Newsgroups: rec.audio

> >What the heck is class D? (Excluding Stereophile's rating list)
> 
>>Class A is full power all the time
>>Class B is power as needed
>>Class A/B is a combination of the two
> 
>>So what is Class D? (And is there a Class C?)
> 
>yes there is a class C ( only used in high power RF applications ) which can not
>be used for audio frequency ( well , I seem to remember that it has about 80%
>THD, so would you use it?? ) ( except possibly Morbid Angel B-)
> 

Well, almost right, sort of.  (God, we've GOT to get the FAQ posting done!!!)

"Class A" means that the amplifying device conducts during the full 360
degrees of any input cycle.  You might be able to claim that ALL audio
amplifiers are "Class A" from this definition, were it not for the fact that
the individual *devices* in the output stages (i.e., the tubes or transistors)
may not themselves be operating Class A even though the amp in total does
provide the full cycle of output.  Before I explain that, more on amp classes:

Class B means that the amplifying device conducts during exactly half (180
degrees) of every input cycle.  Class C means that it conducts during *less*
than half (less than 180 degrees), and Class AB means <360 but >180.  As an
aside - yes, Class C does mean high distortion, and IS generally used only
in RF applications.  But that doesn't means that all high-power RF amplifiers
are Class C, or that such operation is its only use.  Note that ALL modes of
operation besides Class A distort the signal severely, as they cut off at least
some of every input cycle.

There is, by the way, really no "Class D" or any higher class that fits into
this scheme.  "Class D" is a term which has been adopted by some manufacturers
to indicate a switched-mode type of design (meaning that the amp works the
same way as the switching regulator in a switching power supply), but this
really doesn't mesh well with the 360-180-less scheme of the A-B-C class
designations.

The other important thing to realize about the amplifier classes is that the
"higher" the class, the higher the efficiency.  Class A amplifier work by
leaving the amplifying device on *all the time*, meaning that they're using
power even when there is no signal present.  Classes B and C bring the 
amplifying  device out of cutoff only when there's something to amplify; C has
an efficiency advantage over B because it conducts for less of the input cycle,
and can run the device into saturation and still work OK.  The problem for amp
designers in the audio world is to make something that is more efficient at
amplifying than it is at heating the room, while still providing low 
distortion.  This can be done (at least conceptually) by putting two Class B
amplifiers "back-to-back", such that each handles half of the input signal.
This is referreed to as a "push-pull" configuration.  Low distortion at high 
efficiency, right?  Well, not yet.  Unfortunately, the ideal amplifying device 
does not yet exist, and the ones we're stuck with in the real world - tubes a
nd transistors - go through a non-linear region of operation when coming out 
of cutoff.  This means that the signal will be distorted at the times when one 
device is turning off and the other turning on, a phenomenon called "crossover 
distortion."  To minimize this, the devices are actually set up ("biased")
slightly into the Class AB realm, so that one turns on slightly *before* the
other turns off, and linearity is improved.  Most audio amps use this
"Class AB push-pull" configuration for their outputs.

Class A still offers the lowest distortion in theory, as it will have no
"crossover distortion."  But do NOT assume that a Class A amplifier is
inherently better than a Class AB PP design, simply because it's Class A.
There are other ways to introduce distortion into an amplifier besides
crossover problems, and there are both extremely good AB designs and lousy
A designs.  Keep your mind - and ears! - open.


Bob Myers  KC0EW   Hewlett-Packard Co.      |Opinions expressed here are not
                   User Interface Tech. Div.|those of my employer or any other
myers@fc.hp.com    Fort Collins, Colorado   |sentient life-form on this planet.

  

From: davea@hpscit.sc.hp.com (Dave Angelini)
Date: Thu, 20 Aug 1992 20:25:58 GMT
Subject: Audio Gear for sale
Organization: Hewlett-Packard, Santa Clara, CA