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Old 16th September 2009, 09:54 PM   #1191
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John,
The issue you addressed is the never-ending debate between some engineers and audiophiles.
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Old 16th September 2009, 10:59 PM   #1192
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Originally Posted by janneman View Post
Chris,

If I understood Bob's earlier posts correctly, he showed that with a forward path with no PIM, and a linear feedback path, you can generate PIM.
Is that correct? Bob?

jd
Hi Jan,

That is exactly right. The application of negative feedback to a PIM-less amplifier may cause amplitude-to-phase conversion; in other words, some of the AIM gets converted to PIM. The AM-to-PM conversion results because the incremental changes in open-loop gain (AIM) cause the NFB unity gain crossover frequency to move around, causing the closed loop bandwidth to move around, causing the in-band phase to move around, and thus a small amount of phase modulation.

Because the AM-to-PM conversion is of only modest efficiency, it is virtually impossible to have PIM from this mechanism without having AIM. If you drive the AIM down to extremely low levels, you will necessarily drive any PIM down to extremely low levels.

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Bob
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Old 16th September 2009, 11:05 PM   #1193
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Quote:
Originally Posted by Bob Cordell View Post
Hi Jan,

That is exactly right. The application of negative feedback to a PIM-less amplifier may cause amplitude-to-phase conversion; in other words, some of the AIM gets converted to PIM. The AM-to-PM conversion results because the incremental changes in open-loop gain (AIM) cause the NFB unity gain crossover frequency to move around, causing the closed loop bandwidth to move around, causing the in-band phase to move around, and thus a small amount of phase modulation.

Because the AM-to-PM conversion is of only modest efficiency, it is virtually impossible to have PIM from this mechanism without having AIM. If you drive the AIM down to extremely low levels, you will necessarily drive any PIM down to extremely low levels.
Sure, and I see 3 obvious ways to drive AIM down: to linearize each and every stage's complex transfer function, to linearize (or at least decrease) their output complex impedances and linearize (or at least increase) their input complex impedances, and increase bandwidth of each and every stage. Such a way a global feedback will be much more in-phase with errors over the whole audio band, so very small amount of PIM would be created as the result.

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Originally Posted by Joshua_G View Post
John,
The issue you addressed is the never-ending debate between some engineers and audiophiles.
Come on Joshua... Why such wide generalizations are needed here where engineers are honestly searching for ways to improve sound quality?
Who we are in your generalization, heretics who are both audiophiles and engineers, or heretics who are neither audiophiles nor engineers?
I can show my diploma to prove that you are wrong in one part of your statement, and I can show you my equipment to show that you are wrong in another part of it. There is no debates between engineers and audiophiles here. Those who agree about PIM creation mechanisms don't see proof of their presence on the graph, because from that graph it is impossible to distinguish AIM from PIM.

Last edited by Wavebourn; 16th September 2009 at 11:13 PM.
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Old 16th September 2009, 11:41 PM   #1194
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Originally Posted by Wavebourn View Post
Come on Joshua... Why such wide generalizations are needed here where engineers are honestly searching for ways to improve sound quality?
Who we are in your generalization, heretics who are both audiophiles and engineers, or heretics who are neither audiophiles nor engineers?
I can show my diploma to prove that you are wrong in one part of your statement, and I can show you my equipment to show that you are wrong in another part of it. There is no debates between engineers and audiophiles here. Those who agree about PIM creation mechanisms don't see proof of their presence on the graph, because from that graph it is impossible to distinguish AIM from PIM.

I wrote some engineers not all engineers. So, here goes what you refer to as "wide generalizations".

Now, I'm not so sure that all engineers participating in this thread are "honestly searching for ways to improve sound quality". Some definitely do. It seems to me that some others are stuck with their text books. That is, some are negating genuine subjective experiences, unless proven by methods subscribed in their text books. Meaning, they ignore heard PIM (or TIM, or whatever other type of distortion), unless they see it on the graph.

Generally speaking, hard-core audiophiles (with an engineering degree or without it) rely mainly on their ears (actually, the brain-ears mechanism), while engineers who are not audiophiles rely mainly on measuring equipment. Of course, there are engineers who are audiophiles, or audiophiles with a scientific, or technical background however, they are a third group of people who rely on both ears and measuring equipment.

It seems to me that it takes a degree of intellectual honesty to admit that science today don't have all the answers for what impacts the experience of listening to reproduced music.

Yet, obviously, I may have the wrong impression. Possibly, all here agree on the basics, only debating on the means.
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Old 16th September 2009, 11:45 PM   #1195
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Originally Posted by Wavebourn View Post
Sure, and I see 3 obvious ways to drive AIM down: to linearize each and every stage's complex transfer function, to linearize (or at least decrease) their output complex impedances and linearize (or at least increase) their input complex impedances, and increase bandwidth of each and every stage.
What you say may be valid in the context of a voltage amplifier target. Which is not necessary always the case.
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Old 16th September 2009, 11:59 PM   #1196
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Originally Posted by syn08 View Post
What you say may be valid in the context of a voltage amplifier target. Which is not necessary always the case.
Exactly.

In a power amplifier case linearity of each stage, linearity of coupling between them, and high bandwidth of each stage is the way to go. However, I mean complex linearity. Add here minimal power loss between stages and you have the whole equation of a desired amp with minimum of AIM that create minimum of PIM after GNFB applied. By the way, it is the way Otala's pupil designed his amp in order to replace Williamson tube amp version his wife did not like.

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Originally Posted by Joshua_G View Post
It seems to me that it takes a degree of intellectual honesty to admit that science today don't have all the answers for what impacts the experience of listening to reproduced music.
Not at all. Everyone who knows how science does work should know the difference between (the best scientific) models and The Reality.

Last edited by Wavebourn; 17th September 2009 at 12:02 AM.
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Old 17th September 2009, 12:06 AM   #1197
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hi,
does anyone have access to "intermodulation distortion in tape recording" robert z. Langevin jaes july 1963 vol. 11, no. 2.
ygm.
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Old 17th September 2009, 12:11 AM   #1198
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Originally Posted by Wavebourn View Post
Exactly.
Add here minimal power loss between stages and you have the whole equation of a desired amp
Careful here. The minimum power loss (or maximum power transmission) condition is on a collision course with either voltage or current optimal coupling. In audio we care about voltage or current gains (or linearity) rather than power gain (or linearity).
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Old 17th September 2009, 12:28 AM   #1199
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Good to see you here, Joshua.
What is amazing is: If it looks like a duck, quacks like a duck, maybe it is a duck, except everyone here is insisting that it is a swan, or that it is an optical illusion.
When it comes to FM modulation, I will, once again say: The pattern looks like FM, the cause seems to come from Barrie Gilbert's analysis, yet nobody wants to presume that it might be FM distortion. What is the problem? Politics and denial. If true, it 'implies' that traditional op amps can be troublesome. Wow, what a concept! As if we don't know something subjectively about that, today. The problem is that the traditional measurements don't show anything.
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Old 17th September 2009, 12:53 AM   #1200
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Demian,
the harmonic and inharmonic content of musical instrument sounds is for the instrument makers to deal with. Our aim is a faithful reproduction of those sounds so you can discern the difference between a Steinway and a Bösendorfer. As a first step, stay far away from Op-amps.
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John,
regarding who is telling anecdotes and muddying waters, I tend to support you.
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