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
enjoy