A question about Push-pull class-a - diyAudio
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Old 15th July 2010, 11:01 PM   #1
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Default A question about Push-pull class-a

I know the following may be a bit contraversial but I figure the best place to ask here is Mr. Pass' forum as he is the master of Class A

Is not push-pull class A simply "Class AB deeply biased and operating in it's class A bounderies"?

And if that is the case, could it not also be asserted that the term push-pull Class A be a marketing term for that same quoted statement

Before anyone jumps down my throat for saying that note that I'm only talking about the term, not the technique - even at my limited experience I see value in the technique.

I'm just asking as I'm trying to understand what is really gong on compared to what the textbooks are telling me.
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Old 15th July 2010, 11:24 PM   #2
Zen Mod is offline Zen Mod  Serbia
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real PP class A is as long opposite active devices are pushing/pulling - in opposite directions , in exact same amount .

in a moment when amount of pushin/pulling between two opposite devices isn't exact the same - one is decreasing "force" while other is increasing - that's moment of leaving pure class A

there are amps and amps ; some of them are fully in class A , while some other are leaving A , hopping in B .......

that means just one thing ( considering that we are talking about real PP amps solely) that some of them aren't capable , from some reason, to drive own output stage above voltage and/or current needed to jump from established quiescent current/power

nota bene - that's not necessarily bad thing - it can be mater of decision ......

read this , while listening to your fave amp , and you'll be taught how to hear KLUNK!!!



www.firstwatt.com/pdf/art_leave_classa.pdf


edit:

yeah ..... even Papa cheats
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Last edited by Zen Mod; 15th July 2010 at 11:28 PM.
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Old 15th July 2010, 11:32 PM   #3
juma is offline juma  Germany
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Sorry mutter, but it's not like that at all - Class A is a working regime of active amplifying device and push-pull is a topology.

Google a bit more or use more liberaly the search button on this forum, learn the meaning of the terms you use - that will help you to really understand what are you talking about, which is not the case right now.
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Old 15th July 2010, 11:46 PM   #4
Zen Mod is offline Zen Mod  Serbia
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............... Klunk!!! ............

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Old 17th July 2010, 02:08 AM   #5
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"Is not push-pull class A simply "Class AB deeply biased and operating in it's class A bounderies"?

I think that is a perfectly true statement. Take a look at the A75 article by Pass and Thagard. It can be an AB amp, but it is designed to be so heavily biased into class A that it can put out 75 watts in class A. Sounds amazing, too. You can turn down the bias pots and run it with just a few watts of bias. But it definitely sounds better with higher bias.

"And if that is the case, could it not also be asserted that the term push-pull Class A be a marketing term for that same quoted statement"

well, I think it really is a matter of how many watts it puts out in class A. If the amp has really small heatsinks and doesn't list how many watts it puts out in class A, then, yeah, it is probably just a marketing term.

I think there is a thread on the forum called "leaving class A" where you might find some good information.


JJ

Last edited by jupiterjune; 17th July 2010 at 02:11 AM. Reason: I don't think my original wording was accurate
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Old 17th July 2010, 04:25 PM   #6
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"Leaving Class A" is posted on the Pass Labs website.

It's certainly true that a Class AB amplifier is Class A until
one of the amplifying pair shuts off. It would be legitimate
to call it "Class A" up to that point, but as a marketing reality
it's limited. Many heavily biased 100 or 200 watt Class AB
amplifiers would still only be able to talk in terms of about
10 watts Class A.

Even an X600.6, which qualifies as heavily biased, leaves
Class A at 100 watts or so, and that's a peak figure.

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Old 17th July 2010, 04:30 PM   #7
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even a single ended ClassA amplifier goes out of it's ClassA region. We call that condition: overload distortion.
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Old 17th July 2010, 06:31 PM   #8
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Thanks all.

There is definitely a gap between my training and the level that some of these amps are designed at. There is obviously a gap between the simple definitions and the reality of the designs.

I need to learn more...
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Old 17th July 2010, 07:04 PM   #9
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A push pull works in class A as long as it delivers no more than twice its bias current.
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