How Do You Perceive THD at HF? - Page 3 - diyAudio
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Old 25th May 2012, 02:11 PM   #21
Jay is offline Jay  Indonesia
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Quote:
Originally Posted by SY View Post
I think that the question is somewhat meaningless. HF distortion is a symptom, not the disease, so you can't categorize the "sound" of it, independent of knowing the cause. THD in particular is a very incomplete number- it lumps together very different pattens of nonlinearity with different causes, different effects, and different cures.
In simple words, how will you benefit from understanding the numbers?

ADD: I just simulated F5. Like I thought previously, the HF THD is awful... My TSSA is much better.

Last edited by Jay; 25th May 2012 at 02:13 PM.
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Old 25th May 2012, 02:21 PM   #22
jcx is offline jcx  United States
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Default Intermodulation Products

I've mentioned IMD in the past...
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Originally Posted by jcx View Post
Harmonic Theory is OK for music but it really isn’t that important for explaining audible distortion – Intermodulation Products Totally Dominate All “Harmonic” Distortion

Some of my thoughts on the subject:

How much distortion can we hear? - diyAudio

in

P-channel JFETs more linear? - Page 3 - diyAudio

you can see the 1KHz intermod difference product sticking out like a sore thumb, clearly audible and not “harmonically” related to the 2 high frequency tones that caused it

A high order “harmonic” distortion is evidence of a high order nonliearity that will generate vastly hugher Intermodulation distortion components having non-harmonic frequencies, IMD differences create new distortion frequency components below the fundamentals of the music that will be particularly audible because they are not masked

further down in the 2nd thread I give the best reference on distortion I know of:

"Multitone Testing of Sound System Components - Some Results and Conclusions, Part 1: History and Theory" JAES V 49#11 nov 2001 by Czerwinski et al at Cerwin Vega – with 119 references it is a truly thorough and relatively recent review article of the history of distortion measurement back to the dawn of audio reproduction
arguments also work for the IMD of a harmonic distortion generating nonlinearity in the top octave


and distortion cancellation is real - diff pair, complementary or push-pull output stages can cancel even order terms that match between the devices

Dr Cherry has claimed increasing bias to signal ratio, cancellation and negative feedback are pretty much all we have after selecting amplifying devices - and negative feedback is the only technique that can practically be used for factors of >1000x of distortion reduction

Last edited by jcx; 25th May 2012 at 02:50 PM.
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Old 25th May 2012, 06:23 PM   #23
wahab is offline wahab  Algeria
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Originally Posted by stinius View Post
Then you have too few tools in your toolbox.

When I read “distortion cancellation” I translate it to “making the amp as linear as possiblebefore NFB is applied.

Cheers
Stein
As linear as possible using local NFB before GNFB is applied ?..
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Old 25th May 2012, 06:31 PM   #24
wahab is offline wahab  Algeria
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Originally Posted by sgrossklass View Post
Anyway, 10 kHz distortion is just a handy way of getting a handle on high-frequency intermod in simulation. As most designs show distortion increasing with frequency, the point is that if it's OK there, it should be anywhere. Always look at the spectrum rather than just the THD number though.

Incidentally, just about any case where someone thought to have proven the audibility of ultrasonic tones eventually turned out to be caused by plain ol' amplifier HFIM...
A high OLG amp will see THD increase as a square of the frequency
or even following a cubic law if no linearization is applied at OL ,
using degeneration , although the design "choice" will only change
the starting point , that is THD at low frequency , but will left HF THD
ratio unchanged.
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Old 25th May 2012, 11:48 PM   #25
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Quote:
Originally Posted by jcx View Post
and distortion cancellation is real - diff pair, complementary or push-pull output stages can cancel even order terms that match between the devices
I don't consider those circuits to have "distortion cancellation". They merely mirror the non-linearity of a single ended stage into a symmetrical form of non-linearity. The idea that even harmonic distortion is canceled an over simplification of a complex mathematical process.

Distortion cancellation is where one stage has a non-linearity that unbends the non-linearity of another stage in such a way as to produce a more linear result. Another method of distortion cancellation is the careful selection of a device's operating point such that two distortion mechanisms cancel. See the third link.

http://www.firstwatt.com/pdf/art_dist_fdbk.pdf

http://www.firstwatt.com/pdf/art_sweet_spot.pdf

http://www.firstwatt.com/pdf/art_zv9.pdf (see page 4)
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