You're making several assumptions here. Let me start with the first one: Has anyone been able to subjectively distinguish between a non-feedback and feedback amp in a controlled level-matched test? (This assumes both amps have similar frequency response with the chosen speaker load, sufficiently low distortion, and are not clipping)
If not, then theoretical explanations are useless until the actual existence of the phenomenon is demonstrated.
Traderbam,
there you go again:-
To say NFB makes things sound better when properly used is like saying that if two tin cans and a piece of string are designed properly they will perform as a perfect telephone.
Where did I say that?
Read my post again. Carefully. I specifically state 'electrical performance'
there you go again:-
To say NFB makes things sound better when properly used is like saying that if two tin cans and a piece of string are designed properly they will perform as a perfect telephone.
Where did I say that?
Read my post again. Carefully. I specifically state 'electrical performance'
I am assuming an individual is able to repeatably distinguish between two circuits using identical stimulus, output level and loading. That is all.
It is up to the engineer to investigate what the electrical differences are between the two circuits and to surmise whether those differences can be entirely accounted for by changes which are independent of the application of NFB.
For instance, I am prepared to give John Curl the benefit of my doubt that he has sufficient skill and experience and dedication to have minimized the significance of collateral differences such that he feels justified in ascribing the sonic degradation to the use of NFB itself.
Please correct me if I am wrong, John.
That does not prove it is the NFB. Nor does it prove that it isn't. But my personal experience and theory ground-breaking, achieved through a fusion of critical listening experience and engineering education, lead me to believe that the publicly published theory is defficient rather than John Curl's judgement.
Brian
(is that sufficiently disagreeable? 🙂)
traderbam said:
But Brian, that is exactly the assumption that is not confirmed and is disputed. As long as it isn't, any conclusions based on that assumption are tentative at the most.
Jan Didden
this is a really good question.
time for a radical idea - why don't we all spend the next few weeks over the holidays doing some listening by either visiting the audio shops of choice or doing home listening trials of some amps of differing philosophies.
In a few weeks, folks can describe what they heard (or didn't hear) and we can see where things go from there.
spice and handwaving is fine and fun, but it's time to put the cars on the road and drive ...
mlloyd1
time for a radical idea - why don't we all spend the next few weeks over the holidays doing some listening by either visiting the audio shops of choice or doing home listening trials of some amps of differing philosophies.
In a few weeks, folks can describe what they heard (or didn't hear) and we can see where things go from there.
spice and handwaving is fine and fun, but it's time to put the cars on the road and drive ...
mlloyd1
janneman said:
mlloyd, to be perfectly honest, in the face of decades of claims yet to be substantiated (I was hearing the no feedback mantra back in the 70s), I have enough things to do than to try to prove someone else's hypothesis. If someone else actually gets a reasonable controlled subjective result or can propose a reasonable, testable mechanism, I'm all ears, as it were.
<reaches down to lift jaw off the floor > The answer is "no". Indeed, it is not really an assumption it is an experiencial fact. But I can't conceive of why you challenge it so I can only imagine I'm not understanding you.
I do think mlloyd1's prescription is good medicine.
John neither believes in nor performs any sort of controlled subjective evaluation to substantiate his claims. Indeed, he feels (not without some justification) that there's no need to. That's his right.
For me personally, I just couldn't imagine releasing a product without hard data supporting my design choices. I've never done so in my professional (admittedly non-audio) careers.
For me personally, I just couldn't imagine releasing a product without hard data supporting my design choices. I've never done so in my professional (admittedly non-audio) careers.
That's several valid methodologies. There are others, too (ABX is not the only way to do controlled testing), but the key is that the listener has to be able to distinguish between two things with no other cuing than the sound. If he can't, claims of differences are just simply not supported; the claimant can either say, well, I guess I can't really tell the difference, let me now find someone who does, or say, I screwed up the test design and here's the way I'll take care of that objection.
Or the claimant can simply say, go p!ss up a rope, I'm not going to prove anything to anybody. That's OK, too.
This is all pretty OT, so my apologies.
Or the claimant can simply say, go p!ss up a rope, I'm not going to prove anything to anybody. That's OK, too.
This is all pretty OT, so my apologies.
I think we are at cross purposes. I'm talking about the R&D process and I think you are talking about marketing and quality control. Do you design circuits?
No, but I do design other products which require subjective sensory testing, even in the R&D phase (maybe especially in the R&D phase!), so have developed some experience in that area.
traderbam said:
John Curl's JC-1 uses 34 dB negative feedback with a 6 kHz open loop bandwidth, if I recall correctly. Some people would not characterize that as particularly minimal.
Cheers,
Bob
janneman said:
Yep. Precisely.
That way, we avoid some sort of murky phase problems (if my operating hypothesis is correct--which I do not insist upon--if someone else comes up with a better idea as to why/how NFB screws up the image, I'm all for it), yet have pretty decent performance in the measurement department with the arguable exception of damping factor, which I think is overrated anyway. I mean, really, if damping factor were the panacea for all bass ills, every low Zout circuit on the market now or in the past would have phenomenal bass, but it just ain't so. Some do. Some don't. We're lacking a 1:1 correspondence to back that claim and the fact that some higher Zout circuits are capable of good bass also tends to argue against it, so I'm not all that worried about damping factor.
One place where I think feedback may earn its keep is in servo woofer systems, where the feedback loop includes the driver. When we're talking whole percentage points of distortion--I've seen figures approaching 10%--then I'm game to put feedback there. I that case, I see the cost/benefit ratio as being more favorable. In the case of electronics, I'm able to throw most any old circuit together and have it come in below .5% distortion. After that, I fiddle the thing until I get it below .1% and I'm happy. But drivers? Uh, unh! I'm not set up to build drivers from scratch and I haven't got any cute ideas that haven't already occurred to Scanspeak, Seas, et. al. I've heard a few servo sub systems and was generally impressed.
I have an entire notebook that I've filled with scribbled notes, schematics, papers, and such relating to the whole servo thing, but never seem to find the time to try any of them out. But to reduce the gross distortion from, say, 5% to 1% or less...yeah, I can see feedback playing a role in that. One of my ideas might even be fast enough to apply on up into the midrange, but I draw the line at attempting anything above a couple kHz and even that might prove unattainable.
I also want to be able to back out of a servo system cleanly if there turn out to be unexpected problems, either in execution or some unanticipated sonic penalty.
So, it's not that I'm pathologically opposed to NFB under any circumstances, ever, period, no way...it's that I'm unwilling to pay that imaging penalty or the upper midrange glare. And if I can flog a circuit into decent distortion specs without it, what benefit do I get from NFB? Lower distortion? It's already below the threshold. Wider bandwidth? My goal on anything I build on my own is 200kHz minimum, preferably 250. At the moment I'm running twice that (I have measured the bandwidth--it's actually over .5Mhz, but I'm rounding down). Do I really want to go to 2 or 3Mhz? Higher still? Nah. Maybe next time. I just don't see why I would want to claw my way up to where I'm able to put something together that performs that well (albeit by standing on John, Nelson, and Charles's shoulders--thanks guys) then gunk it up by adding bells and whistles. It just ain't elegant, you know?
Grey
I note a deafening silence on the point made that both non-feedback and feedback amps get first class reviews from sophisticated reviewers.
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