Cerzwinski's mutlitone testing paper
but furter research needs to be done, multitone amplifier measurements could perhaps put a practical limit on "hidden" intermodulation distortion, i wouldn't be suprised if it was limited to as little as 10 dB over the prediction from single tone distortion identification of the principle nonliearities - the GedLee metric is probably not too far wrong in that aspect
some people have attempted "patches" - notably Fairchild for switching MOSFETs where a device dissapation-thermal model drives some dependent sources to give Vth and R_on tempcos that vary with a simulated device temperature
my "hidden distortion" sim showed that it is theroretically possible for "coupled" distortion mechanisms can escape single tone THD measurement
http://www.diyaudio.com/forums/showthread.php?postid=492499#post492499
but i had to re-explain it a few pages later
anyone interested really must get Cerzwinski's mutlitone testing paper
but furter research needs to be done, multitone amplifier measurements could perhaps put a practical limit on "hidden" intermodulation distortion, i wouldn't be suprised if it was limited to as little as 10 dB over the prediction from single tone distortion identification of the principle nonliearities - the GedLee metric is probably not too far wrong in that aspect
Are you aware of any BJT versions? Even a crude model would be interesting, to experiment with.
you could "roll your own" models with whatever level of bjt model you choose by using BV, BI equations to implement the equivalent models with your own added temp dependencies at some cost in sim speed
Bob Cordell said:
Bob,
Some time ago, I tried myself to do some Hirata measurements (on some opamp based gain stages), using a MC1455 timer and MAX303 switches http://datasheets.maxim-ic.com/en/ds/MAX301-MAX305.pdf to generate a test signal from a dual DC power supply (I tried all four waveforms), multiturn pots as attenuators, and a HP3478A 5 1/2 digit multimeter to measure the output. I haven't played much though, as I concluded that the sensitivity is quite low. Almost anything better than a LM301 opamp gain stage showed nonlinearities buried in the noise.
Certainly only breadboarding the thing had an impact on the circuit performance, for sure a finished instrument would do much better. However, I am still skeptical regarding the resolution and sensitivity of this method, at least for power amps. I also find Hiraga's statement "It was found that high-quality amplifiers with a total harmonic distortion of 0.01-0.001% show a nonlinear distortion of more than 0.04%" at least confusing, as no correlation with the THD numbers is provided.
It could be a very good tool for evaluating speakers, though. I haven't tried this so far.
