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

I am a newbie in this area. When people say "class A" amp, does it mean it soulds better than a class B amp, more expensive than a class B amp, or ??.

What are the classifications for?


The first five or six hits cover you question in various ways. Search "class" in the archives if the above doesn't work.
A few other good posts are:

Just highlight each of these in turn and copy them to the Address bar above. They are some nice past threads on the topic.
I often read "Stereophile class A" amp. It seems Stereophile class A is not the same as "class A bias" amp.

What does "Stereophile Class A" mean for a piece of an equipment?
You are getting confused between technical operational classifications of amplifiers, and Stereophile's completely unrelated subjective ratings systems from their review staff.

Operational classes of amps are not ratings at all, they are just technical descriptions of how the amp operates. Class A operation means that the amp is not in push pull configuration. Class B is Push pull operation. Class A/B operation is when the amp operates in Class A mode up to a certain wattage, and then slips into Class B operation. As far as I know, there is no such thing as class C operation. Class D stands for digital operation, for example, the bell Canto Evo series amps.

Stereophile has a rating system that has nothing to do with the technical operating modes of the amplifiers. They are divided into classes A,B,C, and D. These are subjective qualitative values rating the sound of a unit that has been reviewed. Class A is best and class D is still pretty good. Mid fi components don't even make it on to their ratings system.
It's supposed to mean that the equipment reproduces music exceptionally well. The jury is out on that since it might mean that the company advertises in Stereophile exceptionally often.
There are Class C amplifiers but i don't know of any manufacturer using them for audio purposes. They are quite high in multiple types of distortion, hence the lack of desire to use such a design. Sean
Esun is pretty correct.

A very important aspect of the operational Class of an amp is:

1 - How much current the amp draws at any given moment
2 - How hot the amp is at any given moment
3 - How efficient an amp is from converting AC wattage to output watts

Amps running in Pure Class A operation suck A LOT of wattage at any given second (even idle) from the wall. They tend to use a constant wattage from AC no matter if they are idle or playing music. They can consume upwards of 10 (most amps less than 10 times) TIMES the amount of wattage idle compared to the wattage they put out at full power. Thus, a 100wpc Class A amp can theoretically suck 1000 watts idle. This means the amp is about 10% efficient. This massive intake of power has to go somewhere and is converted into heat. This is what makes amps that run in Pure Class A HOT even while idle.

Class AB amps run in Class A up to a power level while the rest of their power is drawn as needed from the AC. So lets say the amp is Class A to 10 watts while AB to 100 watts. This amp might draw 100 watts idle. If the amp needs to output more than 10 watts of power this power is taken from the AC as needed. Class AB amps stay a lot cooler than Class A amps because they generally do not draw nearly as much current on a continual basis. This makes Class AB amps much more efficient than pure Class A amps.

Digital amps or Switched amps are a bit different. I am not 100% certain about how to describe these amps operation; however, I do know that these amps are A LOT more efficient than either Class A or Class AB amps. Switched amps (most people call them digital amps, but I have heard that a more correct term is calling them Switched) are about 80% to 90% or so efficient. They can also be amazingly powerful. I have a friend who has had the Bel Canto amps (monoblocked), and he told me they were the most powerful amps that he has ever had.

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
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
Sean: Have you ever seen an AF Class C amplifier? I have not. RF amps, in particular, pulsed nuclear magnetic resonance instrumentation, I have seen many. Ham operators,many. Nada for audio.
No, i've never seen a Class C amp for audio. My main comments in my last post regarding audio amps pertained to Class D amps. Sean
I think the new Rowland Amp 301 is a class D or H switching
amp. So how does an audio company who makes class D amps
get around these problems. I assume the faster you oversample the better, but how much I am not sure. I assume
you would have to decimate the data back into the audio
frequency, again I am not sure. Does anybody know????
Most "decent" switching amps run Class A ( or at least AB ) driver stages and then run the output stage in some type of "switched" mode. They do this to maintain as much linearity as they can at the lower level stages that don't pull as much current while increasing efficiency in the output stage where it is quite current thirsty.

As to the "refresh rate", i have no idea. I remember seeing something about 800 KHz with the Sunfire amps, but that may been something about a harmonic of the primary signal. That seems AWFULLY high to me for an audio amp. Then again, such a high rate would offer greater linearity, both in and above the audible range. Sean