John Curl's Blowtorch preamplifier part II - Page 447 - diyAudio
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Old 26th May 2010, 10:38 PM   #4461
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Quote:
Originally Posted by john curl View Post
Cherry is another major contributor. I tend to not go his direction, however.
I want to tell you a little story about Dr. Cherry.
Back in the late 1960's, Cherry and Hooper wrote the definitive textbook on amplifier design. Something like: 'Low Pass Amplifier Design' and it contained almost 1000 pages of 'everything you would want to know about amplifier design' EXCEPT slew rate, slope distortion, distortions related to rate of change, etc, were NOT IN THE BOOK. Both Walt Jung I looked in our own copies of the book.
10 years later, you have thought that he 'invented TIM' along with Bob Cordell, and he wanted to rename it 'Slope distortion'. Well, he would have had the right to it, IF he had put it forth in his textbook. But, alas, he did not.
Personally, I tend to side or at least see the point of people who find problems in the direction that we normally go, in audio, in order to progress.
It is my opinion that both Bob and Ed go with what might be considered the status quo. This is the major philosophical difference between us.
Hi John,

I am not surprized about your usual diatribe that contains little technical argument. When it comes to PIM, you are a "Clingon" :-).

Slew rate distortion was a well known phenomenon decades before Matti re-named it. I think the name actually derived from the maximum rate of change that a gun turret was capable of.

I never claimed to have invented TIM or slewing distortion; I merely tried to explain it long after the fights had begun. Look at the date of my article on TIM in Audio; I was actually a late-comer :-).

How did we go from PIM to TIM here anyway? This old thread is a big de ja vu.

Cheers,
Bob
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Old 26th May 2010, 10:46 PM   #4462
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I have a article which makes for some interesting reading regarding bandwith and TIM, this is from 1977 wireless world by Bert Sundqvist in response to Otalla s papers. Its titled # Transient intermodulation in amplifiers. Simpler design procedure for t.i.m.-free amplifiers #

Its in pdf format but just over the size limit allowed here, if someone here has pdf editing program I can email the article as it could be smaller as there is another unrelated article from JLH together with it which could be edited out and the article made available here for those interested in reading it.
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Old 26th May 2010, 10:47 PM   #4463
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Originally Posted by PMA View Post
The goal is to get reasonably high OLG -3dB corner without loading VAS resistively.
PMA this is a good point. If one really does want to have high open-loop bandwidth for the global feedback loop, one should use local NFB in some way to keep the gain down and the bandwidth up. However, if one uses very localized shunt feedback, like traditional Miller compensation but in a resistive way, the input stage still has to work harder at all frequencies, just as if the VAS was resistively loaded. However, the shunt feedback approach at least uses the gain thrown away to linearize the VAS.

If, on the other hand, you reduce VAS gain by using quasi-local NFB back to the input stage, you then do improve life for the input stage. This is not unlike the Miller Input Compensation that I used in my MOSFET power amplifier.

Cheers,
Bob
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Old 26th May 2010, 10:50 PM   #4464
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Homemodder, you could try to ZIP the PDF and then you have more space.
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Old 26th May 2010, 10:53 PM   #4465
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Originally Posted by PMA View Post
The issue is Gm variation, which leads to corner frequency modulation and PIM.
Correct. Closed loop corner frequency modulation, to be exact. No modulation of the open loop corner frequency is necessary to have PIM by this mechanism.

At the same time, it is important to bear in mind the other mechanisms that can cause PIM, some of which do involve modulation of the open-loop corner. One example is nonlinear Ccb in a VAS transistor. Another is nonlinear Cgd in a MOSFET that is driven directly from a VAS without a buffer.

Cheers,
Bob
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Old 26th May 2010, 11:00 PM   #4466
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Here it is.
Attached Files
File Type: zip Power Amplifier Design Notes.zip (186.0 KB, 124 views)
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Old 27th May 2010, 04:43 PM   #4467
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Moving on, TIM was pretty well solved by 1980. Walt Jung, Matti Otala and I, all agreed that 5V/us was probably enough for a preamp, and 50V/us for a 100W power amp. This recommendation has not changed much, and for several years, the PIM question remained in an unresolved, (at least by Walt Jung and me), question.
However, when Barrie Gilbert did his analysis of a typical low noise op amp input stage, in the late 1990's, it resurfaced, at least for Walt and me.
Added to this, I got feedback from two FM designers who demanded, if I designed anything for them, I had better get the FIM or (PIM) distortion right. So there we are. I still design most of my products with significant negative feedback, but my high end competition often doesn't use it, and sometimes edges me out in listening comparisons.
The best that I can do is to get as high an open loop bandwidth as possible, and still get very low distortion, perhaps not the lowest in the industry, but low enough to meet THX specs. Seems to work pretty well, but I think there is more to learn about this area of design.
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Old 27th May 2010, 10:17 PM   #4468
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Quote:
Originally Posted by Bob Cordell View Post
I think the name actually derived from the maximum rate of change that a gun turret was capable of.
Cheers,
Bob
I have to compliment you on this correct bit of trivia.

Shooting down aircraft was one of the major engineering projects of the 40's, often unrecognized for all the knowledge we as engineers gained form it. Back then 40 degrees per second was a good slew rate for anti-aircraft guns.

Of course the basic math came out of Bell Labs from feedback amplifiers.
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Old 27th May 2010, 11:27 PM   #4469
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Of course, it was slewing rate, then, I'm pretty sure. You know, slew those guns!
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Old 28th May 2010, 01:04 AM   #4470
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I found some measurements that Stereoplay ( German HiFi Magazine ) did on high and low NFB amps. I try to scan them tomorrow and post some.
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