No global negative feedback

The deepest challenge of designing high fidelity equipment is finding the common ground between truth and beauty, left brain/right brain, or more plainly, reconciling the interior experience of listening with the technical world of measurements. If you can't reconcile the two, or insist that only one exists, you are flying blind.
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http://www.nutshellhifi.com/library/FindingCG.html


The fault is not with the subjective perception of the listener, but rather in the measurement itself. Nothing new in that; you can measure all you want, but a mass spectrometer isn't going to find a lot of difference between lunch at a high school cafeteria and the best dinner at a four-star restaurant. To foolishly assert that the mass-spectrometer is right, and the restaurant customers are all deluding themselves with some kind of "placebo effect," is an example of simple ignorance trying to cover its nakedness with the fig-leaf of Science.
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?

Stereo Mic said:
You see that's where we will disagree. You now seem to think all feedback is the work of Satan whereas had you simply increased the output impedence you might well have got the same results for all we know. I can think of too many wonderful sounding amplifiers that use NFB to take the cultists views seriously - and I still wait for someone to provide me with any proof whatsoever that properly implemented NFB is anything but beneficial.
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Do people really buy an amplifier because of it's specs? ;)
 
Another excert from the above link:

Stereo Mic, maybe you can understand this...

Norman Crowhurst wrote a fascinating analysis of feedback multiplying the order of harmonics, which has been reprinted in "Glass Audio," Vol 7-6, pp. 20 through 30. He starts with one tube generating only 2nd harmonic, adds a second tube in series (resulting in 2nd, 3rd, and 4th), and then makes the whole thing push-pull (resulting in 3rd, 5th, 7th, and 9th), and last but not least, adds feedback to the circuit, which creates a series of harmonics out to the 81st. All of this complexity from "ideal" tubes that only create 2nd harmonic!

With real devices there are even more harmonics. In terms of IM, actual amplifiers have complex and dynamic noise floors thanks to the hundreds of sum-and-difference IM terms. That's not even counting the effects of reactive loads, which adds a frequency dependency to the harmonic structure! (With reactive loads, additional harmonics appear due to the elliptical loadline seen by the power tubes. The elliptical load-line dips into the very nonlinear low-current region, resulting in an instantaneous increase in upper harmonics. This spectral "roughening" is most audible with strong low frequency program material and hard-to-drive horn or vented bass drivers.)

As Crowhurst noted, feedback mostly reduces the 2nd and 3rd harmonics, leaving the upper ones more or less alone, or sometimes even greater than before. Feedback fools the simple THD meter, but the spectrum analyzer sees through the shell game. Too bad raw power and almost useless THD measurements became the end-all and be-all for more than 50 years. If more engineers and reviewers had access to spectrum analyzers, the misleading nature of raw THD measurements would have been discovered earlier, and amplifier design might have taken a different course.
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Cables dont work, but... :rolleyes:

Mainstream AES-school engineers have ridiculed "audiophile" power cords for many years, but EMI emission from solid-state-rectifier power supplies is no joke. It's hard to identify on a scope (the trace just looks a little thicker), but a wideband spectrum analyzer clearly displays the comb spectra created by the switching devices. The fancy power cords may be doing their greatest benefit by partially shielding the dirty power supplies from other solid-state equipment and CD players.
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melorib said:
I am not clever, at least I admit it... :crazy:
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No need to admit it Antonio. Just keep posting.

I fully understand your posts and those of Thorsten. What I have asked for is a real world example of an amplifier that displays audible levels of higher harmonic distortion because of it's NFB loop.

Sadly despite all of the theorising and bluster, I'm still waiting.

Good to see you posting scientific findings though. Next you'll be convinced all CD players sound the same and cables only vary in their LCR charateristics :D
 
Antonio, can you think of a product that displays the problems I have asked about? Genuinely rather than just having a childish tit for tat. The question remains as to whether there are audible panalties for employing NFB. Until I see otherwise, I will assume there isn't.

For your reference, I measure and listen as most do. That way I can understand why my system sounds good/bad and can make a mental note of what works. I find that rather more educational that simply blindly swopping or worse cables.
 
I did my education in different areas, this is only an hobby, but it doesnt take a genius to sujjest you to listen to your only amplifier with 12db feedback than with 3db, if you cannot disconnect it completely, then you will know the difference more or less feedback does... :bookworm:

I dont go around testing amplifiers, particularly since I am in Riga, so I cannot give you any examples, except for mine and Thorsten's, as he mentioned already... :SLEEP:
 
I've spent 6 pages trying ever so hard to explain that the only audible consequence of what you did would have been the increase in the amplifier's output impedence and the consequential frequency non linearities when driving your loudspeakers. I've also hypothesised that the critical damping of the bass unit could well be affected too.

And after all that effort you still write:

but it doesnt take a genius to sujjest you to listen to your only amplifier with 12db feedback than with 3db, if you cannot disconnect it completely, then you will know the difference more or less feedback does...
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It's disappointing and glaring proof that some only read what they want to read.
 
Stereo Mic said:
I've spent 6 pages trying ever so hard to explain that the only audible consequence of what you did would have been the increase in the amplifier's output impedence and the consequential frequency non linearities when driving your loudspeakers. I've also hypothesised that the critical damping of the bass unit could well be affected too.

And after all that effort you still write:

It's disappointing and glaring proof that some only read what they want to read.
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Yet, never occurred to you that your explanations might be wrong, and Thorsten's + the other links posted might be right... :confused:

It's disappointing and glaring proof that some only read what they want to read.
 
Now I see what Ian meant!

Antonio. Of course I could be wrong. That's why I've repeatedly asked for someone to show me an example of a current amp with audible high order HD caused by the NFB.

I can happily show the effects of a high output impedence on FR so I know my position is secure. I am waiting for you or someone else to provide proof that your claims are also true. In the absence of that, I can only assume you are merely hypothesising.
 
A friend.

Now shall we put this on hold until someone can give me an example as requested? There's not really much of interest until we see that..
 
Stereo Mic said:
I've spent 6 pages trying ever so hard to explain that the only audible consequence of what you did would have been the increase in the amplifier's output (impedance) and the consequential frequency non linearities when driving your loudspeakers.
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It does not follow that you'll increase the drive unit's non linearities (i.e., in certain cases you may tame them -- as in an over-damped driver...)

And you seem capable of seeing that:
(...) I can happily show the effects of a high output (impedance) on FR
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I assume this can also lead you to determine an "optimum" range of damping for any particular drive unit?
The question remains as to whether there are audible (penalties) for employing NFB. Until I see otherwise, I will assume there isn't.
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Audible or not, there may be serious penalties if you don't use some f/back loop on a transistor amp. Otherwise, your assumption is, I imagine, perfectly fine in the context of this thread -- the original subject of which was about a small but satisfactory modification melorib performed on his amp:) Cheers
 
Not that illuminating I'm afraid Antonio. Cheever is, like Geddes, trying to put forward an alternative metric that has yet to be universally accepted.

I'm looking for a test where the feedback circuit was removed from an amplifier and higher order harmonics were reduced whilst there was objective agreement that there was a sonic improvement. All whilst keeping the output impedence constant.

That's the kind of evidence that will make me sit up and take notice. It would be worth bearing in mind though Antonio that the same kind of documents as you present, are often used to show that all CD players sound the same and that there is no magic in cables (although the latter does tend ot be based on accepted thinking rather than pure hypothesis). Accept one and you must surely accept the other?
 
Maybe without feedback cables stop making noticeable difference, hmmm... :rolleyes:

Now back to topic... ;)

So far I removed what is in the red boxes, selector, volume and NFB...

My friend wants me to also remove the "input capacitor" C1, he says the gain will go back to close to the original value, and increase input and output impedance, besides eliminating a "poor quality" electrolitic CAP...

What do you guys think, is it safe, can it improve my amplifier... :confused:

Classic_schema_mod.jpg
 

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