John Curl's Blowtorch preamplifier part II

[janneman]

Originally Posted by anatech

[snip]Given that a feedback network is a linear network, it cannot create anything (which is my point). I suspect that you meant that these things occur as a result of global, negative feedback being applied. But any of these effects must occur in the forward gain path as that is the only place where non-linearity can exist. I am assuming that the resistors used in the feedback network are not defective (ie they are linear in all respects).[snip]-Chris

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

[janneman]

Originally Posted by 1audio

[snip]This whole issue has become pretty abstract. It started with a claim (fm modulation in op-amps) and a reference to some artifacts in a measurement that were not explained when the measurement was made. [snip]*Given our focus on mathematics I'm puzzled that the non-harmonic tuning of a piano has not been touched on. http://en.wikipedia.org/wiki/Piano_t...etched_octaves. Perhaps just being harmonic is the problem.

Demian,

Actually, the artifacts were explained in the paper, its that they just were not labeled in the graph. There's a lot of sense in your post but I don't think that trying to focus on several issues at the same time instead of step-by-step getting to grips with the one that seems stubborn right now is a good career move.

BTW Funny that you mention non-harmonic tuning. When I visited Jan Lohstroh last week, he explained that concept to me (he has a beautifully restored 1942 Steinway model C, and I had no knowledge of these things). It does of course NOT mean that any non-harmonic frequencies are generated by IM or from a single source. It does mean that two snares that should generate harmonic frequencies are de-tuned so that the two tones are slightly non-harmonic. The way I understood it (I haven't looked at the wikipedia entry) this is necessary to fit the twelfth-root-of-2 steps into an octave.

jd

[john curl]

Well, PMA, at least you are trying something useful. I wish everyone else was trying something similar, even with IC's.

[syn08]

Originally Posted by 1audio

.

*Given our focus on mathematics I'm puzzled that the non-harmonic tuning of a piano has not been touched on. http://en.wikipedia.org/wiki/Piano_t...etched_octaves. Perhaps just being harmonic is the problem.

Demian,

Mind you, it's not the same issue. Creating a non-harmonic signal is not the same with processing a harmonic signal.

Inharmonic oscillations in mechanical resonators (as strings or, as a matter of fact, quartz crystals) is a well understood phenomena. As you invoked math, the reason for this inharmonic behaviour is the violation of another essential condition in the Fourier theorem: the signal periodicity. And physically, mechanical resonators are generating aperiodic signals because of the nonlinearities in their mechanical properties.

You would agree that generating an aperiodic signal is different from transforming a continuous and differentiable periodic signal in an aperiodic signal (as JC's extraordinary claim implies). Such a transformation would ultimately violate the causality principle.

[john curl]

Demian, it is true that perfect tuning doesn't really exist, but humans have had thousands of years to perfect what works pretty well.
The real point is: Lower order harmonics, i.e. 2'nd, 3'rd, and 4th, at the very least, can be tolerated in very high amounts or else both vinyl records and analag magnetic tape would have been found unacceptable, from the very beginning. This also includes direct radiator loudspeakers, as well.
Many designers have wondered about this for multiple decades, and studies going back to the 1930's. at minimum, have given tolerance levels of distortion which can be very high, with lower order harmonic/IM products.
However, the dilemma exists that negative feedback, especially with op amp type designs, can lower MEASURABLE harmonic/IM distortion to almost nothing at all, perhaps 1 part in 1 million, YET the sound quality of the electronics can still sound compromised, even almost annoying.
The reasons why this is so, is much of my quest for a better approach to making audio electronics. I say AUDIO electronics and not test equipment, etc electronics, because it appears to be something that the human ear detects, that most engineers and scientists don't yet completely know about, that is more important than very low typical measured distortion. This is what people have been working at for decades, either by making sonically acceptable products, just by brute force, or in trying to create a test that is outside the boundary of normal test methods, that will give a visual indication of what we hear. This is the point of this discussion.

[syn08]

Originally Posted by john curl

However, the dilemma exists that negative feedback, especially with op amp type designs, can lower MEASURABLE harmonic/IM distortion to almost nothing at all, perhaps 1 part in 1 million, YET the sound quality of the electronics can still sound compromised, even almost annoying.
The reasons why this is so, is much of my quest for a better approach to making audio electronics. I say AUDIO electronics and not test equipment, etc electronics, because it appears to be something that the human ear detects, that most engineers and scientists don't yet completely know about, that is more important than very low typical measured distortion. This is what people have been working at for decades, either by making sonically acceptable products, just by brute force, or in trying to create a test that is outside the boundary of normal test methods, that will give a visual indication of what we hear. This is the point of this discussion.

The ultimate tactic: when everything else fails, move the discussion in the muddy waters of subjective experiences.

Now waiting for the GEB to jump in and "clarify" the situation.

[PMA]

I do not think there is any tactic. It is the same experience of many of us - circuits that measure extremely low THD and IMD, have wide CL bandwidth and reasonably high slew rate still sound very different. It is up to you, syn 08, to find an appropriate method that would explain anything. I take it as a fact, experiment is above model to me.

[janneman]

Originally Posted by PMA

I do not think there is any tactic. It is the same experience of many of us - circuits that measure extremely low THD and IMD, have wide CL bandwidth and reasonably high slew rate still sound very different. It is up to you, syn 08, to find an appropriate method that would explain anything. I take it as a fact, experiment is above model to me.

Not really. Its up to whoever makes the claim to prove that the claim holds water.

This is the claim:

"circuits that measure extremely low THD and IMD, have wide CL bandwidth and reasonably high slew rate still sound very different" due to the use of neg feedback (or due to the use of ICs; take your pick).

To be proven:
a) this is a fact;
b) if it is a fact, it is due to neg feedback (or due to the use of ICs; take your pick).

After that, we will worry about why it could be so.

jd

[AndrewT]

Originally Posted by janneman

..................

To be proven:
a) ..
b) ..
After that, we will worry about why it could be so.

is this back to front?
Don't experiments often lead to unexpected results and lead to further experiments to try to find what is happening?
Then the experimenter starts to formulate a hypothesis to explain the phenomena.
The theory and the proof often follow afterwards.

[albin]

ot maybe
wolves,snow,lemmings.
2 cents
regards Max