Explain Class A amp to non audiophile friends

OK, here's an explanation for your non-audiophile friends & for you:
class-A amplifiers are a class of power amplifiers where the output transistors (or output stage) driving the loud speakers are always on & are always dissipating the maximum wattage that the power amplifier is rated for. Due to this fact, the heat sinks of class-A power amplifiers get very hot - 54C is considered the norm. Class-A power amplifiers actually run a bit cooler when the music is playing because a significant portion of the output wattage is delivered to the loudspeaker (rather than being dissipated as heat when no music is playing). The advantage of class-A power amplifiers is that, since the output stage is always on at full power, there is minimal amount of signal distortion (THD).

in a class-AB power amplifier, the output stage driving the louspeaker is biased at a very low level in comparison to a class-A power amp such that the output stage is considered to be weakly on (as compared to being full-power on in class-A and completely off in a class-B power amp). The reason for having the output stage weakly on is to avoid what is called as "cross-over distortion". Cross-over distortion occurs when the power amp needs to follow a music signal that is going from a positive cycle to a negative cycle or vice-versa and some time is required for the transistors servicing the negative cycle to turn on (pos cycle --> neg cycle). The advantage of a class-AB power amp is that it consumes much less power during idle & remains warm to the touch (as opposed to hot/scalding to the touch for a class-A power amp). The cross-over distortion is minimized in a class-AB power amp but I believe that it is not eliminated.

There are several well-known amp designers who subscribe to the view that a class-AB power amp can sound just as rewarding as a class-A power amp & that it is all in the implementation.

Hope that this helps.....

here's my attempt at a semi-laymen view of the issue.

with Class A, a single output device (eg, transistor) produces the full output wave. That is, both + and - side of the sound wave.

with Class AB, the + side of the wave, and - side are produced by 2 separate output devices.

The result is Class A generally produces more coherent sound because the 2 waves in Class AB don't always match up perfectly resulting in "crossover distortion". Further, Class A is less efficient, and generates more heat because of the way the output devices need to be biased to operate in Class A. Conversely, Class AB is more efficient to operate, and to manufacture (all other things being equal).

Don't explain, as there is nothing non audiophile's will ever understand about your obsession. Non audiophile's can only understand what they hear, better or worse. So, leave them with A is better, then let them hear so.

Thanks Atmasphere,
It would seem(at least in theory) that class A operation although much less efficient is a more pure and less 'altered' form of signal propagation.Implimentation probably has much to do with the final sound quality when comparing class A vs class AB.

The last responder got it right technically speaking. I see confusion regarding Class A and Class A/B all the time. Some believe that Class A means the bias is such that the output (could be any devices in the amp) are turned on all the time. This is techically true. But, others don't understand that there is class A and class A. One is the output devices (or pre-drivers), input devices, etc. can be biased such that they are always "on". This could be the equivalent of a 100 wpc amp biased "on" at 2watts. it is still class A because the drivers are "always on". However, there is the other class A, where it is biased "on" at rated output power. This means that without an input signal present the output drivers are biased "on" at sufficient current that the drivers total output produces the current for 100 watts. This means that you really need a huge, beefed up power supply with expensive large transformers, etc. and expensive heat sinking. So, there is class A and class A. Class A/B is as discussed previously. So, like any electronic device, especially amps, you really have to read between the lines on the specs, to get the real truth. I have seen amps touted as class A at 150wpc, which were totally untrue. Because without large transformers, and stupidly large and expensive heat sinks, there is no way on this planet that they would be biased at 150wpc class A. Maybe 2wpc class A, but not the other. The old saying is still true. you can really tell the quality by how much it weighs. Get on Nelson Pass' web site on the DIY site, he really gets into detail to explain Class A and heatsinking and power supply design to the lay person. This is Engineering 101. There is no magic here. it is what it is. lots of manufacturers outright lie or stretch the truth regarding specs. others, such as Nelson Pass tell it like it really is. Seriously, his explainations are really good for non-Engineers to really understand amp design.

enjoy