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Old 24th December 2011, 11:32 AM   #21
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by toprepairman View Post
if the bias is such that the output devices are always conducting
I don't accept "always conducting" as a sufficient definition for ClassA operation.
There are many "cheat" ClassA, ClassAA etc circuits that deliberately hold the "OFF" device at a non zero current. and use this to claim they have a lower dissipation ClassA circuit.
the upper and lower devices of a ClassA PP stage must always actively control the output current. And further I think that the waveform variation of the current in the upper and lower devices must follow the audio waveform, for the output to be Real ClassA.
That last sentence is in effect telling us that as the doubled Ibias is approached the accuracy of the waveform following is dropping off and that the ClassA nature of the output is debatable. eg. KSA50 is baised to 1.9A. The peak output current is 3.8Apk just as one half of the devices are switching off.
I think that maximum ClassA applies to a slightly lesser output current. Maybe 3.7Apk or 3.62Apk or some other value that can be technically defined after some experimentation to prove when the accurate ClassA "actively controlling" starts to deviate from Real ClassA.
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Old 24th December 2011, 11:36 AM   #22
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by toprepairman View Post
THe JLH is biased for half peak output current, and being pushpull this is a proper class A design.
The JLH is not a conventional Push-Pull output stage.
It is nearer to a SE output stage with an active source/sink.
The clever bit is how that active source/sink is modulated to vary the bias.
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Old 24th December 2011, 04:40 PM   #23
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Quote:
Originally Posted by Nico Ras View Post
Hi Kenpeter, tried it and your idea works. It runs cold, only thing is the distortion is very much audible now. I tried different values of R which changes both the power dissipation and distortion. The higher the dissipation the less the distortion. When I say audible it is quite obvious, probably 5% or more (subjectively).
Checklist:
You are taking both output and feedback from midpoint of Schottkys?
You are taking bootstrap (if any) from the top emitter, not midpoint?
R1 sized to conduct more current than your bootstrap (if any)?
(R1 purpose to prevent bootstrap from shutting any transistor off)
You used only Silicon Schottky, not Silicon Carbide Schottky?

If feedback trace still goes directly to emitter (or collector) and not
the new midpoint, will definitely hear diode distort. Would believe a
.5% distortion, but 5% seems way much. Something simple might
have been missed...

Last edited by kenpeter; 24th December 2011 at 05:09 PM.
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Old 24th December 2011, 05:35 PM   #24
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Quote:
Originally Posted by AndrewT View Post
I don't accept "always conducting" as a sufficient definition for ClassA operation.
Agree, and when I abuse such tricks, always want to be clear about it.

However, there is subtle difference between those that have no current
variation over half cycle and hard knee, and those that express smooth
complimentary curvatures over the majority of the full output swing.

R1 in my example was not purposed as an "always conducting" cheat.
Even though it is clearly exactly such a cheat...

But mostly there in-case you have to drive a bootstrap, not cause
curve of two Schottkys somehow knees over abruptly at quiescent.
Schottky quiescent (about 300~900mA) is hotter than hot AB, but
not hot as JLH (humped in the middle beyond even linear class A!).

Don't even need the extra 50~150mA that R1 provides to plausibly
call this class "Aa". If your JLH had no bootstrap, you could possibly
dispense with R1 entirely. But R1 also prevents reactive loads from
accidently shutting off a transistor, so I don't think this "cheat" is
anything bad.

"Aa + CCS" might be an accurate description of this class?
I've worked too hard to be lumped in with mere "AB + CCS".
So, not true class A, but then neither was our original JLH.
Can make linear class A by use of resistors instead of diodes,
but heat reduction from JLH to Linear A won't be as significant...

In 24V rails with 8 ohm load example, 50W is saved by restricting
JLH's enriched "aA" overcurvature down to Linear A. Another 50W
to be saved by restricting to Square Law vs Square Law class "Aa".

Last edited by kenpeter; 24th December 2011 at 05:55 PM.
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Old 24th December 2011, 05:59 PM   #25
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You can see the original JLH "aA" curves (bent due to 2n3055 beta droop)
equally as warped as Class "Aa" (bent due to Schottky diodes) . But there
is a waste of 100W difference between these classes.

Both of these might technically be Class A, and neither seems to cheat.
But neither of these curves should ever claim to be "Linear Class A".
The distinction Andrew's bitchin' about make plenty of sense.

Could say, "I'd never bias my JLH as hot as that first picture". But fact
of classic transistor droop, is that it will clip before reaching the rail if
you don't. Second pic allows plenty of generous bias to handle droop,
all the way to the rail, and discards any excess in the quiescent state.
Attached Images
File Type: gif EnrichedA.gif (33.1 KB, 144 views)
File Type: gif HyperA.gif (32.3 KB, 142 views)

Last edited by kenpeter; 24th December 2011 at 06:27 PM.
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Old 24th December 2011, 06:35 PM   #26
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Quote: Originally Posted by toprepairman
THe JLH is biased for half peak output current, and being pushpull this is a proper class A design.

Unless you are using modern transistors, unchallenged by droop,
or accept an amp that clips before reaching the rail, I'd say not...
Low rail 10W versions maybe do not challenge 2n3055 droop?

High Watt JLH with classic transistors will see this challenge for sure.
If you bias for half "calculated peak" at quiescent, you will never have
enough bias to actually reach that peak. Beta droop devil gets you.

Last edited by kenpeter; 24th December 2011 at 06:47 PM.
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Old 24th December 2011, 07:01 PM   #27
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Quote:
Originally Posted by toprepairman View Post
Hi Nico. Andrew is correct, regardless as to whether it is single ended or pushpull, if the bias is such that the output devices are always conducting (and not any strange sliding bias idea) then it is class A.
Who has been arguing? Me? Never said anything, all I say is KSA50 is over biased class B. Obviously you are only reading what you like...
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Old 24th December 2011, 07:04 PM   #28
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Quote:
Originally Posted by toprepairman View Post
THe JLH is biassed for half peak output current, and being pushpulll this is a proper class A design.
A single ended design has other considerations depending on whether it is choke/transformer coupled/loaded or resistive or constant current fed.
Okay so you know how class A works, single ended and push pull.
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Last edited by Nico Ras; 24th December 2011 at 07:13 PM.
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Old 24th December 2011, 07:09 PM   #29
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Quote:
Originally Posted by kenpeter View Post
Checklist:
You are taking both output and feedback from midpoint of Schottkys?
You are taking bootstrap (if any) from the top emitter, not midpoint?
R1 sized to conduct more current than your bootstrap (if any)?
(R1 purpose to prevent bootstrap from shutting any transistor off)
You used only Silicon Schottky, not Silicon Carbide Schottky?

If feedback trace still goes directly to emitter (or collector) and not
the new midpoint, will definitely hear diode distort. Would believe a
.5% distortion, but 5% seems way much. Something simple might
have been missed...
Hi Kenpeter, yes did as you said. I also mention subjectively 5% since I can hear it, could be overstating it. Your idea works very well in fact as the dissipation very little at idle and increases as the signal increases. Very nice actually.
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Old 24th December 2011, 07:19 PM   #30
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Kenpeter, found a lousy 6mm spade connection which caused very audible distortion. Since changing it, it is heaps better subjectively.

I simulated your modification and theoretically the modification increased THD by a factor of 8.

I think some attention to you proposal would be time well spent Kenpeter, this is just the right formula to introduce class A to the masses allowing them the opportunity ti hear real hi-fi at moderate cost
Attached Images
File Type: jpg JLH_.jpg (251.3 KB, 138 views)
File Type: jpg JLH_KP.jpg (229.5 KB, 117 views)
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Last edited by Nico Ras; 24th December 2011 at 07:34 PM.
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