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Old 5th October 2001, 05:03 AM   #1
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Does anyone have any links to sites with the basic theory of class B amp operation, or perhaps someone can give me a brief description here. I would also be interested in some links to simple class B schematics on the net. I'm just curious how class B operation is different from class A.

Cheers

Dan
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Old 5th October 2001, 05:56 AM   #2
paulb is offline paulb  Canada
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Check out the amplifier articles at http://www.sound.au.com. Also check some of the other links at this (diyaudio) site.
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Old 5th October 2001, 07:39 AM   #3
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Dan,
In a nutshell, (assuming a push-pull circuit, which is necessary for all classes other than A) a class A circuit is like two guys on one of those old-fashioned crosscut saws, each putting his strength into both the 'push' stroke and the 'pull' stroke. In a class B circuit, one fellow is pulling the saw, but the other is just standing there with his arms folded, resting for a second. This allows him to save strength and cool down for a brief time which sounds like it might be a good thing, but the fly in the ointment comes when he lays hands on the saw to take his turn pulling the saw back his way--as he grabs hold, and as the other guy lets go--there's a moment there when the saw is getting wiggly and neither one of them has good control over what's going on. This, in electronic terms, is called crossover distortion; a brief spike of really obnoxious distortion (this has nothing to do with tubes vs. MOSFETs vs. bipolars, it's simply a question of how they're used in the circuit) that's difficult to measure (using standard measurement techniques--it's not that hard to see with other methods) because it's of short duration. The problem is that the ear isn't fooled.
There are in-between classes, too. The guys can, instead of each doing 50% of the work (strict class B), do something more like two-thirds, which will involve a certain amount of overlap where they both have their hands on the saw for a portion of the cycle, but their rest period gets shorter. This leads to class AB. Note that AB isn't one set percentage, it's a sliding scale all the way from 50/50 (class B) up to 99/99 (meaning each guy gets this tiny little rest period of 1% of the saw stroke--hardly worth the trouble, really). Once you get to 100/100, then you're back at class A, with both guys pumping 100% of the time.
There are other classes, but how useful they are for high fidelity is open to debate. Class C, for instance, is slap-full, rail-to-rail clipping...all the time. Now, what the hell good is that? Well, it turns out to be really cool if you're running a radio transmitter (or a PC, for that matter...t'ain't nuthin' but a square wave, and they're really nifty for timing purposes, but we ain't building computers, here). Class D chops the signal into a skillion little bits (digitizes it) and then more-or-less amplifies it as class C, then reassembles the pieces to make something like a sine wave again. Other classes exist, but they're based on sliding bias or other oddities that get pretty slippery when you want to implement them cleanly.

Grey
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Old 5th October 2001, 05:12 PM   #4
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Grey, though I'm kind a against you in the matter of forcing your opinion on others(meant no offense), the class of operation was well and vividly explained.

-XL
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Old 5th October 2001, 06:52 PM   #5
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Merci...

Grey
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Old 6th October 2001, 07:42 AM   #6
hifiZen is offline hifiZen  Canada
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Douglas Self has a decent website which goes into a little more technical detail, and gives a very good overall view of class-B. If you read the section on power amplifier distortion, there's a bunch of stuff dealing with diff pairs and the VAS, and about half way down the article, you'll find the section dealing with class-b output stages.

Now, I have to warn you - Doug tends to take the opposite extreme position to Mr. Rollins, dismissing everything subjective without giving much consideration to the possiblity that his test instruments aren't showing all the effects experienced by the listener. Anyway, Mr. Self does present a good method for optimizing class B to remove as much distortion as possible. While at one time, class B distortion was poorly understood and handled (leading to terrible sounding amps), modern designs can effectivley implement class-B outputs with very little distortion. In fact, enough class-B distortion can be removed to (IMHO) render it nearly inaudible. I say <i>nearly</i> because there is a difference between how class-A and good class-B amps sound - which i think has something to do with non-monotonic distortion (distortion which decreases with amplitude, instead of increasing like it should). That said, I regularly listen to a class B amp which sounds positively wonderful.

Class-B is a worthwhile pursuit if you're willing to invest the extra time and attention to deal with it's ugly side. But, with proper design, construction, and testing, class-B will deliver a substantial gain in efficiency, bringing down the cost of power supply components and output devices/heatsinks, which together form the bulk of an amplifier's cost. So, if a high-power amp is your gig, then going class-B can save big bux. For low-powered amps, the difference is much less, and I'd recommend sticking with class-A.
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Old 6th October 2001, 01:48 PM   #7
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Good job Grey, but I still don't understand where the little men live in my amp when the're not pulling or pushing?
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Old 6th October 2001, 03:49 PM   #8
jam is offline jam  United States
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Pete,

Usually in the power supply capacitors (condominiums).

Jam
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Old 7th October 2001, 11:31 PM   #9
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Thanks for the info everyone.

I have been reading up on this quite a bit and believe I now understand the subject a lot better. One thing I have been thinking about also is the whole amp/speaker interface. This is what I have arrived at:

A speaker is a current controlled transducer which produces movement of the cone by F = Bil.

The speaker is a reactive load with increasing impedance with frequency.

All amplifiers that I have seen are voltage amplifiers with current handling capabilities up to (and often exceeding for class A) their maximum power rating.

Controlling the voltage across a reactive load to produce current as a by-product doesn't sound like a good solution to me. I would think that a low distortion current waveform should be supplied to the transducer and let the voltage do whatever over the frequency range of the system.

I would like to hear some comments about this. Am I missing the point or trying to over simplify things?

Cheers

Dan
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Old 7th October 2001, 11:46 PM   #10
Geoff is offline Geoff  United Kingdom
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Dan

Rather than repeating information available elsewhere, I will just draw your attention to the following articles at the ESP Audio Pages which cover current drive as opposed to voltage drive:

http://sound.westhost.com/z-effects.htm

http://sound.westhost.com/project56.htm

A thing to bear in mind is that all commercial speakers have been designed to work from normal commercial amplifiers and are therefore balanced for voltage drive.

Geoff
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