Class A, B, C? What do they mean?

The "system operation" refers to the transistor on-time. The transistors are what amplify the power in a power amp and so are the critical part most talked about. Class A means the transistor is on all the time, in varying degrees (i.e. volume level and transients from the CD or LP). Few high-power amps are pure Class A - generally 50Wpc is the limit. Class B is on only half the time and so the transistor efficiency doubles over Class A. Class D is a totally different animal (and the topic of my dissertation so I can ramble on forever here) where you transistion from a linear circuit/amplifier to a nonlinear circuit opeation. The transistors are now switched like your room's light switch, either totally on or totally off. When you switch it very fast (compared to very slow for the other 2 classes) you get huge harmonics which degrade sound so this class is rarely used in audio equip (except for PS Audio et al.) outside of some power supplies.

Class A amps are theorectically limited to 25% efficiency max. This is due to a fundamental circuit limitation. Actual efficiency is generally much lower (around 12%). Hope I have not confused you more. Arthur

Class C amps are used mainly in RF amplifers ie from
around 5Mhz to 1Ghz. After a 1Ghz you start using wave guides for amplification of a small voltage signal. Ham
operators use them. I do not believe it a distortion
problem per se, but a question of turn on/ turn off. A class
C is either turned on to MAX output or turned off. In
order to attenuate the signal you use a filtered network,
which would in that case cause distortion. But a ham operator, wants power on, and on full.

When it comes to Class C as an RF amp, they are pretty filthy when it comes to out of band harmonics ( THD ) and in-band splatter ( IMD ). That is, unless used in AM mode with limited percentage of modulation. Otherwise, the constant switching off and on of SSB and over-modulated AM produces a lot of garbage due to the crossover distortion involved. Some good designs are able to get around this, but they are far and few between. Sean
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Sean: In power amplifiers operating at radio frequencies distortion of the rf waveform is relatively unimportant.
RF amplifers usually operate with tuned circuits and the
selectivity of such circuits filter out the rf harmonics resulting from distortion. Of course, a wideband class C
amplifier has tremendous amount of distortion, because the
operation angle is less than 180 degrees. A push-pull
amplifier operating at pure class B is exactly 180 degrees
and you take the the two waveforms one from the push and
one from the pull and add to give a 360 degree waveform.
A class C amp since each side of a push-pull is less
than 180 degrees can never give a 360 degree waveform, thus
the huge amount of distortion. A Class A amplifiers since
by definition gives a 360 degree waveform,does not need
a push/pull to define a full waveform. You only need one
output tube for class A operation, two for class AB or B
operation

Using tuned "tank circuits" on tube gear and harmonic traps on SS gear will help to reduce THD, but that does nothing to get rid of IMD or "in-band splatter" when using a Class C amp. Class D will also suffer from this, but with slightly different results that would vary with design. All of these "problems" occur in audio amps using these design variations also. Sean
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