John Curl's Blowtorch preamplifier part III

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#6,243
john curl said:
This is the cause of added FM distortion, and that separates the winners from the 'also rans'
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John, I keep wondering if you missed Ron's second article or Bob Cordell's measurements, modern IC's and amps with high ft devices have no measurable FM. It was only 741 era IC's and Dyna ST70's etc. that did. Oh BTW no feedback designs can have plenty.

I'm certain Damir's amp has little or none.
 
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#6,245
john curl said:
If Damir's amp is the one built by Richard Marsh, then its high open loop bandwidth will minimize the generation of FM distortion. This is based on first principles, not in any existing measurement method used to test it.
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First principles?
What mechanism is causing the FM distortion?
Why is high open loop bandwidth the solution?

Jn
You understand that "I don't know" is a valid answer, right?
 
#6,251
Nelson Pass said:
Siegfried asked me to develop an analog crossover, as described at last year's BAF, and which will be a kit in the diyAudio store next month. The documents to be published shortly show his comparative acoustic results.
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I found the paragraph my brain had partially recalled
My neighbor here, NP, a seasoned Class A amplifier designer, who listens at much lower volume levels than I prefer, summed it up: "Give me one of these and one of those a customer might say". But we both believe that analog should be at least as accurate as digital. So tomorrow we will get together in his workshop to perform a bunch of electrical tests on the ASP.4 to identify a reason for the sonic difference, which we hear.
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I was interested as I do have a couple of ASP.4 boards stashed for just in case my mind start playing tricks with me and I cannot live with a digital crossover.

My bold as it made me smile 🙂
 
#6,256
john curl said:
It all starts here.
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So, theoretical stuff from 38 years ago.

Since then, has anybody confirmed the existence of this FM distortion by way of test? Replication is of course, the basic tenet of scientific advance.

I would love to read some of his peers confirming the existance of such modulation, so if you could, a link to anybody who has measured this would be great.

Jn

DPH said:
http://www.cordellaudio.com/papers/another_view_of_tim.pdf

This is a very good start.
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I must say, all this talk so absolutely mirrors the motion equations and frequency response of large mechanical objects under computer based PID control.

Most interesting is that all these motion guys generally have never heard of Bode. They tell me that if the gain is too high, the error jumps wildly and the system crashes...they do not understand phase margins, negative fb, and gain in fourth and sixth order systems..or that with sufficient gain, feedback goes positive.

When I read Otala, I am reminded of them. Like video race car gamers who never actually drove a real car.

There are real reasons for amplitude modulated phase shifts, but Otala is not in the ballpark..sorry John.


Jn
 
#6,257
Recently, I have been refining the design of a couple of new power amps. I just can't get them 'perfect' like my critics seem to think they can easily do. Of course, I would like to measure the FM distortion, but my test equipment won't do that, just AM distortion. Oh well.
 
#6,260
john curl said:
And how do you separate AM from FM distortions, and what sort of test conditions will give the most FM distortion?
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No need to separate. If here is nothing new in spectra (or something below -110-120dB), what want you to separate (and hear..)?? So simple it is. You may use single tone, CCIF ,multitone, square or DIM signal. Nothing "magic" to reveal.
 
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