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Old 28th June 2003, 01:39 PM   #11
Joel is offline Joel  United States
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Quote:
Originally posted by SY
Well, dumb question- if you want to go the Class B route, why not use a more suitable tube...
Which would be...?
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Old 28th June 2003, 01:42 PM   #12
316a is offline 316a  England
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Default DA42

Try a DA42 , good for 175watts of zero bias class B

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Old 28th June 2003, 01:52 PM   #13
Joel is offline Joel  United States
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Also, please explain what you mean by "PSU rectification problems" in class B/C amps.
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Old 28th June 2003, 01:59 PM   #14
316a is offline 316a  England
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Default What happens...

...to current draw in a push-pull output stage when one tube is shut off and the other draws current compared to idle ?

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Old 28th June 2003, 02:03 PM   #15
Joel is offline Joel  United States
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Quite obviously there are large swings in current demand from the output stage. Equally obvious is that you need a tightly regulated power supply. What does that have to do with the rectifier?
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Old 28th June 2003, 02:52 PM   #16
SY is offline SY  United States
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Quote:
Which would be...?
Besides the usual zero-bias suspects (e.g., 811), there's sweep tubes. I use 'em, they're easy to get, and they're (mostly) not stupid-priced.
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Old 29th June 2003, 08:06 AM   #17
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Class B usually doesn't get much respect, but the overturning of the audio myth that "chip-amps can't sound good" should have us rethinking the possibilities. Those little chip-amps are pretty much Class B.

The most aggregious issue with Class B is the distortion at the turn-over, how do you minimize the notch -- is it possible to eliminate?

And if it doesn't work out, i guess you could always dial in a bit more bias and you have the standard fix -- Class AB.

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Old 29th June 2003, 10:55 AM   #18
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Hi,

Quote:
Class B usually doesn't get much respect, but the overturning of the audio myth that "chip-amps can't sound good" should have us rethinking the possibilities. Those little chip-amps are pretty much Class B.
O.K. but let's not forget a few things here:

Those chips quite likely use fast-switching D-MOS power MOSFETs that you can even run in Class D for audio frequency.

When talking about classes of operation we have basically four different classes namely:

Class A, B, C and D.

If you think to "fix" the inherent problem of Class B for audio use by biassing the output devices for a given amount of Class A operation you effectively have entered a subclass of operation called Class AB.

Other subclasses exist but that's not the issue.

The point is simply: it's either Class B or it must be something else:

"Class B operation is the opposite of class A. Both output devices are never allowed to be on at the same time, or the bias is set so that current flow in a specific output device is zero when not stimulated with an input signal, i.e., the current in a specific output flows for one half cycle. Thus each output device is on for exactly one half of a complete sinusoidal signal cycle. Due to this operation, class B designs show high efficiency but poor linearity around the crossover region. This is due to the time it takes to turn one device off and the other device on, which translates into extreme crossover distortion."

Snippet taken from a glossary found on the web.

Back to class B, the only way I can think of using tubes in Class B and still have acceptable audio results is the use of sweep tubes, as SY pointed out, still the x-over distortion would be there and having an OPT won't help matters either.
Which leaves us with PP OTL amplifiers which in turn have their own set of problems due to the non-existence of complimentary tubes.
Solid state devices do have complimentary pairs which at least allows for a simpler solution to that problem.


Quote:
The most aggregious issue with Class B is the distortion at the turn-over, how do you minimize the notch -- is it possible to eliminate?
Well, allow me to say that it's a very old problem inherent to this particular class of operation. One which led most audio designers to compromise on sound quality (Class A) and power efficiency (Class B) by using judicious amounts of both classes.
This is essentially class AB operation and it's deeper subclasses.

To this day no design using only class B as an operating Class has succesfully tackled this problem using tubes as the amplifying/output devices.

While there may well be a solution to this, I remain skeptical about its validity for audio use both from a sonic as well as an economic POV.

This is of course just my opinion on the matter, YMMV.

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Old 29th June 2003, 03:47 PM   #19
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Quote:
Originally posted by fdegrove
While there may well be a solution to this, I remain skeptical about its validity for audio use both from a sonic as well as an economic POV.
And i too, have the feeling Joel will end up with a Class AB amp after all is said and done.... but like when doing a good feedback amp, you build a really good no-feedback amp 1st, i think it is worth having a crack at getting Class B as good as possible before turning up the bias.

Joel is doing all the work... it won't hurt us to give him a hand. We can learn from his successes (& at least as much from the failures -- a good example of that over in the buffered gain clone thread )

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Old 29th June 2003, 04:52 PM   #20
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Hi,

Quote:
Joel is doing all the work... it won't hurt us to give him a hand. We can learn from his successes (& at least as much from the failures -- a good example of that over in the buffered gain clone thread )
No problem.
Let'sjust keep in mind that semis and tubes are not the same.

Switching distortions are moving targets and are very hard to tackle.'Fraid that NFB alone won't cut it.

As far as GCs go expect some Class D and E designs for audio any time soon.

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