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Old 7th January 2010, 02:07 PM   #501
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Originally Posted by traderbam View Post
Speaking of quotations, here are a couple that were not made in the context of audio but I think are very apt in the audio world:

“If you can't explain it simply, you don't understand it well enough”

“Unthinking respect for authority is the greatest enemy of truth.”

Guess who?

These are both true statements, but let's remember the following. These are my responses to the above.

If you CAN explain it simply, you MIGHT understand it well enough. Simple is not always correct. I've met many who can explain things in a simple and concise manner, while still getting it WRONG.

Unthinking DISrespect for authority is also a great enemy of truth. Too many on this forum (I'm not implicating any specific person) are too willing to tell the pros and profs that they got it wrong. Questioning authority is neither good nor bad, because the critic could be right, or maybe wrong.

Those who encourage others to question authority should be equally willing to have their own beliefs questioned. Unfortunately, the most vocal critics of authority have little tolerance for their own critics. That's how it is, sadly.

Still, your comments are appreciated.
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Old 7th January 2010, 02:41 PM   #502
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[snip] Too many on this forum (I'm not implicating any specific person) are too willing to tell the pros and profs that they got it wrong. Questioning authority is neither good nor bad, because the critic could be right, or maybe wrong.[snip].
Agree, but telling ANYONE that they're wrong is not disrespect, *provided* the telling is backed up by factual arguments (which may or may not be correct).


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[snip]Those who encourage others to question authority should be equally willing to have their own beliefs questioned. [snip].
But of course!

jd
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Old 7th January 2010, 02:42 PM   #503
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Old 7th January 2010, 02:47 PM   #504
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Back to the original question,

Since there are evidences about the good performance of correctly designed amps using old and relatively slow devices at output (fT near 4MHz, as an example). Someone show here a Blameless original design using MJ802/4502 with no more than 0,01%DHT at 20kHz, I´m asking if the sustaining gain capacity at high current demands would be more important than the fT, in these cases.

Any clues?

Regards,
Do you mean that if you wanted to "improve" the amp by changing the output transistors, should you seek ones with better high current gain or with higher ft?
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Old 7th January 2010, 02:52 PM   #505
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The answer to this one probably fits into 'it depends' category. Each amplifier and application probably has it's own tradeoffs where Ft may or may not be more important that high current performance.

igh current performance good if peak transient response is d
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Old 7th January 2010, 04:07 PM   #506
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Yes, see here: diyAudio - FAQ: General Forum Usage

Good idea - better than getting angry

GREAT!!!!

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Old 7th January 2010, 04:51 PM   #507
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Hi Andy,
May I clarify my understanding of your post? In essence, you appear to be saying three things:
a) If an open-loop system has an infinite series of harmonics then putting NFB around it will not create any novel harmonics.
Yes. I guess you could call this belaboring the obvious, but what's not always obvious is that many circuits will have an infinite number of harmonics, yet it's sometimes claimed that "circuit X has no harmonic components above the Nth", when what is meant is "circuit X has no measurable harmonic components above the Nth". That is, "no harmonics" gets treated as being equivalent to "no measurable harmonics", yet increasing signal level (but below clipping) will cause the high-order components to show up in the measurement. One very, very smart person who claimed that source degeneration in a JFET circuit didn't increase high-order harmonics was fooled because he didn't crank the signal level up high enough to see them above measurement noise.

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b) The distortion products of a single-tone will be monotonically reduced as NFB factor is increased.
Not what I'm saying. Bob found for the specific case a class AB BJT output stage with a typical bias that this was true. What I'm saying is that each case needs to be looked at individually. In fact, Baxandall's BJT example is a good counterexample to the "a single-tone will be monotonically reduced as NFB factor is increased" argument. BTW, my tanh() example was a bit silly. Since exp(x) is an infinite series, it could be argued that in the absence of some (unknown to me) technique for canceling the high-order harmonics, any BJT circuit will have an infinite number of harmonics.

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c) This is a complicated area to understand.
Yes, but maybe "complicated" is not the best possible word or phrase. Certainly it's a mess mathematically. Baxandall was only able to get a closed-form solution for the amplitudes of all harmonics for the case in which the open-loop amplifier produced only second harmonic. So it means simulation or measurement becomes necessary to find the distortion components for non-trivial circuits. This tends to prevent the kind of clean prediction that can be obtained from a closed-form mathematical solution. It's also messy because there doesn't seem to be a clear rule that can be applied in advance to determine what's going to happen. People naturally gravitate to these clear rules, but the sometimes surprising effects of feedback on different open-loop configurations tend to violate such rules. For example, Bob's findings for the BJT class AB stage shows the "feedback increases high-order harmonics" assertion to not hold in all cases.

Last edited by andy_c; 7th January 2010 at 05:12 PM.
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Old 7th January 2010, 05:10 PM   #508
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Originally Posted by traderbam View Post
Do you mean that if you wanted to "improve" the amp by changing the output transistors, should you seek ones with better high current gain or with higher ft?
I think go for the higher FT, another caracteristic I look at is lowest Cob, if you need higher power capability use 2 pairs or more, it would give lower distortion too.

I have found that I do like no global feedback designs but this is only for mid and high frequencies, for bass I find a feedback circuit to have a better controlled bass.

Last edited by homemodder; 7th January 2010 at 05:14 PM.
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Old 7th January 2010, 05:13 PM   #509
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Originally Posted by traderbam View Post
Do you mean that if you wanted to "improve" the amp by changing the output transistors, should you seek ones with better high current gain or with higher ft?
I´m just asking if the necessity of very high fT devices at output is something that must be placed ahead of other questions like better gain characteristics etc. Sometimes it seems to be the case on discussions.

regards,
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Old 7th January 2010, 05:14 PM   #510
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Originally Posted by andy_c View Post
Yes. I guess you could call this belaboring the obvious, but what's not always obvious is that many circuits will have an infinite number of harmonics, yet it's sometimes claimed that "circuit X has no harmonic components above the Nth", when what is meant is "circuit X has no measurable harmonic components above the Nth". That is, "no harmonics" gets treated as being equivalent to "no measurable harmonics", yet increasing signal level (but below clipping) will cause the high-order components to show up in the measurement. One very, very smart person who claimed that source degeneration in a JFET circuit didn't increase high-order harmonics was fooled because he didn't crank the signal level up high enough to see them above measurement noise.
'belaboring' or 'belabouring', great word. Never seen it used before.
I agree with your point. The point I was making is that in cases where an infinite series of harmonics are not present in the OL system, application of NFB can create new harmonics.

Quote:
Not what I'm saying. Bob found for the specific case a class AB BJT output stage with a typical bias that this was true. What I'm saying is that each case needs to be looked at individually. In fact, Baxandall's BJT example is a good counterexample to the "a single-tone will be monotonically reduced as NFB factor is increased" argument. BTW, my tanh() example was a bit silly. Since exp(x) is an infinite series, it could be argued that in the absence of some (unknown to me) technique for canceling the high-order harmonics, any BJT circuit will have an infinite number of harmonics.
Ok. I would add that I consider single-tone analyses pretty irrelevant.

Quote:
Yes, but maybe "complicated" is not the best possible word or phrase. Certainly it's a mess mathematically. Baxandall was only able to get a closed-form solution for the amplitudes of all harmonics for the case in which the open-loop amplifier produced only second harmonic. So it means simulation or measurement becomes necessary to find the distortion components for non-trivial circuits. This tends to prevent the kind of clean prediction that can be obtained from a closed-form mathematical solution. It's also messy because there doesn't seem to be a clear rule that can be applied in advance to determine what's going to happen. People naturally gravitate to these clear rules, but the sometimes surprising effects of feedback on different open-loop configurations tends to violate such rules. For example, Bob's findings for the BJT class AB stage shows the "feedback increases high-order harmonics" assertion to not hold in all cases.
"Mathematical mess" rings true. I don't think Baxandall had the miracle of LT Spice . There are exceptions, but I think we agree that one cannot make a blanket statement like "NFB never adds new harmonics".

The cautious, maybe even belaboring, comment I am making to folks is not to assume that stable NFB always makes a circuit perform better in all respects. Rather, NFB changes the nature of the performance and one can choose certain aspects/measurements that improve and certain ones that deteriorate. In particular, changing the spectral content, including adding new spectra, are common characteristics of the application of stable NFB.
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