Distortion and Negative Feedback

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#321
jcx said:
although "single ended pentode" is likely a pretty good aproximation to a "no feedback amp" - as long as you don't use a cathode R (and bias up those "extra" grids like the tube designers intended)[/url]
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You can't have it both ways.

If triodes have internal feedback, then so do pentodes,
since they exhibit a finite plate impedance due to their
"feedback".

😎
 
#322
As engineers we can perhaps argue over what defines "good approximation" in terms that make sense to both of us

I am happy to engage in the original meaning "dialectic" - not that I think we really have uniformly "opposing" positions or that every engineering issue can be reduced to simplistic "two sided" sets of propositions with simple binary true/false resolutions

(which doesn't mean that there's no definitions, standards that have to be agreed - and are agreed by the EE community - some statements can be "nonsense" or "true/false" by the rules that a community like professional EEs use to "construct"/express/represent knowledge )

for tube internal feedback we could continue in my thread in the tube forum?

the better measure of internal feedback in vacuum tubes is probably (inverse) "mu" - "mu" is easily 100x higher in pentodes vs triodes

so pentodes would seem to at least qualify as ~ "100x better approximations" to "no (internal) feedback" amplifying devices by having ~100x less internal feedback?

my tube expertise is pretty limited to having been the only one with any idea of how to replace/rebias/maintain a Dynaco tube amp in a community music room in my college days so I could easily be wrong – care to put up some numbers to compare or point to some good tube modeling sites?
In one of those life ironies George Valley hung around the undergrad program I was in but before I realized I wanted to do electronics he fully retired

Since my education in control theory started with Mech E courses and my career has involved motion control with a variety of motors, hydraulic valves as well as electronics I do think I can figure out diverse "gain device" parameters if anyone wants to “put some numbers” on the floor to debate

this source kind of fades away before reaching pentode properties: Tubes 201 - How Vacuum Tubes Really Work


last 2 posts added to http://www.diyaudio.com/forums/tube...e-internal-negative-feedback.html#post2492053 if Nelson really wants to keep this thread closer to the subject of his article tube internal feedback debate should continue there
 
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#327
jcx said:
[snip]some others here have as Jan points out apparently have gotten hung up on detailed "semantic analysis" of simplified "feedback" definitions that require "feeding back" the OUTPUT signal to the input ("in anti-phase" seems to be another stumbling block)
[snip]
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Linear Audio Vol 1, which will be published 1 April, has an article by Bruno Putzeys under the apt title: "The F-word".

His definition:

“Feedback is an arrangement where an amplifier is made to respond to its own output signal in addition to the wanted input signal and any unwanted disturbances. When the response to an unwanted disturbance is smaller with feedback applied than without it, we call it negative feedback.”

Now I fully realize that if you're not coming from an engineering discipline, if you enjoy building audio amps for a hobby, you most probably aren't interested in such a formal definition. But it seems to me that anybody would benefit if we all speak the same language. Suppose you go to Spain and start to talk about your ''carro" to the car dealer. After much confusion they tell you that in Spain they use 'coche' for car. Are you going to insist that they follow your 'carro' because that's what you think it is?

jcx said:
[snip]it can be seen as consistent in a very fundamental way when you apply Blackmann's relations to calculate input and output impedance as a function of "loop gain", when you use Bode's sensitivity analysis, calculate bandwidth, or the distortion reduction/harmonic order multiplication (as explained yet again, at length in the Cordell Feedback thread)

then you can go o the lab and see these behaviors in real circuits do follow the theory

its worth repeating: "there is nothing so practical as a good theory"
[snip][/B]
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That was my point also. If you assume it's feedback, and you calculate and measure, and it acts as if its feedback, it seems folly to persist it isn't. Except for marketing reasons 😉

jcx said:
[snip]"there is nothing so practical as a *really* good theory"[snip]
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Ludwig Bolzmann IIRC?

jan didden
 
#330
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#332
jcx said:
As engineers we can perhaps argue over what defines "good approximation" in terms that make sense to both of us
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We are of course actually in good agreement about reality -
the discussion is about terms. The marketplace of ideas
being what it is, we could as easily be talking about the
weather - there is no probable resolution.

I too have developed a greater interest in tubes lately,
as I have been playing with SITs, which have a very
triode-like characteristic. Rather than thinking about
feedback, my focus is on their sonic signature, which I
find fascinating. If your tube discussion wanders in that
direction, you will find me lurking there.

BTW, given that this whole endeavor is entertainment,
I would be properly classified as an artist, not an
engineer or scientist.

😎
 
#333

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#334
janneman said:
... Suppose you go to Spain and start to talk about your ''carro" to the car dealer. After much confusion they tell you that in Spain they use 'coche' for car. Are you going to insist that they follow your 'carro' because that's what you think it is?...jan didden
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After getting that straightened out, the dealer shows you what he calls an SUV but you think it's just a station wagon. What he calls a sportscar is just a sedan with 2 doors to you...they have the same issues!
 
#339
careful, you want hip boots when walking near some audiophile "artists"

but (at least when slightly pressed) Nelson seems to show perspective


the simple approximation is that the follower has "open loop gain" of gm*R_load

R_load includes Mosfet source terminal bias R in parallel with amp circuit load

the closed loop gain is then

gm*R_load / (1 + gm*R_load)

as long as gm*R_load product is >> 1 then small changes in load or device gain have only ~ 1/Aol, ~ 1/(gm*R_load) effect on the closed loop voltage gain

in a "no feedback" amp any change in load or gm would give a 1:1 change in circuit gain


I would call audio power amp design mostly "conceptual art" at this point - people pay for "concepts" embodied in the marketing story even when controlled listening doesn't reveal perceptible differences

not to say that there are no amps that sound different "by design" - but the 1st order dbt established audibly perceptible difference appears mostly "explained" by frequency response (which may include amp output Z/speaker load interactions)
Mr Curl seems to claim he never chooses distortion/"coloration" despite being in the low/"no" feedback camp
Pass amps appear to strive toward "transparency"/engineering accuracy - with the FirstWatt and diy designs/articles mostly showing low measurable distortion with "simple" topologies

the big audiophile magazine reviewer "night and day" differences seem to be clear only to people that do sighted listening without level, frequency response matching, blinding protocols

likewise the stated and by now "assumed correct" "official audiophile position" is that low/no feedback is "audibly superior" - but as far as I know this hypothesis hasn't been shown to be supported in controlled listening tests
 
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#340
jcx said:
likewise the stated and by now "assumed correct" "official audiophile position" is that low/no feedback is "audibly superior" - but as far as I know this hypothesis hasn't been shown to be supported in controlled listening tests
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I'm not even sure that 'low/no feedback' can be listened to in isolation. Too many other circuit aspects change when feedback is changed. So the best that could be achieved by controlled listening would be that low feedback is found to be correlated with perceived sound quality, not that it causes better quality.
 
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