John Curl's Blowtorch preamplifier part II

Status
Not open for further replies.
AX tech editor
Joined 2002
Paid Member
You may have 2 circuit solutions, both with 100Hz OLG corner. One would have 0.5V/us and the second would have 100V/us. Both really do exist. So, it is pointless and wrong to state that it is OLG corner that defines TIM. It is neither OLG corner, nor feedback factor. It is only slew rate that defines TIM, and slew rate depends on circuit design and components used. Yes, stability is a must, but again stability depends on circuit design and compensation scheme.

As much as I hate to admit it Pavel :D I agree 100% with you here.

jan
 
AX tech editor
Joined 2002
Paid Member
"hate" :D is never good, Jan.

Actually is is a case of hating myself and sometimes something good can come from it ;)

Anyway, to add to your post, we should remember that ol gbw and ol corner are typically small-signal parameters, while slew rate is a large-signal parameter. So you can easily have the same ol corner but different slew rate and vice versa. Connecting SR to OL corner as if one determines the other is incorrect. But you know that.

jan
 
Last edited:
John keeps retruning to PIM despite the analysis, measurements by Cordell and others - and in the 30 yrs no one has produced a psychoacoustic study showing PIM is any more objectionable than "AM" Intermodulation Distortion products


PIM can be measured, minimised by design - and is reduced by high negative feedback - reguardless of loop gain slope/corner frequency

sloping loop gain only converts some static nonlinearities into "phase modulating" nonlinearity - but there are also inherent phase modulating nolinearities in nonlinear device capacitances so "flat" loop gain doesn't eliminate PIM sources per se

the only "evidence" for PIM importance is John's "intuition" - and he rejects DBT as "too insensitive"

the repetition would be less annoying if there actually was some intellectual justification for being concerned about PIM or if it were a circuit design challenge
but there is no mystery on the circuit design side, and no evidence for over weighting PIM vs any IMD from psychoacoustics

but our guru seems to be stuck in his groove

Mike,

Yes, we are saying the same thing: If open-loop gain gets modulated by a large signal, the amount of NFB will vary, and therefore the closed loop bandwidth will get modulated. The phenomenon is real, but we need to plug in the numbers and do the measurements to see how much it matters.

I built an instrument to do that about 25 years ago. It was a coherent IM analyzer. It first used a phase locked loop and synchronous (I) detector to synchronously "demodulate" the 6 kHz "carrier" of a SMPTE IM signal. This yielded a very high-sensitivity conventional SMPTE IM analyzer. I then added a second quadrature phase detector to synchronously demodulate the phase modulation on the 6 kHz carrier. The output of this "Q" demodulator was then calibrated in r.m.s. nanoseconds. I measured numerous op amps and a test power amplifier (the latter under different conditions of NFB). It was described in a paper given at an AES convention. The design description and data taken are on my web site at CordellAudio.com - Home

I have never heard of anyone else that built such a PIM analyzer, and have not seen anyone specify the PIM of their amplifiers.

Anyway, the PIM of the crappy old 741 op amp at gain of 10X at 6V rms into a 10K load was only about 3.9 ns. The PIM of a TL071 was about 1.6 ns.

The PIM of a 70-watt power amplifier of 1970 vintage and unsophisticated design was less than 10 ns at any level below clipping (it was about 6 ns at 50 watts). An experimental 35 watt power amplifier without negative feedback had PIM of about 50 ns to just below clipping. That same amplifier with NFB had PIM of about 10 ns to just below clipping. My MOSFET power amplifier with error correction had PIM of less than 0.1 ns (yes, 100 picoseconds).

Bob Cordell
 
Last edited:
a amplifer that measures good on any IMD test has low PIM

visual analogies are poor for audio - we simply can't put two sounds "side-by-side" for comparison

there is a deep literature of psychoacoustics that results from scientifically exploring hearing itself - why not become familiar with the actual relevant subject material?

"just listen" is a naive approach to gaining insight into what we hear, perceptual psychology has developed protocols over the more than 100 years that the physics of sound waves and our hearing have been explored systematically with the scientific method


one part of my argument with John's repeated appeal to "PIM" as "explaining" what he believes is "wrong" with high feedback amplifiers is that there is no evidence for our hearing "PIM" differently from any IMD product

secondly he is simply wrong in claiming it can't be measured - Bob Cordell's special apparatus is only needed if you want to separately measure "PIM" - any measure of IMD includes "PIM" as part of the vector sum of each IMD product frequency - get IMD products down to say -100 dB and it is guaranteed that PIM doesn't exceed -100 dB
AP's FastTest is perfect for "capturing" "PIM" with its multitone test signal - "PIM" is not "invisible" to common IMD tests

and Otala was wrong to claim that "flat loop gain, "high corner frequency" is the cure all for PIM - properly applied, very high loop gain reduces all IMD products

as engineers designing audio amplifiers "PIM" is only useful if we have a psychoacoustically established level or threshold to reduce this class of distortion to - where's our target number John? 10 microseconds?, 10 picoseconds?

how does the amplifier PIM compare to Doppler distortion from the speakers?
 
Last edited:
I played with "PIM" simulation.. It looks like ordinary IMD, so it is clearly measurable. No sidebands, no PIM.
 

Attachments

  • PIM.png
    PIM.png
    22.5 KB · Views: 220
BV, at least you measured something useful. The sidebands CAN coexist with NORMAL IM sidebands, BUT they will have their own frequencies, yet they well be symmetrical around one of the major signal components, just like your simulation shows! Now, just superimpose YOUR simulation with normal IM components and see what you get.
 
PMA, I am only searching for the BEST way to make audio products. Marketing has little or nothing to do with anything in my life. I have never personally advertised these ideas or asked someone to mention PIM in a promotion, for the very good reason that virtually all of my cost effective products are compromised with a modest open loop bandwidth and a fair amount of feedback. Certainly, I strive for high open loop bandwidth, but I am seldom happy with what is practical. My job as an audio designer is to make best use of the components at my disposal, to make GOOD SOUNDING products.
As newer and 'better' test equipment becomes available, I am hoping that the FM component that is added to every piece of audio equipment will become more easily available to easily measure, so that we can go forward by comparing the amount and character of the FM distortion with actual listening performance shown by comparing products with different amounts and noting the subjective difference.
This is much like just looking at the higher order components of the harmonic distortion waveform at moderate levels. That is the FIRST thing that I look at when comparing another audio product and how it is received in the marketplace. Works for me, and you should do it too, if you do not already.
My subjective experience, so far, points to wide open loop bandwidth as being a positive subjective attribute to making a successful audio design.
I have 4 products shown at the Munich show, how about the rest of you? '-)
 
Last edited:
AX tech editor
Joined 2002
Paid Member
[snip]My subjective experience, so far, points to wide open loop bandwidth as being a positive subjective attribute to making a successful audio design. [snip]

Really? Then why are there orders of magnitude more audio products sold that have relatively low ol bw and high feedback factors? What makes you think you are on the right path?
If your ideas about 'succesfull audio design' are the road to sound nirwana, why are they not scooped up by the big guys and manufactured by the millions, putting a Blowtorch in every home and you filthy rich? Is capitalism failing big time here?

jan didden
 
Last edited:
Really? Then why are there orders of magnitude more audio products sold that have relatively low ol bw and high feedback factors? What makes you think you are on the right path?
If your ideas about 'succesfull audio design' are the road to sound nirwana, why are they not scooped up by the big guys and manufactured by the millions, putting a Blowtorch in every home and you filthy rich? Is capitalism failing big time here?

jan didden

Really?? I think you know the answer to your question.
Yes ,capitalism failing big time here to.
The masses buying MP3 and this is not a measure of quality.
 
In actuality, Charles Hansen, Nelson Pass, and I make more 'A' products than the size or scope of our companies would predict, IF we did not know a bit more than many of or direct competitors. The only real competition are the vacuum tube based products, that I often find a pleasure to listen to.
The reason, as I said, many times before, was because the AES refused to publish Matti Otala's PIM paper in the JAES, citing it too controversial, even though it was just pure mathematics and NO actual measurement values were stated. What was the harm? To the 'professors' who cited that double blind tests had proven that everything essentially sounded the same, it was not necessary to acquire new knowledge in this direction. THEY controlled the JAES at this point. And so it goes.
 
While on this topic, I would like to (once again) relate some audio history, where we got started, where we went wrong, and then got right again.
The history of TIM comes from 2 separate inputs. First, an accident where a power amp was put together wrong and it sounded BETTER than the correct design, based on op amp theory. This amp apparently had two changes: Higher slew rate and higher open loop bandwidth. In any case, this stuck with a young Matti Otala.
Second, in 1966 (a long time ago) a young graduate student Daughority(sp) and Dr. Greiner (well known, even today) published in the IEEE 'Audio' publication that HIGH OPEN LOOP was necessary for low distortion audio, even class A audio. I read this at the time and was impressed by the mathematical effort to show this. Still, I did not really understand it.
This IEEE article was cited by Matti Otala in his first papers, and initial efforts on his part showed that a high open loop bandwidth that tended, as well to create a high slew rate, as well, power amp sounded better.
However, before Jan L. and Matti O. built their amplifier design, I was designing complementary differential, pp throughout, complementary output power amps for the Grateful Dead group. I, then in 1971, 5 years after the D&G paper, was asked by a colleague then working at LLL, how to make a faster slew rate power amplifier. He needed on for a project. I thought about it, and even with my experience of making amplifiers from 10W-2000W in both 2 quadrant and 4 quadrant symmetry, I could NOT think of a better way to speed up my amps, except to use faster output devices, which would increase the gain-bandwidth, the open loop bandwidth, and the slew rate in proportion. He came back to me later, that somebody HAD made a really fast amplifier that overcame all his efforts, but they were not saying how. NOW, we can presume that they JUST degenerated the Gm of the input stage in order to increase the slew rate, without increasing the G-B, or the open loop bandwidth. What a breakthrough! At least it had not been OBVIOUS to me, at the time. Within the year, attending advanced engineering classes at UCB, the equations were shown, and all was clear. However, please remember, my engineering colleague at LLL did not have access to this knowledge in early 1971, or else he would have not have had to ask me what to do. It was just not that obvious in those days, what is so obvious now. (more later)
 
But there was more subtle question/suggestion under lines. In case we want to argue by commercial success as a measure of circuit design and sound quality, then MP3 is the winner, and not the Blowtorch.

Say's Law. Supply creates demand.

People are selling music, not MP3s and Blowtorches.

MP3 delivers more music - more supply, more demand.

Jacco's right.
 
BUT they will have their own frequencies,

NO, NO, and NO they will have the same frequencies. There are people here I assume that are interested in some of the basic principles of engineering. John just ask directly THIS question to Ron Quan and his advisor Tom Lee (an old friend) they will set you straight.

Volterra analysis extends to arbitrary non-linear systems even those with menory. The output with input at f1 and f2 can ONLY be at n*f1+-m*f2. Simple amplifiers do not fall into the non causal non time invariant category.
 
Last edited:
Status
Not open for further replies.