John Curl's Blowtorch preamplifier part II - Page 1251 - diyAudio
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Old 27th May 2011, 05:46 PM   #12501
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Quote:
Originally Posted by simon7000 View Post
Yes that is one reading of what I wrote.
The question was "are you aware of feedback math that does not rely on linearized small signal models"? In what way was my answer a "reading" of what you asked.

Your example in particular of back to back diodes (as in a distortion pedal) does NOT yield at all to this kind of analysis. It would yield to a harmonic balance simulator. The circuit you showed can only be analysed in a large signal sense.
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Old 27th May 2011, 05:48 PM   #12502
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Quote:
Originally Posted by john curl View Post
Whatever, but please PROVE it.
As soon as you PROVE it does.
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Old 27th May 2011, 05:56 PM   #12503
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Quote:
Originally Posted by scott wurcer View Post
As soon as you PROVE it does.
No no, Scott, that's not how it works.

It's YOUR job to DISPROVE the UNSUBSTANTIATED claims made by others.

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I met a man who wasn’t there
He wasn’t there again today
I wish, I wish he’d go away...


se
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Old 27th May 2011, 06:01 PM   #12504
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I am just asking that we should attempt to measure it to the sensitivity that it deserves.
I say this because I still have problems with IC designs that are reasonably fast, measure well, quiet, BUT do not perform as well as my discrete designs, NO MATTER WHAT I DO. This is why we, that is (or was) Matti, Walt Jung and me, for example, still pursue this PIM or FIM distortion. IF we could find IC's that worked WITH NEGATIVE FEEDBACK that sounded as good as our discrete designs with some negative feedback, there would be progress. IF we could make both IC's and discrete designs with negative feedback sound as good as designs WITHOUT NEGATIVE FEEDBACK, I could retire, kick back, and enjoy my royalties.
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Old 27th May 2011, 06:02 PM   #12505
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Quote:
Originally Posted by scott wurcer View Post
The question was "are you aware of feedback math that does not rely on linearized small signal models"? In what way was my answer a "reading" of what you asked.

Your example in particular of back to back diodes (as in a distortion pedal) does NOT yield at all to this kind of analysis. It would yield to a harmonic balance simulator. The circuit you showed can only be analysed in a large signal sense.
I was answering "Where have you seen people using maths which implies the opposite?" as we were discussing published papers. The meaning you got was certainly there and is quite valid, but not the intended one.

As to the diodes I used for the distortion model. I really did look carefully but they really don't seem to behave like real diodes in those circuits. In the second set of experiments the voltage across them was .5 volts or less to keep them out of the hard clip mode as I wanted a softer distortion.

I am not interested in analyzing those circuits as they are not practical audio circuits. I will try to construct more typical audio circuits for the hardware tests.

The idea is to produce a simple article that points out there is a need for proper distribution of signal gain and feedback. As I keep pointing out the flamers who insist on no feedback, local feedback only, lots of global feedback etc, don't really get even the basic issues.

Most readers who understand a simple article will probably model their designs and play with where they place feedback to get better results. I do not intend to aim for a rigorous general case, as even the concept of what the residual distortion should look like is an area of OPINION.
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Old 27th May 2011, 06:08 PM   #12506
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Ed, why not simple common-source FET voltage amps?
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Old 27th May 2011, 06:45 PM   #12507
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Originally Posted by john curl View Post
I say this because I still have problems with IC designs that are reasonably fast, measure well, quiet, BUT do not perform as well as my discrete designs, NO MATTER WHAT I DO.
This is OK, but why do you suggest that the PIM would be the reason?
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Old 27th May 2011, 06:55 PM   #12508
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Regarding Otala, his articles are extremely controversial. He has had good points, and wrong explanations as well.

His key article "Circuit design modifications for minimizing transient intermodulation distortion in audio amplifiers" starts with this statement
Quote:
Transient intermodulation distortion appears in power amplifiers when a feedback loop is extended over several stages and the open-loop cutoff frequency of these stages is lower than that of the preceding preamplifier stages.
and he uses the image attached. This statement is completely wrong, because TIM does not depend on OLG corner frequency at all, it depends only and only on slew rate of the circuit after closing of the feedback.
Attached Images
File Type: jpg preamp.JPG (36.7 KB, 181 views)
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Old 27th May 2011, 07:13 PM   #12509
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Ed, why not simple common-source FET voltage amps?
Because if I had the time to do a thorough project that would require getting distortion spectra from all of the common gain stage configurations and using similar for the test amplifiers!
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Old 27th May 2011, 07:14 PM   #12510
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PMA, which came first, the need for feedback stability or TIM?
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