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

As a "generic" rule of thumb, Class A amps typically average about 40 - 50% efficiency. That means that you have 50 - 60% of the power being drawn at any given time being dissipated as heat.

As far as Class AB amps go, their efficiency levels will vary depending on how "richly" they are biased into Class A. Some AB amps might stay in Class A for a watt or two while others may switch over after 5 - 15 watts. As such, efficiency suffers so long as the amp is run at low level. Once the amp is pushed beyond the point of crossing over into Class B, efficiency rises somewhat. The harder the amp is driven, the more efficient it becomes ( in theory ). AB amps are typically considered to be about 65% efficient in terms of power drawn vs actual power output. As mentioned, this figure can go up or down as a whole depending on the overall bias level.

As far as switching or "digital" amps go, efficiency levels can be VERY high. Some of this will vary with how the driver circuits are set up to operate. Since minimal power is lost in most switching designs, there is a minimal amount of heat build-up within the amp itself. This is due to the fact that the circuitry is only "active" a very small percentage of time ( greatly reduced duty cycle ). The drawback to this gain in efficiency is that one runs into a massive increase in several different types of distortion and "typically" a loss of resolution ( especially at lower power levels ). The more that you "pile drive" a switching amp, the less noticeable the side effects will become.

As such, the hotter an amp runs, the less efficient it is. Having said that, it is "probably" also more "linear" than an amp of lower bias all things being equal. Getting all of the variables "equal" is a whole 'nother ball of wax though... Sean
>

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
>

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