Bob Cordell Interview: BJT vs. MOSFET - Page 103 - diyAudio
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Old 18th April 2007, 12:15 PM   #1021
SY is offline SY  United States
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Quote:
Good grief John, you have just avoided answering my question with a total furphy.
Well, we're even, you did the same for mine. Unless, of course, you were serious and think that a low static THD at one frequency into a test load is a sufficient criterion. Load tolerance, stability, source impedance, IM, bandwidth, noise, overload recovery, clipping characteristics, none of that (or anything else) seems to be taken into account.

BTW, that Ayre spectrum is very unimpressive. I thought that their stuff had much lower distortion than that. It makes my tube amp look like a specs king.
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Old 18th April 2007, 12:32 PM   #1022
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Originally posted by AKSA
...Perhaps you misunderstand my intent. I suggest a two stage development here; first get the engineering right, focus on low distortion, accuracy, flat response, THEN address the subjective issues within market constraints and try to make the connection between the 'coloration' we hear in a null test as you describe and the topology/dimensioning/component choice. .....

Hugh,

I did understand your point from the start, as you elaborate further in the post quoted above.

My point is, once you finish what you call the first stage, the engineering process leading to certain performance parameters, any further modification must involve deviations form the just attained baseline.
Otherwise, being objectively measured improvements then they qualify within the first design stage.

Be it that way, then any further tweaking to meet subjective listening goals implies loss of accuracy. There is nothing wrong with this in my view, as long as there are listeners willing to adjust their listening experience to suit their tastes. Music enjoyment is after all a subjective, personal and emotional experience.

Only, out of intellectual honesty, I should not like to hear manufacturers claiming implicitely "true" rendering when in fact knowingly or not they are in fact coloring the result.

Rodolfo
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Old 18th April 2007, 12:59 PM   #1023
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Quote:
Originally posted by estuart
designers of such heaters should be punished by cutting off one ear.
Silly man.
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Old 18th April 2007, 01:26 PM   #1024
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Originally posted by jacco vermeulen
Silly man.
Have you lost your sense of humor?
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Old 18th April 2007, 02:22 PM   #1025
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Quote:
Originally posted by AKSA
Hi Rodolfo,

Thanks for your post.



Ah yes, thank you, this slipped by. Good point.



Perhaps you misunderstand my intent. I suggest a two stage development here; first get the engineering right, focus on low distortion, accuracy, flat response, THEN address the subjective issues within market constraints and try to make the connection between the 'coloration' we hear in a null test as you describe and the topology/dimensioning/component choice. This is the aspect which requires deep experience of the market and the practicalities to my way of thinking, although I do not decry the engineering. It's tough to get it right and clear proof is a quiet walk through the silicon crematorium.

You know, I might come across as mystical in my approach, but I have never in my design experience seen an improvement in linearity without a commensurate improvement in sound quality. This is a clear and obvious proof that accuracy is good. But while I appreciate that audio engineers stand their ground without apology I do not decry the objective approach and do not intend to fight city hall, I merely believe there is more to it than engineering, a little art, if you will. After all, pcb layout is called artwork, this is no coincidence.....

This is recognised in other fields of engineering; bridges, autos, architecture, even chemistry. I would offer that the difference between a competent design and a great one is art and passion. Why not audio? And I have noticed that even the most hard-bitten technologists here like to describe how their amps 'sound'. This is subjective, and demonstrates we are all artists at heart.

Cheers,

Hugh
Hi Hugh,

These comments are right on target, and I agree completely. Measurement and listening - beautiful together.

Bob
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Old 18th April 2007, 05:29 PM   #1026
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Folks, I'm for hot and heavy amps. That's what I like to make. I know of no better way to make the first 10 watts sound smooth and unfatiguing, than to make that first 10W class A. At higher power levels, I am not so concerned. For example, an ALL class A power amp, of greater than 350W, might require special house wiring, much like an electric stove, and special cooling as well.
However, only time will tell if certain lower heat amps actually do as well in long term listening.
When it comes to distortion, please read more thoroughly:
Matti Otala published a paper in the 1970's called: 'Correlation Audio Distortion measurements' By Eero Leinonen and Matti Otala , in the 'AES' Jan/Feb 1978, that is most definitive about this.
Another paper is 'A comparison of Nonlinear Distortion Measurement Methods' By Richard C. Cabot, preprint 1638 AES Convention 1980.
Also, what about Hirata distortion, or IIM, or PIM?
I know that some of you think that anything but harmonic distortion is not very important, but if that were so, many amps would be 'good enough' for quality listening, just like the ABX people have told us for decades. ;-)
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Old 18th April 2007, 05:39 PM   #1027
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Quote:
Originally posted by john curl
Folks, I'm for hot and heavy amps. That's what I like to make. I know of no better way to make the first 10 watts sound smooth and unfatiguing, than to make that first 10W class A.
That`s absolutely correct.
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Old 18th April 2007, 06:02 PM   #1028
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Originally posted by G.Kleinschmidt






Do you honestly think that listeners of the ‘ExtremA’ amplifier are going to be suffering listener fatigue from the vanishingly small high-order distortion products ultimately produced by it’s BJT’s, or for that matter listeners of Bob Cordell’s lesser-performing MOSFET design with its BJT VAS, driver stages and 0.001% THD-20?


Cheers,
Glen

Hi Glen,

I hate to be picky, but the ExtremA amplifier does not appear to outperform my MOSFET amplifier published in 1984. At the very least, there is not enough information in the ExtremA measurements to make that statement. Check out the measurements in my paper on my site at www.cordellaudio.com. Look especially at Figures 13 and 14.

At 6 kHz, 50W and in a 20 kHz BW, ExtremA reports 0.0004% THD.
At 6 kHz, 50W in a 60 kHz BW, I report 0.00025%.

At 20 kHz, 40W, 80 kHz BW, Extrema reports 0.0006%.
At 20 kHz, 50W, 200 kHz BW, I also report 0.0006%.

At 1 kHz, by spectral analysis ExtremA reports second of about 0.00008%.
At 1 kHz, 50W, 10 kHz BW, I report 0.00017%, all harmonics and noise combined.
Perhaps this 1 kHz measurement is where you thought ExtremA was superior?


I also don't use Class A to achieve these results.

Cheers,
Bob
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Old 18th April 2007, 06:11 PM   #1029
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Bob,

should these only numbers explain different sound?? I do have experience with different sound of link stages where one would measure quite NOTHING in THD.
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Old 18th April 2007, 06:15 PM   #1030
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(Sorry Pavel, something went wrong. This 'yes' was meant for Bob)
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