??? WHY WHY WHY ??? Class A vs Everything Else

(AB) Significantly higher efficiency, greater power output (typically 3X or more) and an implied requirement for local or global feedback to linearize the switching transition in the output devices.

@lynn_olson I think most people haven't thought this bit through. If there is a switching transition in the output devices (producing crossover distortion) then the amp is actually biased class C. 

Class B is defined as exactly 1/2 of the audio waveform. If its more than that the output device is biased AB, if less than that its class C. Class B has really only been theoretical on this account until perhaps recently when microcontrollers could be used to actually really get the output device(s) to do exactly 50% of the waveform. 

EV used to sell amps back in the 1950s they claimed were class B, such as the A20 or A30. Indeed they made double the power you would expect out of a pair of the same power tubes in class A and the tubes ran cool. They had no zero crossing artifact at any power level you could measure (I have an A20 so I've put it through its paces)...

This all says they were really very lightly biased AB amplifiers. AB amps won't have a zero crossing artifact unless there is a design flaw (usually a problem in the driver circuit). Feedback won't fix a zero crossing artifact since there is no loop gain at the zero crossing if the outputs are in cutoff. So if the amp is AB there's no worries since there's loop gain. 

Put simply, most of the time when people talk about a zero crossing artifact such as 'switching transition' they are talking about an amp that is biased class C. Douglas Self wrote about an amp he designed called the 'blameless amplifier', which is an example of a class C push-pull amp, although he claimed it was class B. But if you really think about it, you know the class B in the real world can't exist without computer control. 

Try Accuphase A36, A48, A75 or A80 depending on your need.  They run cooler than Pass Labs counterparts.

Ralph, I hate to disagree, but a pure Class B amp (like the Quad 405), switches from the upper set of transistors to the lower set with no region where both are operating. Class AB has an intermediate region where both are operating.

In a typical Class AB amp, whether tube or transistor, there are three operating regions: the plus direction, which activates the upper output transistors (or tubes), the middle zero-crossing region, which activates all devices at once, and a minus region, where only the lower devices are operating.

The size of the middle, zero-crossing region is at the discretion of the designer. If this middle region is so large that the B regions are never activated, it becomes a Class A amplifier (by default).

The Quad 405 had no A region, and relied on the feedforward system to supply current and voltage for +/- 0.7V region where all output transistors were turned off. As a result, it ran quite cold, but if you had a good enough distortion analyzer, you could see the switching region along with a spray of harmonics.

Class C is reserved for radio frequency transmitters ONLY. This has massive distortion since not all of the waveform is amplified ... there’s holes in it. It doesn’t matter in RF applications because tuned circuits filter out all of the harmonics, and Class C is more efficient than Class B or AB.

Class D is a switching amplifier, akin to a switching power supply. Pulse-width modulation converts the incoming analog signal into PWM, which is applied to very high speed switching transistors. A lowpass filter at the output removes most, but not all, of the ultrasonic grunge. It is normal to apply substantial feedback to linearize the PWM modulator and correct for small timing errors in the switching devices. (In PWM, timing errors translate into distortion when lowpass filtered.)

Here's a succinct explanation on Wikipedia:

Amplifier modes of operation

Nice to see mention of the Classe DR3-VHC.  IIRC they only made 133 of them and I have 3, a pair running bridged and a single running in stereo in another system.  The bridged VHC puts out 500 watts into a 1 ohm load all day long.  I bought mine from a system using Apogee Scintilla speakers - there were very few amps that could run those into 1 ohm.  Great amps

Valvet Audio also makes class A SS amps that don’t get hot. They sound wonderful!