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Old 13th March 2013, 12:45 AM   #8621
jcx is offline jcx  United States
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Ed Cherry's JAES articles are a good source on these issues
without a input buffering ef the added degeneration of a current mirror driven "VAS" Q is largley a wash - the increased open loop input Z increases loop gain with the current mirror drive by about the same amount as the degen reduces the gm
Cherry does claim some dgen aids high frequency stability in his analysis of the standard RCA/Lin audio amp topology
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Old 13th March 2013, 02:53 AM   #8622
a.wayne is offline a.wayne  United States
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Gentlemen Mr Gordon Gow ......!

Gordon Gow & the McIntosh Labs Unity Coupled Transformer - Audio Classics Ltd. - YouTube
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Old 13th March 2013, 07:54 AM   #8623
dvv is offline dvv  Serbia
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Quote:
Originally Posted by DF96 View Post
I don't care if it is 4Hz! I'm with Self, Putzey et al on this issue.
Fine, no problemo, to each his own.

Obviously, I am sitting in the other camp.
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Old 13th March 2013, 07:57 AM   #8624
dvv is offline dvv  Serbia
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Won't play for me?
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Old 13th March 2013, 11:23 AM   #8625
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Originally Posted by DF96 View Post
It is local feedback, but only at low frequencies where Cdom does not do anything. How much feedback it applies at LF depends on how the stage is driven. The usual current-mirror in the LTP collectors will provide a high impedance drive so the resistor won't do much except to raise the VAS input impedance, which could raise LTP distortion by a tiny amount. A simpler resistor drive from the LTP will be affected more, but these are not seen so often nowadays.

If the 16R doesn't do anything then you won't hear any change! The bare transconductance of the VAS BJT will drop from about 0.3A/V to 0.05A/V, which will affect things even when Cdom is active. Careful simulation may show exactly what happens.

" Doesn't do anything " is a statement about my measuring equipment . The nicest compliment I had , I took to be an insult . An amplifier I built for a lady friend , she had measured by another friend in Austria ( already furious , I was gutted ) . I had no idea he had an Audio Precision measuring station . His comment was in German " what planet is Nigel from " . I was so upset and I don't mind saying that question has been part of my life especially when at school . After about 4 years I asked exactly what he meant by it ? He said something like " I have studied all my life and built many things , I never built one like this " Apparently he just couldn't find a flaw in it and was very depressed about it . When he asked I said . I only use measurements to keep me on coarse . Mostly I listen . Sometimes it takes me 20 years to know what I did . I will never accept bad is good . If bad does sound good I ensure good sounds just as good . Recently for valve designs I have allowed good to be 1% distortion . My amplifier I made for the lady would probably in 1970 be taken to be a 0% distortion amplifier . It sounds sweet and not unlike a 211 amp .

My problem with measurements and simulators is it is " painting by numbers " . However I don't object to a camera obscurer being used ( you choose what that is ) . BTW DF 96 , you are 100 % right , however I do not have enough days in my life for measuring equipment to catch up with my ears . Not least a test station I could buy . Most of my test gear I built . J L Hood comes closest to my hero if asking .

A Paul Kemble web page - working with sound, some tips.
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Old 13th March 2013, 11:41 AM   #8626
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Originally Posted by dvv View Post
Fine, no problemo, to each his own.

Obviously, I am sitting in the other camp.
4hz is fine . I bet your amp can be made into 4 Hz . Just sling those emitter resistors away .

Seriously . As long as the closed loop works why worry ?

I was watching about dust mites today and thinking about the ones that live on cheese . I rationalized this by saying the mites are made of cheese . The weirdest bit is I eat them with only kind thoughts , we both love cheese . It' s a choice and is my way of coping with the reality that science brings into my life .

Like amplifiers etc . The more nasty a cheese is sometimes the better it is ( for nasty read French ) . If not our hi fi will be like processed cheese . Doubtless the best cheese in the world is processed , the measurements I suspect prove it ? I rather like Philadelphia so not complaining . I never mistake it for cheese and I don't mean the state of if asking .
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Old 13th March 2013, 01:28 PM   #8627
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One other thing . I had this argument with my boss years ago about measurements . He had trained to the highest levels as an engineer . 7 years of study which was ruined by the war . Mostly he was a paper engineer except one famous connector called Hypertac ( still in production ). He did concede one point . He said even the smallest tyre maker would have a man who would know exactly what all the others know in the other companies . For all that tyres were an art . His statement to me was that empirical engineering can be a fast track to success . The mystery to him was how the empirical side seemed to be dominant , however the production side used science to make it repeatable . I thought it a very fair way of seeing it . I forgot to say " Easydrive " eventually bought by Toyota was one of his . The worst problem he had was being a dreamer and being stopped by people supposedly his superiors from running his bit of the company ( Smiths Industries ) . When I went to work for him he asked me to build him an organ . We never got past the asking .

His favourite question to me about an amplifier in his deliberately broken English ( he could speak 21 languages , 7 fluently including Russian ) " Nigel does it have the discerning ability " . I loved that and it sort of makes sense . For a true scientist he could bend a little . He worked on the JET project for nothing as a translator . It probably kept it in the UK as the Germans had no idea we were so advanced in fusion ( his opinion , I can beleive it ) . Even though the Germans team spoke English Emeric could speak technical German . That work came from him visiting Harwell one day trying to sell them something ( high speed video ) . He came away with an unpaid job . He understood Fusion at the level of the engineers , he bored me to tears about it ( just kidding ) . When I got married he wrote the formulea for fusion in my wedding photo album saying here is your future ( tritium and helium ) . He died the night Hallie's comet passed our planet ( I was flying over Beograd Dvv at the time , honest ) . I fancy it came for him.
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Old 13th March 2013, 01:45 PM   #8628
fas42 is online now fas42  Australia
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We need the dreamers ...

Thanks for that, Nigel,
Frank
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Old 13th March 2013, 01:52 PM   #8629
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I know . I even caught broken English from him ( truly ) . He tried to teach me Latin without success . If I got it wrong it was because I am " English " . If we annoyed him he would refer to his time in intelligence and silent killing . He said it to one of my friends who instantly pinned him to the floor and said prove it . That sort of sums up the paper engineer , it's all talk .
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Old 13th March 2013, 07:17 PM   #8630
dvv is offline dvv  Serbia
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Nige,

From our personal correspondence, we both know we have similar views regarding engineering by numbers and by ear. We agree measurement is an excellent way to keep us on track and limit our flights of fancy.

Unlike DF96, I do care what my open loop bandwidth is and it has to be at least 50% wider than our hearing bandwidth, i.e. it must not be less than 30 kHz. Given my age, I guess I could be happy with say 24 kHz. More is better, but I won't go head over heels to make it ultra wide at any price. If I can do say 100 kHz, I will gladly trade it off for simple, plain, old fashoned stability. If it's not reliable, I don't want it.

I do not believe in extremes regarding global NFB, I neither like zero, nor 60 or 80 dB of it. I believe in moderation. I do not claim to be the Buddah of it all, that's just my view, no more. A rule I choose to live by, but by the same token, others will choose their own rules.

My reason is simple - I believe this approach has a better chance of producing a good sounding amp than either of the extremes, but I agree that maximum global NFB can produce excellent results here and there. Zero global NFB has not, to the best of my knowledge, produced any memorable (for me) results yet

I understand why say Thorsten loves FETs so much, it's just that I am not convinced that FETs by default sound better. Surely they can here and there, but so can BJTs as well, and tubes, etc.

Only my hearing is "by deafult", everything else varies. As I grow older, my deault will tend to decrease, and there's nothing I can do about it except wash my ears regularly.
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