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Old 19th September 2012, 02:19 PM   #1021
SY is offline SY  United States
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George, this was also covered thoroughly in "Valve Amplifiers." In tube amps (or any other amp which uses RC coupling), it's even a bigger problem, since the loss of feedback jams the amp into a blocking distortion which can take a second or two to resolve.
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Old 19th September 2012, 02:23 PM   #1022
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Quote:
Originally Posted by gpapag View Post
May be this is one of the potential distortion mechanisms you are addressing:



(P.364 from the excellent book of Bob Cordell: Designing Audio Power Amplifiers )

This may look exotic to me that I am an - non EE - amateur, but for a cautious designer, it should be a more homeland one.
Short time clipping certainly has the potential to occur in listening sessions quite frequently and get unnoticed.
The acoustic signature of clipping and subsequent sticking will be attributed to increased feedback from a person that adjusts the amount of feedback on an amplifier he is building/optimising and then evaluating by listening to music.
It is difficult to spot short term clipping while measuring, as the source material is rarely a music piece. The crest factor on some music is high and we may underestimate the overhead actually required.
Therefore, it is even more difficult to locate the stage that distorts first, then think of the underlying mechanism and decide on the remedy.

Regards
George
Indeed, and that is why a competent designer will take anti-saturation measures in his design, such as baker clamps, which, incidentally, are also used if not recommended by Bob C.

jan d
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Old 19th September 2012, 02:58 PM   #1023
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It is more than that, George. I don't anticipate clipping of any kind in most of my designs, without having the police called for too much noise.
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Old 19th September 2012, 03:36 PM   #1024
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Originally Posted by janneman View Post
Indeed, and that is why a competent designer will take anti-saturation measures in his design, such as baker clamps, which, incidentally, are also used if not recommended by Bob C.

jan d
Or you could simply design an amplifier with a good clipping behaviour without using clamping.

Sorry for beeing OT, but this is a power amp and not a discrete opamp.

BTW: THD at rated power (100W into 8 ohm, about 1 dB before clipping) is less than 0.5 ppm, (almost only 2nd and 3rd). Rock stable into 2 ohm complex load.
No exotic parts.

Cheers
Stein
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Old 19th September 2012, 04:17 PM   #1025
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Quote:
Originally Posted by gpapag View Post
May be this is one of the potential distortion mechanisms you are addressing: (P.364 from the excellent book of Bob Cordell: Designing Audio Power Amplifiers )

The acoustic signature of clipping and subsequent sticking will be attributed to increased feedback from a person that adjusts the amount of feedback on an amplifier he is building/optimising and then evaluating by listening to music.
Regards
George
That is a good point to remember -- I tend to forget about it because I use high power amplifiers to prevent clipping and use high effeciency speakers. I suspect a lot more of clipping goes on than has been assumed; compressed recordings will bring us closer to the threshold of frequent clipping when played at moderate to high listening levels. And, low effecint speaker with low-moderate power amps will make it easier to clip, as well.

Amount of neg feedback, of course, affects the produced harmonic distortion structure when clipping does occure.... adding to the amps 'sound' character. The lack of this is often noticed in very high power amplifers.... say greater than 250W/cnl; Often described as an effortless and easy quality..... or just cleaner sounding. In fact, it may be one of the major reasons why amps are described as having a sound difference from one another.

Magazine reviewers and consumers swap in various amps and speakers during listening evaluations without regard to efficiencies and amp power rating differences but listening at same spl. -Thx RNM
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Old 19th September 2012, 04:47 PM   #1026
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Speaking of Bob, he has a no feedback pre-amp in Linear Audio Vol. 4. A JFET diff-pair (gm) feeding a cascoded high gain stage, with an open-loop unity gain buffer. That would be an interesting variant, internal feedback, or no feedback at all, or maybe even a combo.
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Old 19th September 2012, 05:06 PM   #1027
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It seems so far off from the OP requirements for this thread to be speaking about distortion in power amplifiers here. But at the same time we have to look at the paradigm that the current marketing has created in consumer audio. Ever smaller speakers and enclosures with low power multichannel amplifiers running on a single power supply and expected to drive a sub-woofer. Now the consumer thinks that they can turn up the system and listen at any type of realistic level and what do we have, pure mud. That is so far from what we as a DIY group are expecting and designing for. But we also have to look at the differences of a horn loaded high efficiency system vs. a low efficiency direct radiator system with wide bandwidth devices. The power requirements are different by a exponential rate between these two opposing camps. One may be fine with a small high quality amplifier and the other will need a high current high power amplifier with much more headroom to handle those compressed bass passages that are being reproduced. So the actual needs are quite different between the two camps.

Now back to your discrete opamp design.
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Old 19th September 2012, 05:27 PM   #1028
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Quote:
Originally Posted by scott wurcer View Post
Speaking of Bob, he has a no feedback pre-amp in Linear Audio Vol. 4. A JFET diff-pair (gm) feeding a cascoded high gain stage, with an open-loop unity gain buffer. That would be an interesting variant, internal feedback, or no feedback at all, or maybe even a combo.
My copy arrived yesterday, and there is a lot of good stuff within.

One thing I noticed about Bob's non-global-feedback MM preamp: it assumes the cartridge is connected differentially*. This I suspect in large part accounts for the reasonable distortion numbers, as even harmonics will nearly cancel. The MC preamp is single-ended except for a quasi-differential common sense, despite using groups of differential pairs. But of course for almost all MC cartridges, the voltages will be much lower.

I could go on about other aspects of that article, as well as a lot of the other articles, but it would get hopelessly off-topic.

Brad

*for most existing equipment this will require rewiring the tonearm with shielded twisted-pairs. I can hear KOJ saying "the way they OUGHT to be" Keith also had his preamp in the base of his AR turntable "where it belongs!"
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Old 19th September 2012, 05:42 PM   #1029
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Quote:
Originally Posted by scott wurcer View Post
Speaking of Bob, he has a no feedback pre-amp in Linear Audio Vol. 4. A JFET diff-pair (gm) feeding a cascoded high gain stage, with an open-loop unity gain buffer. That would be an interesting variant, internal feedback, or no feedback at all, or maybe even a combo.
I still like the use of the exact same circuit and its power supply and only varying the OL gain to vary fb amount and to do so as you listen. Can you hear a difference? -RNm
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Old 19th September 2012, 07:12 PM   #1030
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Quote:
Originally Posted by SY View Post
George, this was also covered thoroughly in "Valve Amplifiers."
You mean Morgan Jones book. Yes, and on some others as well, less thoroughly maybe, down to the old Radiotron designer's handbook.
But my point was to trickle Mr. Curl for his use of the “more exotic” term.
I certainly do not qualify to make a statement but I am confident that “exotic” is as far as it should go for an expression as to any form of distortion. Beyond that, is the magician’s land.

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It is more than that, George
It has to be, I agree.

Quote:
Originally Posted by john curl View Post
I don't anticipate clipping of any kind in most of my designs, without having the police called for too much noise.
Mr. Curl, I am not addressing your designs. But too much noise is the result of average SPL. You know that you can have the cop at your door with your amplifier outputing on average anything between 2 and 20 Watts depending on your speakers sensitivity. What about some short excursions 12-16db higher?
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That is a good point to remember …


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Originally Posted by janneman View Post
Indeed…..if not recommended by Bob C.
jan d
The beauty of this work for me Jan is that, all the myriad small but important details in the design of an amp are rationally presented and adequately explained.

Here I would like to make a question which I think is relevant to the subject of this thread.
Which of all the potential technical-design- issues met on a power amplifier are not to be faced in the design of this discrete op. Amp (assuming appropriate scaling of power) ?

George
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Last edited by gpapag; 19th September 2012 at 07:15 PM.
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