Bob Cordell Interview: BJT vs. MOSFET - Page 102 - diyAudio
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Old 18th April 2007, 07:11 AM   #1011
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Quote:
Originally posted by john curl
Glen, anyone can design mid fi. Why even bother to talk about it? There are 100's of completed amp designs, both new and used, that should suit your needs. Perhaps, PA applications are where you are really going to be helpful. Even then, we would put tube amps on the tweeter array, when we could. It just sounded better.
For you to come to some conclusion about harmonic distortion and the harmonic series shows an amazing lack of interest in the subject, and is even in contradiction to what was mentioned in the 'Radio Designers Handbook' in 1941, and even earlier. Haven't you ever been exposed to listener's fatigue from higher order distortion products present in the electronics?



What on earth are you talking about??

I think that you need to read my post again – carefully. You’re attributing to me the opposite of my view. As I said, I AGREE that odd harmonics/high order distortion products are less desirable than even harmonics in principle. But I don’t believe that anyone is going to suffer listener fatigue from higher order distortion products produced by a modern, state-of-the-art solid state amplifier with <0.001% THD-20. Perhaps the harmonic composition of the distortion is a more important factor for you, since your JC-1 amp is rated somewhat higher at 0.15% THD-20.

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?

Also, to the best of my knowledge, 0.001% THD-20 wasn’t easily achievable in 1941, so I think this design requirement was a bit more relevant then.


Cheers,
Glen
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Old 18th April 2007, 07:17 AM   #1012
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Glen, there is a good chance that some other distortion is being generated that is not directly measurable in high feedback circuits, just like SMPTE IM distortion failed to show TIM. However, if you can hear .15% distortion at 400W output, you are a better listener than I am. I look at the 1-10W level to get best performance, with a little global feedback as possible.
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Old 18th April 2007, 07:18 AM   #1013
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Glen,

this is lost. You will be always taught about high order harmonics produced by NFB, though they are -120dB and less. And the preferred "non-NFB" amp would measure like this:

Click the image to open in full size.

That means high order harmonics 10-100x higher.

To mask is the case, not to amplify
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Old 18th April 2007, 07:26 AM   #1014
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Quote:
Originally posted by john curl
However, if you can hear .15% distortion at 400W output, you are a better listener than I am.
Even 0.05% and less of thd is audible, in case that it is caused by crossover distortion. You can make your own simple test, you would hear it on pure sine tone like 1kHz, provided the test signal is clean.
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Old 18th April 2007, 08:15 AM   #1015
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Quote:
Originally posted by john curl
Glen, there is a good chance that some other distortion is being generated that is not directly measurable in high feedback circuits, just like SMPTE IM distortion failed to show TIM. However, if you can hear .15% distortion at 400W output, you are a better listener than I am. I look at the 1-10W level to get best performance, with a little global feedback as possible.

Good grief John, you have just avoided answering my question with a total furphy.

Let’s suppose than a well designed amplifier with 0.0004% THD-20 at full power (not a 1941 single-ended tube design with 15% THD) was indeed producing such distortion. Of what magnitude would you guess it might be, particularly at normal listening levels? I’d appreciate an answer, especially so since you readily dismiss 0.15% THD as inaudible.

Cheers,
Glen
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Old 18th April 2007, 08:27 AM   #1016
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Originally posted by PMA
Glen,

this is lost. You will be always taught about high order harmonics produced by NFB, though they are -120dB and less. And the preferred "non-NFB" amp would measure like this:

Click the image to open in full size.

That means high order harmonics 10-100x higher.

To mask is the case, not to amplify

NNNOOOOOoooooooooo...........NFB BBBBBAAAAAADDDDddddddddddd........

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Old 18th April 2007, 08:38 AM   #1017
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Quote:
Originally posted by PMA
This is a common semantic discussion here, returning in cca 2 year period.
Of course emitter follower has 100% negative voltage feedback.
For some reason hifidesigners mean Global Voltage negative feedback around the whole amplifier from output to input when they say "Feedback".
Hi PMA,

Actually, it wasn't my intention to start such semantic debate all over again, rather to comment on the Ayre V-3 amp.
However, I was forced, or at least invited by Bob to clarify my view on the definition of FB. Although we both agree that, starting from a strict definition of FB, it is impossible to build a good amp without FB, I'm not inclined to go any further on that matter.

There are much more interesting things to discus. That's why (in my reply to Hugh) I put a link to a variant on Cherry's NDFL amp.
Regrettably, nobody has responded until now. Assuming it has just been overlooked, I'll put it here again:

What is nested feedback, how it realy works and some examples...
post # 29

Cheers,
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Old 18th April 2007, 09:04 AM   #1018
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Hi Edmond,

thank you for the reminder, I will read it.

The amplifier that really impressed me is this one:

http://www.hardwareanalysis.com/content/article/1842/

Regards,
Pavel

http://www.hardwareanalysis.com/cont...diy_amplifier/
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Old 18th April 2007, 09:11 AM   #1019
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Quote:
Originally posted by john curl
Glen, there is a good chance that some other distortion is being generated that is not directly measurable in high feedback circuits, .....
Hi John,

Not directly measurable? What on earth should that be?
Please, explain.
BTW, as Glen has pointed out, all kind of distortions are inter-related to a degree. So, if thd=zero, all other disturbances, including your mysterious brand of distortion, are zero too.

Cheers,
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Old 18th April 2007, 11:42 AM   #1020
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Quote:
Originally posted by PMA
Hi Edmond,
thank you for the reminder, I will read it.
The amplifier that really impressed me is this one:
http://www.hardwareanalysis.com/content/article/1842/
Regards,
Pavel
Hi Pavel,

I did have a look at that amp and indeed, the thd figures are pretty good. But I'm sorry to say, I'm not impressed. IMHO it is a rather 'insipid' design, as it is quite easy to obtain such a low thd by reverting to a power hungry class-A stage (hence it's named ExtremA?) and using an inverting input stage. Clearly, the designer of this amp wasn't challenged by the hurdles that other designers are willing to overcome.

Analogue to incandescent lamps, there should be a ban on class-A amplifiers and designers of such heaters should be punished by cutting off one ear.


To all,

I'm looking for vertical fet's like the 2SK1530/2SJ201 pair, but with a much lower Cgd. Any suggestions?

Cheers,
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