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Old 1st October 2012, 07:56 PM   #7341
dvv is offline dvv  Serbia
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Nige, global feedback does nothing for the behavior of individual stages, which could compromise transients even up to the level of indivual stage overload, and in the extreme, clipping.

I agree with Wave.

I still maintain that no one single approach could ever be ideal, it takes a balance of local and global to produce truly ideal sonic results. This also means tons of work, especially fine tuning after it's electrically operational.
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Old 1st October 2012, 07:58 PM   #7342
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BTW, Nige, of exactly what use is a unity gain power amplifier?

To give some oomph to an op amp output?
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Old 2nd October 2012, 09:13 AM   #7343
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Hi DVV , Hi wave .
I love that EL84 amp . I must build that as I have all the bits . It is like something I had in mind .

The idea of unity gain power amplifier is to see if it will work just as a buffer . It will struggle . My feeling is we should be able to ask it to do that . In inverting mode perhaps ?

The point about local feedback is it is a useful thing and sometimes is the key to the difference between competent and excellent amplifiers . My brother modified many commercial amplifiers using it . His amplifiers would be more musical some said .

When I use local feedback I draw that resistor or circuit in purple to say it was designed by ear . This means I found a critical valve . My 8 mA VAS often have 15R purple VAS resistor . This is not strictly speaking feedback . It is to lift the VAS up 0.12 V which helps the current mirror ( 0.65V - 0.2V = 0.45 if you are lucky then - 0.1 V balancing resistors , + 0.12 V is useful ) . I can not say for certain the sound improvement is due to mirror balance , easier to drive VAS input impedance or local feedback , or all 3 ? All I know is it works . Below 47 R all measurements remain about the same . The difference is the sound becomes softer as the resistance climbs . If listening to rock music all night to short out the 15 R would be preferable . Note I can not measure any real difference . The VAS is correct even at 0R I should add . I usually try to get 1:3 impedance ratio if I can , it might be 1:1 at 0R . The most complicated VAS would be something like a Darlington cascode . With a little care a 1 x 2N5551 will do the job . The interesting thing is the VAS is described as a trans-impedance amplifier . That is it is neither a voltage amp nor current amp . It is a form of I to V converter . The preferred option is to make it into a voltage amp especailly if using a Darlington . My instinct is to say what a very bad idea . My experience says do it . So I design to make it > 1:1 . Many amps were < 1:1 . I suspect when TID was talked about that ended ? My ears did not like it although it measured fine .

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Old 2nd October 2012, 11:54 AM   #7344
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@Nigel

Well, if it's simple cuorsity, that's understandable, why not try it anyway?

But honestly, I see no further point to a unity gain POWER amplifier, unless it is in fact a simple current boost stage to whatever is amplifying the signal. On the other hand, if it's an op amp doing the signal amplification, there are far more practical ways of current boosting them.
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Old 2nd October 2012, 12:18 PM   #7345
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I suppose it is like having a super low gear on a car . When will it be useful ? As an engineer it is always good to know . The number of times I have been asked a question and have been able to say " it's funny you should ask that ".

As I said before my clock stopped at age 10 . When I write here I usually have some child's project on the go . This week it is playing with 555 timers . Still didn't make a crude class D that worked . I have a blown up inverter I would like to replace . I hoped to make something powerful of about 0.5% distortion .
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Old 2nd October 2012, 03:45 PM   #7346
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Quote:
Originally Posted by nigel pearson View Post
Hi DVV , Hi wave .
I love that EL84 amp . I must build that as I have all the bits . It is like something I had in mind .
I believe it has a typo in the article, it should use EL34 tubes, not EL84. For EL84 470 Ohm in cathodes is too much.
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Old 2nd October 2012, 05:24 PM   #7347
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EL 34 is fine . Mine came out of Chipping Norton Studios , they are antiquated with good music . The topology was my interest . Things I have tried but not in this mix .

Thanks anyway . Like seeing a Delta-wing it's the shape I need . The bottle I would have worked out . The triode like anode to grid shunt feedback unimpeded by the impedance of the driver stage I like . I think that's right ? Nice to see transistors lending a hand .
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Old 2nd October 2012, 05:39 PM   #7348
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This one is better: no transistors that add to emitter current base current dependent on beta (non-linear). Schematic drawn by Alexander from diyaudio.ru forum.
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File Type: gif hybridinstramp.GIF (18.9 KB, 89 views)
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Old 2nd October 2012, 05:51 PM   #7349
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That's very nice and a little bit like what I have been building . Mine is SE . I gave up with the Alex Kitic type feedback on mine . I kept my notes so can always go back to it . The best I saw was a DC coupled 1000 V Fet driving 5881 in a similar circuit . Half grid bias and half cathode bias with a 180V zener to G2 . The distrotion spectrum a near perfect exponential . I think I might have it somewhere . Forgive any errors , this is similar . If people need more info I will find the original and it's author ( Michael Kloster is it ? ) . Forgive as it is to show an idea only .

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Old 2nd October 2012, 07:38 PM   #7350
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Are you sure it was calculated for KT66?

Gu-50 fits here ideally.
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