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Old 18th August 2012, 05:12 PM   #101
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Quote:
Originally Posted by CopperTop View Post
Anyway, although EQ is still pre-distortion by certain definitions, it's off topic for this thread! (But I feel better now I've got it off my chest).
EQ of speakers is the only pre-distortion that worth discussing, the rest is from the field of fantasy.
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Old 19th August 2012, 08:04 AM   #102
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Originally Posted by CopperTop View Post
If you took a conventional amplifier with NFB and 0.005% THD or whatever, are you saying that there is nothing you could do to 'null out' the remaining THD (and IMD of course) with just a smidgeon of the right pre-distortion?
I saw a simple MOSFET amp with single input transistor take a distortion analyzer to it's sensible limits . It had a simple moderate current SE FET amp stage drive the class B dead band in feed-forward via a capacitor . This is something I have tried many times without any positive result . The guy if I remember dropped it as he felt he could hear no difference . I was doubly impressed as a piece of Douglas Self dogma was proved wrong . There was no significant distortion by using a single transistor input . I must say the supposed single input extra stability people talk of has no reality in the world I live in . When it works it is due to high input capacitance transistors .

I was told if great care is taken to make the Quad 405 work to the best possible null then magic happens . In the opinion of the man who did it Quad were making a cheap amplifier and ruined the concept . Sansui did it better and my Kenwood KR 750 also ( not sure , no circuit dia ) .

Pre- distrotion is something to know of . I must build a new oscillator as the - 80 dB I get from mine is not good enough .

Last edited by nigel pearson; 19th August 2012 at 08:07 AM.
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Old 19th August 2012, 09:59 AM   #103
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My memory of the circuit I mentioned seems slightly wrong ( feed-forward ) . However the reading is good . What I like here is all of the no-no's of modern thinking are rejected ( MOSFET's , single input , capacitor coupled ) .

MJR-8 Mosfet Audio Power Amplifier
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Old 19th August 2012, 03:20 PM   #104
jcx is offline jcx  United States
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I have posted working sims showing Quad's and a "parallel Black's Feedforward version from the Vanderkooy and Lipshitz paper

this web site has publication references http://digilander.libero.it/paeng/fe...d_concepts.htm

Quote:
Originally Posted by jcx View Post
I've read up on Black's Feedforward, Quad, Vanderkooy (…and Annison, Danyuk, Sandman, Stochino) paper's circuits, simmed a few alternatives

after thinking about it I come to the question that if you can measure the error to amplify and add in to cancel the main amp distortion - why don't you just use more feedback?

for audio power amps we have the ability to use plenty negative feedback loop gain over an extended definition of audio frequency range if allowed to use RET or MOSFET output Q to push unity loop gain intercept up and use higher order compensation, or nested feedback (including Bob's, Hawksford's EC)
feedback error disappears into noise for really high loop gain feedback amps except for the last few octaves of audio

the feedforward schemes can knock down the >20 kHz errors ~ 20-30 dB – but at some cost in parts, design complexity for arguably inaudible “improvement”

and the cancellation is very sensitive to gain/power coupling network tolerances

remember also that the audible IMD products are reduced by the feedback at the product frequency – so any high frequency difference products folding down into audio are reduced by the high loop gain feedback

feedforward may just reduce Bob's THD 20 kHz metric without adding to/giving any of the implied "goodness" at actually audible frequencies that we hope the THD20 correlates with


Alternative topologies


High loop Gain Composite Op Amp Circuits
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Old 19th August 2012, 03:52 PM   #105
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The essence of current dumping is feedback. When 2 transfer functions start overlapping feedback linearizes the result. That "bridge" is the result of usage of slow opamp and slow output transistors. The author did not expect them to work nice, so made frequency response worse, but predictable, to balance it better. In 1970'th I independently made similar amp in class A+C, without any "Bridge": I used faster opamp and faster transistors. Nested feedbacks did the magic charm.
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Old 19th August 2012, 08:24 PM   #106
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The positive thing I read about feed-forward is it doesn't require massive bandwidths to be able to get a reliable 50 kHz of usable bandwidth ( even in germanium ) . The example I show is a very simple amplifier . Please go to the home page and read on . The complexity is low compared with class D . I feel that the Quad was bad news for a good idea . If the class A was up to 10 watt and used MOSFET's so as to have gradual turn on all would be better . I completely go with no absolute need for the bridge . All my versions were that way .

JCX I read your description before and like it very much . I seem to remember Mr Black was on a Mississippi river boat when negative feedback occurred to him ( 1929 ? ) . Mostly to stabilize gain . Apparently after 5 repeater telephone amplifiers the overall gain was highly unpredictable ( voltage of PSU etc ) . Negative feedback meant telephone lines needed virtually no maintenance after the invention of NF , just the occasional valve change . I as you can understand never met the man so can not vouch for this account .
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Old 22nd August 2012, 12:43 PM   #107
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I like this description . To say feedback is pre distortion is the simplest explanation . If very fast it must be very similar ? I guess complimentary input topologist look to eliminate the work of the feedback circuit ? My guess is very fast feedback might be preferable ? The idea that NPN and PNP might be the same and do cancellations looks a bit dodgy . Good old negative feedabck will sort it out in the end . Doubtless the harmonics die down quicker if good cancellation before feedback ?

http://www.tubecad.com/articles_2001...pInvDisAmp.pdf
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Old 22nd August 2012, 06:49 PM   #108
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Yes. If the amp, each stage, had been linearized before applying global feedback, and if it's bandwidth is good for good feedback on the top end of the frequency band, it is the right direction. However, local feedbacks linearizing each stage decrease open loop gain decreasing "amount of global feedback", but it is the matter of optimization.
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Old 23rd August 2012, 01:47 PM   #109
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It occurs to me that pre-disortion should be very slightly nicer than feedback as there is no time delay . Various valves seem very similar so the fit of the antiphase curves can be reasonably good . However output impedance is higher without the loop feedback .

Distortion can almost meet DIN 45500 ( 8 W 1% ) without loop feedback ( SE amp KT 88 ) doing this . Certainly so at 2 watts . Without pre-distortion 10 % is more typical .
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Old 30th August 2012, 09:20 PM   #110
JZatopa is offline JZatopa  United States
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Originally Posted by Wavebourn View Post
However, DSP-controlled analog EQ that uses motorized potentiometers, inductors, relays, would be a real dream-machine...
I don't want this to digress into a discussion on EQ and DRC but could you tell me what benefit you would get doing this in the analog domain instead of the digital domain? It seems to me that the con's far outweight the pros when it comes to EQ.
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