John Curl's Blowtorch preamplifier part II

[Bob Cordell]

Originally Posted by john curl

There is current thinking from several sources that getting a PhD does limit creativity in many cases. It has to do with left brain over-dependence.

John, I think this is complete hogwash. I am not a pHd and can design circles around many of them, yet I respect them and would love to have gotten one myself (so I could teach at a place like Stanford). At the same time, I have rubbed elbows with many people with Masters and Bachelors that had little creativity.

Cheers,
Bob

[Bob Cordell]

Originally Posted by john curl

Gee, guys, a silly 10Hz difference. However, where did 2F2-8F1 go?
For the record, the 741 is NOT clipping in example 3, and this can be shown in the results for the 741 in table 2 in the same paper where TWICE the output voltage was used in the measurement and 3.8% was totaled for 10V pp.
Now, we have to look at the next set of symmetric components, then the third set, etc. just to be sure.
Yes, there are several sets of symmetrical components, where did they come from?

Hi John,

I don't recall anybody saying the 741 was clipping. However, it was perilously close to, if not into, slew rate limiting in that test. Slew rate limiting, of course, is a form of internal clipping.

2F2-8F1 would be much less likely to show up because it is an even-order product, and the nonlineaities that lead to feeback-generated PIM seem mostly to be odd order (think input differential pair). Nonlinearities that lead to open-loop PIM may be more likely to be even-order, such as junction capacitance nonlineairity.

Cheers,
Bob

[Bob Cordell]

Originally Posted by BV

2f2-9f1..

And,

11f1-2f2 = 4.98 kHz.

Bob

[Bob Cordell]

Originally Posted by BV

This is simple diode limiter with "DIM100" signal (3200sq+15000sin)so no SR limitations, only nonlinearity..The same "footprint".

Nice; great idea, good illustrative results.

Cheers,
Bob

[gerhard]

Originally Posted by Bob Cordell

Nice; great idea, good illustrative results.

But dangerous. Some bozo will sooner or later use this for his claims
about better sounding power rectifiers.

rotfl, Gerhard

[Wavebourn]

Originally Posted by gerhard

But dangerous. Some bozo will sooner or later use this for his claims
about better sounding power rectifiers.

Wait for PIM-free rectifiers in Stereophile.

[Bob Cordell]

Here's a thought for the day to put PIM in perspective. Let me know if you agree with me.

PIM in Class D amplifiers.

Class D amplifiers necessarily have a series inductor in the output filter. A typical value is 10-20 uH. These are often realized with a powdered core. It is generally accepted that distortion in the output filter is a problem for class D amplifiers because of this when the feedback loop is not closed around the output filter (which is hard to do because of stability issues).

Any nonlinearity in the inductor usually manifests itself as current-dependent inductance. Thus, we have a pole moving back and forth with the signal current, causing in-band phase shift changes. Voila, PIM.

Note that this is a version of PIM that is not dependent on being created by negative feedback - or even by any active devices.

Suppose the filter is 3 dB down at 100 kHz. If the inductance varies by 10% as a function of signal, the PIM created will be ten times worse than the PIM created by a feedback amplifier whose open loop gain moves around by 10% with signal if that amplifier has a NFB gain crossover frequency of 1 MHz.

(yes, this is VERY back of the envelope).

Cheers,
Bob

[john curl]

Well, here we are again. Thanks again, Gerard for doing a creditable re-creation of the test. It looks like multiple tests, like we originally did in the laboratory with incrementally increasing and decreasing output levels, is in order. This should clear up the matter. Again, I regret that my original tests were destroyed in the firestorm, or I could have looked at them, and added further info, one way or another.
It should be remembered that if the numbers don't fit, then maybe they are derived from the wrong equations.
Personally, this situation reminds me of something that happened around WW1, when Einstein's theory was being discredited. Apparently, some great German physicists could emulate Einstein's predictions by asserting the presence of a planet in our Earth orbit, of a certain size, and 180 degrees opposite the Earth so that it was always hidden by the Sun. Some people will go all out to discredit something that shakes their 'world view'.

[Wavebourn]

What shakes my world view, is creating of non-harmonic content from 2 periodic signals, when no other source of the signal present. As I said before, if some LF instabilities due to kind of thermal feedback with delays present, there will be a hidden signal playing the role, but non-linear stable system can't create in-harmonic content from 2 distorted and intermodulating periodic signals.

[Wavebourn]

Originally Posted by Bob Cordell

Suppose the filter is 3 dB down at 100 kHz. If the inductance varies by 10% as a function of signal, the PIM created will be ten times worse than the PIM created by a feedback amplifier whose open loop gain moves around by 10% with signal if that amplifier has a NFB gain crossover frequency of 1 MHz.

But you said before that you believe that a negative feedback around a thingy that itself creates PIM helps to decrease it, so may we assume that a class D amp with an additional feedback loop from speaker out will have less of PIM caused by filter cores?