Hawksford

[john curl]

Actually, he was. Of course, in the '60's, slew rate was a known (but obscure) term. I did a search, many years ago. What Matti was concerned with, was distortion generated WELL BELOW slew-rate limiting. I asked him about this directly, in the 1970's. Every amp and preamp design has a different distortion buildup profile, BELOW actual slew-rate limiting. This is what is most important, and WHY Matti often refused to relate slew-rate limiting to TIM, even though they are related to each other.

 

[john curl]

I would like to relate a true story regarding Richard Cabot.
Decades ago, I was at an AES meeting featuring Richard Cabot. He, even then, attacked Otala's work on distortion. He claimed that he did NOT get the same measured results as Otala. However, I got up and asked him WHY he did not use the SAME circuit topologies as Otala, if he expected to get the same results. Don't get me wrong, Richard Cabot is a smart guy. When we meet, on occasion, we are friendly, BUT he had it in for Otala from the get-go. The same goes for Bob Cordell. This does NOT free Otala from any responsibility. Once, after one of his AES lectures, I wanted to get one of Paul Klipsch's "BULL****" buttons that he handed out at AES conferences and pin it on Matti ! Why? Because Matti is a political guy. He did NOT always make it easy to understand his work. In this case, he deliberately avoided relating slew-rate with TIM. This annoyed me as much as the other engineers, but I let it go, because he still contributed so much to understanding audio design.
As far as the name 'TIM' was concerned, it was just as good as 'SID' or 'slope distortion'. It had to be called SOMETHING! And he was first.

 

[mikeks]

...... Every amp and preamp design has a different distortion buildup profile, BELOW actual slew-rate limiting. This is what is most important, and WHY Matti often refused to relate slew-rate limiting to TIM, even though they are related to each other.

This is merely due to the increased loading of the dominant pole comp. capacitor on the diff. pair, pro rata with frequency.....always been known.....and TIM is certainly a meaningless discription of this elementary phenomenon......

Use double-pole compensation, and render Otala even more irrelevant...

 

[andy_c]

Linear or invariant, Andy?(...)

Linear. That is, for constants a and b, and arbitrary input signals x1 and x2, a relationship f() is linear if and only if:

(1) f(a*x1 + b*x2) = a*f(x1) + b*f(x2)

A memroyless circuit that creates distortion might have:

(2) f(x) = c1*x + c2*x^2 + c3*x^3 +... (a Taylor series expansion)

It's simple to show that linearity doesn't hold for (2) by replacing x by a*x1 + b*x2. You won't be able to massage it into the form of (1) unless the only non-zero constant is c1. Further, any circuit described by a Taylor series expansion in this way can't distort all signals the same, since increasing the input signal amplitude will increase the percent distortion. So I don't think the concept of invariant that you're describing is common in the real world.

 

[Christer]

Steady on! .....it was always called Slew rate limiting , or slew overload......so good old Matti was not the first to divine this very old problem......

Isn't that the point that Gilbert Barrie makes in one of those
articles, that the phenomenon was already well know in
electronics, except that audio designers didn't know about it
before Otala? I am also a little bit surprised nobody mentions
Marshall Leach. To me, as a non-expert on this, he seems to
make some very good points on how to design away TIM in
his text describing the Leach amplifier. LP filter the input to
limit the signal slew rate, have sufficient bandwith to handle
this maximum slew rate and use heavy emitter degeneration
on the input diff pair to extend its "linear" region. I am taking
this from memory, so please forgive me if I got it wrong.
However, I cannot see that Leach's recipe automatically
prescribes low feedback. Also, I am not trying argue against
anybody in particular, just bringing this up for feedback so
I can relate it to the discussion and learn something from it.

 

[andy_c]

Regarding the Cabot publications:

Suppose you're an objective person (which may be different from an "objectivist" 🙂 ). Person A writes a paper postulating the existence of a distortion mechanism that could escape conventional distortion measurement techniques. Person B works for a company that makes conventional distortion analyzers. Human nature being what it is, do you expect person B's point of view regarding person A's paper to be truly unbiased? Or is there a potential conflict of interest?

 

[jarek]

Indeed, your loudspeaker system is exceedingly unlikely to audibly reproduce 1.2mV of anything.....

0.003%(30ppm) 3rd harmonic in an amp. delivering say 100 Watts(40Vpeak), is equivalent to 1.2mV...

Yea, all with ears close to the speakers😀

 

[Christer]

Objectivity is a difficult issue. One cannot and shouldn't trust
anybody with a commercial interest to be objective or even
wanting to be. They may have a hidden agenda. Now, don't
get me wrong. I don't mean I assume them all to be deliberately
non-objective, just that one has to be aware of this conflict of
interests. OTOH, academics are supposed to be objective and
put truth before anything else and before any personal interests.
unfotunately, my experience from working in academia is that it
far from always works that way. Many scientists have their own
hidden agendas and personal feuds with other scientists. This
problem is small in some areas of sicence and bigger in others.
I have no idea or opinion how reliable and objective the audio
academia is in general since I haven't worked in that area.

 

[Pedja]

Audibility

Packed wav files are sinewaves:
1kHz 0dBFS (100%)
2kHz -60dBFS (0.1%)
2kHz -80dBFS (0.01%)
5kHz -60dBFS (0.1%)
5kHz -80dBFS (0.01%)
Each file lasts 10 seconds. To make the files convenient for upload a dither is not used, but there are still no other components above -100dBFS, and this is mainly 3rd harmonic of the base signal in any file.

Now, play the first file and tune the volume. Then play the others and tell what you hear.

Pedja

 

[PMA]

0.003%(30ppm) 3rd harmonic in an amp. delivering say 100 Watts(40Vpeak), is equivalent to 1.2mV...

Somehow i don't think you'll hear a 1.2mV odd harmonic in the presence of 40V clean signal across your transducer....

Indeed, your loudspeaker system is exceedingly unlikely to audibly reproduce 1.2mV of anything.....

Well - in case that you want to stay in "academical discussion" 1.2mV makes 22.5dB of acoustical pressure for 90dB/W/m/8 Ohm speaker and this is audible 😉 .

Which is not the real case, of course. Not audible for 3rd harmonic. Drier and colder sound for those 7th-15th harmonics of that magnitude.

 

[Pedja]

Lower and higher harmonic components

Now, harmonically distorted 1kHz sinewave. Files are made by the simple mix of the pure sinewaves.

Packed wav files are:
1kHz with 0.1% of 2nd harmonic distortion
1kHz with 0.01% of 2nd harmonic distortion
1kHz with 0.01% of 5th harmonic distortion
1kH with 0.01% of 9th harmonic distortion
Each file lasts 10 seconds. Notice about the hash is the same as in my previous post.

Now you can listen to the different spectral content of the harmonic distortion. A report about it is welcome.

Pedja

 

[PMA]

Yea, all with ears close to the speakers😀

The small but important difference is that I did not speak about audibility of simple 3rd harmonic of 0.003%, if you kindly read the posts more carefuly. BTW - what is your experience in comparing of amps with similar distortion and different harmonics contents? We might like to learn something here.

 

[millwood]

Some resources on audibility of higher harmonics and human ear self distortion may be find here:

http://w3.mit.edu/cheever/www/cheever_thesis.pdf

I do not speak about amplifier stuff at this link.

I thought it was very interestin read. and found the literature review up front quite good. the ear self-distortion is something new to me. thanks for the link.

 

[PMA]

Pedja,

I have no chance to play the files, but one question - is not it quantization distortion what will be heard from the first file? (-60db and -80 dB signals).

 

[millwood]

http://diyaudio.com/forums/showthread.php?s=&postid=195349&highlight=#post195349

I would agree with your point. the cary example, if taken on its face, can be argued against the tube guys and anyone who believes in "even harmonics are good".

It is in my view more of the product of a less-then-vigorous marketing-oriented review, so typical of certain mags.

 

[Pedja]

Pavel,

Just unpack the files and use some player like Windows Media Player or WinAmp. Or burn them on CD to play the through the system

I could make them dithered so they can be basically clean above some -130dB (in the bigger part of the audio range in fact below -140dB). However, as said, dithered files can not be that successfully packed, any of them would have a few hundreds of kB and as such couldn’t be uploaded here. Undithered signals are not that good, but all unwanted components are still below 100dBFS, so they are still usable for this purpose.

Pedja

 

[Pedja]

One more file, it is the 1kHz 0dBFS distorted with 9th, 11th, 13th and 15th harmonic, all these components being at -80dBFS.

 

[millwood]

Re: Lower and higher harmonic components

Now you can listen to the different spectral content of the harmonic distortion. A report about it is welcome.

Pedja

they sounded absolutely the same to me. I suppose it would be even harder if the "base" signal is real life music (or noise).

Another interesting test would be for you to mix those files, without telling us which is which, and let us vote for the "worse" sounding and "best" sounding files. It would be interesting to see which one comes on top in such a test, 🙂

 

[Nelson Pass]

I wanted to get one of Paul Klipsch's "BULL****" buttons that he handed out at AES conferences and pin it on Matti ! Why? Because Matti is a political guy. He did NOT always make it easy to understand his work. In this case, he deliberately avoided relating slew-rate with TIM.

Matti made TIM into a holy crusade for the marketers, I'll give
him credit for that. Too bad he gave up his academic soap box
to dance with Harmon. 😉

Every few years something comes along that explains and solves
everything. When jitter became the buzzword in digital, I said,
"Here we go again!"

 

[millwood]

Every few years something comes along that explains and solves
everything. When jitter became the buzzword in digital, I said,
"Here we go again!"

so true.

as long as we don't think critically of what is fed to us, that will continue to happen. It was tim, jitter, zero feedback, and whatever they tell us tomorrow.