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Old 12th March 2013, 10:03 AM   #8601
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Quote:
Originally Posted by ginetto61 View Post
Hello !
I have a curiosity.
Let's take, as an extreme, a single ended zero feedback tube amp
then select a speaker that sounds very good with the SET above mentioned.
Then take a solid state high feedback amp and use it with this speaker
What would it be the sound ? bland and clinical ?
I do not know really, but maybe it could be a worthwhile test
The comparison must be made with same speakers.
I think it is almost never the case
Kind regards,
gino

Sid Smith told everyone that they should add an 8R resistor to any amp then listen again if preferring SE valve amps . I am so lazy , I never did . I made friends with Sid when I said " Inside every good triode there is a good pentode trying to get out " . I won my argument after 2 hours at a diner in Long Island . Sid gave in and said the best triode he knew of was the EL 34 . Sid made a version of the Model 9 using 300B to satisfy a friend . Personally I preferred the EL34 version ( much tighter ) . Within reason a modernized Model 9 is the best amplifier I ever heard ( for accuracy ) . It should be said I hate euphonic amps although love it when barely obvious . The best amp of all perhaps is one made by Mr Furiama using 2A3 and what looks like an op amp in valves as the driver . It is SE. No hint of euphonic traits and crystal clear . Mostly I dislike valve amps , not least my own design .
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Old 12th March 2013, 11:05 AM   #8602
DF96 is offline DF96  England
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Quote:
Originally Posted by ginetto61
Let's take, as an extreme, a single ended zero feedback tube amp
then select a speaker that sounds very good with the SET above mentioned.
Then take a solid state high feedback amp and use it with this speaker
What would it be the sound ? bland and clinical ?
It would depend on what was meant by 'very good' with the SET amp.

If 'very good' means flat, well-controlled frequency response (despite the high amp output impedance i.e. low damping factor) then the speaker must have high mechanical damping. In this case running it with a typical SS amp will result in poor bass, as it will be overdamped.

If 'very good' means 'plenty of bass' (due to an underdamped resonance) then the speaker might sound fine when used with more appropriate DF from an SS amp. The SET-lover will wonder where the bass has gone, having become used to a bass peak which should not be there and is no longer there.

My hunch is that the latter is more likely, although both are possible.

Quote:
Originally Posted by nigel pearson
I was tempted to say degenerate the VAS a little . Arguments rage about that and as Douglas Self wrote to me it is the worst idea of all as it doesn't bring the expected benefits . I do it because it helps match stages better ( lifts the base voltage for one ) . Sound wise 0R = punch 47R = soft and spacious 16 R = just right .
In most amps, above the open-loop pole, the VAS acts as a transimpedance stage. Adding degeneration will degrade this, raising its output impedance and so bringing the possibility of more crossover distortion due to the non-linear input impedance of the output stage. It may be that what you were actually doing was adjusting the LF damping factor by reducing global NFB, until the amp and speaker combination matched whatever your ears decided was the right amount of bass resonance damping. In my view this is not the way to design hi-fi, although it could be fine for a guitar amp with build-in speaker.
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Old 12th March 2013, 11:50 AM   #8603
dvv is offline dvv  Serbia
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Quote:
Originally Posted by nigel pearson View Post
... I prefer 33078 over NE5532 . The one thing I would like is a MC33077 single version with all the 5534 tweaks ( comp , offset ) .
One day, Nige, you'll discover the virtues of op amps such as AD 826 and AD 829, to name but two.

I had many a similar discussion with Graham Slee some years ago. He also stuck to The Great British Tradition of NE op amps. While it is indeed a good op amp, it was top flight like 30+ years ago.

People have learnt and worked for the last 30+ years, you know.

And, as I have said many times, I DON'T think it's the wild voltage slew rate of 200+ V/uS, it's their short settling time, well over 20 TIMES (sic!) shorter than in case of the NE family.
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Old 12th March 2013, 12:01 PM   #8604
dvv is offline dvv  Serbia
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Quote:
Originally Posted by DF96 View Post
...

In most amps, above the open-loop pole, the VAS acts as a transimpedance stage. Adding degeneration will degrade this, raising its output impedance and so bringing the possibility of more crossover distortion due to the non-linear input impedance of the output stage. It may be that what you were actually doing was adjusting the LF damping factor by reducing global NFB, until the amp and speaker combination matched whatever your ears decided was the right amount of bass resonance damping. In my view this is not the way to design hi-fi, although it could be fine for a guitar amp with build-in speaker.
Surely, that depends on how much degeneration you throw in?

And, I suspect much more on whether that degenration is an afterthought, or was designed for?

Which brings me back to my initial statement - low global NFB will really work only if the amp was designed for it from scratch. Reducing it as an afterthought will usually produce a mish-mash sound, neither here, nor there. It will also wreak havoc on the amp's stability, methinks.

On the topic of local feedback (degeneration), it should also be used with as much prudence as global feedback. Ultimately, most amps will use about the same amount of overall feedback (or thereabouts, assuming nothing went seriously wrong), it's just a matter of how you disribute it among local and global.

This is something you have to have a firm stand on before you even start the simulator. In the course of your work, things might change a bit here and there, but generally, you need to stick to your initial idea and work at it.

This is why I usually say I believe in moderate global NFB, somwhere in the region of 20-26 dB approximately, but I wouldn't object a 1 dB more or less if it does the job.
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Old 12th March 2013, 12:22 PM   #8605
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That's right and Mr Self is right . I am very glad you said trans-impedance as some say voltage amplifier stage ( OK VAS and why not ) . However sometimes it works to corrupt the VAS a tad . I wrote back to Mr S and said in real life his current mirrors do not balance . I ventured to say 0.55 to 0.7 V is too close to the typical Vce of his current mirror . Adding a small bit of emitter resistance might do wonders . I did find a BCV 61 ready made current mirror would work better ( ideal ) . Some would look at it's modest voltage spec and reject it . Daft as the 0.7 V VAS clamping solves that problem . Next there is the effect of the VAS input impedance . Lets say the current is 8 mA and u = 100 , that is circa
Z in = 320 R . Whilst it is not always true it would be ideal to feed that from at least 100 R . In the Goodman's I gave as example the resistance was somewhere on a 2K5 pot . To be honest a bit of current mirror mismatch might help an amp ( more 2 nd harmonic albeit it a small amount ) . I was being pedantic to tell him that .

Some people put a series resistor into the VAS base . Often with no comp cap ! I can see why . It also looks nicer on a scope . The standard VAS usually looks like a triangle wave if sine-wave in ( VAS base ) . That is because it is working as I to V . I still think the triangle wave causes a ghost in the machine if below 1 : 1 ratio . One thing people get wrong is transistors are great current amps ( look at the curves ) . The modern transistor amp hardly does any voltage amplification ( and doesn't have to ) . Thus when carefully forced to be a current amp it works with near perfect linearity . Class AB throws a fly in the ointment . Also the VAS ( TIAS ? ) .

I think the LF factor is unlikely if taking Mr Self's graphs of that very thing . It looks as if there is a 40 dB surplus . He shows a way of degenerating the amp so as to have equal feedback over the audio band ( all from memory so forgive if I am wrong ) . As he says all very interesting but what is acheived ? Having said that it would be consistent with some of the qualities I heard . Certainly to loose HF correction by doing it is pure folly . 16 R seems to show no measured difference at HF ( strange but true ) . 47 R I didn't like . I don't seem to need any frequency compensation in my feedback loop so maybe what I loose one way I gain in another ? I also find excessive gain works rather well . I use a gain of 100 in one of my 100 W amps so as to feed in any source via a pot . A freind who insisted on using the amp with 100 dB sensitivity speakers had excessive hiss . Even that could be cured . I proposed to use 6 x 2SA 1085 for the input with bootstrapped input resistors and gain switch ( 2K ) . Even with a gain of 100 distortion was excellent ( even at 50 kHz ) .

I have looked at getting BCV 61 and BF 720/21 encapsulated with wires . A job for India ?
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Old 12th March 2013, 12:36 PM   #8606
jcx is online now jcx  United States
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Quote:
I believe in moderate global NFB, somwhere in the region of 20-26 dB approximately
its interesting how these low numbers of global feedback keep getting bandied about - this is still in the region where Baxandal's "harmonic multiplication" math shows some higher harmonics may be increased by the negative feedback

while >40 dB over audio bandwidth is quite practical today with RET or MOSFET output Q speed - the Linear Audio Putzeys article is unusually clear but the result that negative feedback should be maximized when used at all if you value accuracy is a standard feeback theory result

http://www.linearaudio.net/index.php...d=67&Itemid=89 (you have to cruise the site for the free sample of the article)

and at higher loop gains the higher order harmonic "hump" gets pushed into the noise floor - all visible harmonics are decreasing with order

Last edited by jcx; 12th March 2013 at 12:41 PM.
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Old 12th March 2013, 12:38 PM   #8607
dvv is offline dvv  Serbia
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Quite few years ago, I read a paper on amp design by somebody from Harman Kardon, don't remember who. Worse, I failed to keep that file for some reason other than sheer stupidity on my part.

Anyway, the gentleman had a clean and clear point. Initially, let your input stage work with a degenerated gain factor of 20 dB, but make sure it's working with better than 1 mA of bias current per trannie and into lower collector resistors (do not go above 2k for a power amp, preferably 1.2, 1.3 or 1.5 k). Assume all is well and go on to complete the VAS and the rest of it.

When testing, you cannot avoid some comensation here and there stability's sake, but the final adjustment of overall gain should be done only by degenerating the input stage. It should not have a gain factor of less than 14 dB and no more than 20 dB for best overall results.

About the only thing he didn't mention was how high up can you move with bias current for the input stage - OK, no less than 1 mA/trannie, but how high up: 1.1, 1.3, 1.5 mA, how far?

I took his advice and am to this day grateful for it.
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Old 12th March 2013, 12:38 PM   #8608
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One thing that occurs to me . Would an amp with an adequate feedback factor cope better with a complex load than one with " excessive " ? My instinct it to think it would . I would expect that amp to have better stability ? How many of us are brave enough to use complex loads when testing ? I' m not . I don't mean 8R 2 uF .

Excessive = where laws of mechanics says not required . Too much of hard fact in electronics would not be so in mechanics . Damping factors of 3 are thought to be effective in mechanics ( WW circa 1955 ) .
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Old 12th March 2013, 12:42 PM   #8609
dvv is offline dvv  Serbia
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Quote:
Originally Posted by jcx View Post
its interesting how these low numbers of global feedbak keep getting bandied about - this is still in the region where Baxandal's "harmonic multiplication" math shows some higher harmonics may be increased by the negative feedback

while >40 dB over audio bandwidth is quite practical today with RET or MOSFET output Q speed - the Linear Audio Putzeys article is unusually clear but the result that negative feedback should be maximized when used at all if you value accuracy is a standard feeback theory result

Putzeys Bruno (you have to cruise the site for the free sample of the article)

and at higher loop gains the higher order harmonic "hump" gets pushed into the noise floor - all visible harmonics are decreasing with order
I can't speak for others, but I do not "bandy" these number just for the hell of it.

I own two amps, one using 17 and the other 12 dB of global NFB, both by HK, and I find them to sound better than most of the others I get to hear.

The few units I built myself also sounded better with lower global NFB, but then, they were designed rigth from the bat to need less global NFB.
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Old 12th March 2013, 12:53 PM   #8610
jcx is online now jcx  United States
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Cherry, Cordell took on "interface distortion" speculation in JAES articles - there is no "problem" with feeback at audio frequency and output load within the design's drive capability

what can happen is that feedback gain stages "wind up" while the output is clipping as the feedback tries to push the output harder to correct the error it sees

then on recovery from clipping there is delay - "rail sticking" as the feedback gain stages have to slew back to normal operating range

standard RCA/Lin dominant pole amps with unity gain output seem to be acceptible, higher order "2-pole" compensation can give oscillation burst or ringing on clipping recovery
clipping recovery can be improved with various clamping arrangements, diodes sometimes seen across diff pair collectors, current limit in VAS/TIS


if you have enough power, limit the input signal before the amp then even higher order compensation can be used if the amp itself will never clip
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