What is 'class A'?

[OldCopper]

I'm shopping for a decent mic pre. Takin' my time, checking it out. It's a big purchase for me.

Anyhoo, I keep finding references to Class A amplifiers? What does this mean. What are the characteristics of these units? I also see occaisional references to Class B amps. So what's the diff?

Also some units boast about transformers (sometimes "Jenson") on mic inputs? Same questions as above...

Thanks for your help!

 

[Emeric]

Amp Classes:

http://www.rane.com/digi-dic.html#amplifier_classes

 

[Tapehead]

Class A is used mainly in high end electronic amplifiers or preamplifiers. Tha main idea, simplified...A sound wave has a peak and a valley. In a class B amplfier one part of the electronics amplifies the peaks and another the valleys. This is efficient but it also creates some distortion where it switches between one and the other. Class A amplifiers add bias to the signal so both the peak and valley are continuously amplified without switching. Theoretically cleaner sound, but very inefficient because the amp is always working to maintain the bias even when there is no signal present.