Anything over 30-40 watts being sold as Class A is not pure class A, it’s class A biased.
This statement is misinformation. If an amplifier is class A biased, its a class A amplifier. The output power has nothing to do with it.
The trade off to a Pure Class A amp and the sound it produces is the limitation of the wattage and it's ability to drive a wide array of speakers.
Here are some simple facts about class A operation:
Class A simply means that the output device or devices do not shut shut off at any point during the waveform to be amplified.
A class A amplifier can be made of any power. We make one that is 500 Watts/channel (So the quote above doesn't make any sense).
The output impedance of the class A amplifier can be quite variable- depending on if the amp is tube or solid state and how much feedback is used.
On top of class A, if you are talking about tube amplifiers, there are three forms of class A: A1 in which no grid current flows, class A2 in which grid current is seen during part of the waveform, and class A3 (recently patented) in which grid current also flows.
Class A can be either single-ended or push-pull. It can be tube or solid state. Triodes and pentodes can be biased class A.
Any amplifier that is class A to a certain output power and then is in class B above that power is an AB amplifier, regardless of how 'enriched' the bias on the circuit actually is. IOW an amp that is class A to 50% of full power is an AB amplifier.
