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

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.

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.

KF

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