Dynamic Headroom

Class of operation has everything to do with headroom. A class A amplifier has by definition 0 db of headroom.

I have always looked at the headroom spec as a way to make cheaper amps look better due to the term itself. IOW 'headroom' seems to imply that an amp that *has* headroom is better than one that does not, but IMO/IME the opposite is true. Theoretically and often in the real world, class A is as good as it gets.

In a Class A amp the output devices do not go into cutoff within the amp's linear region. Sure, you have the amp biased heavily, but that does not affect output voltage.

Many people are surprised to find out that the bias point does not affect the output power (that is a function of the power supply- class AB amps usually have a higher Vcc or B+ than class A amps do so you don't roast the output devices with too much dissipation). Bias affect the *distortion* of the amplifier- class A allows a push-pull amplifier to cancel harmonics at any point in its operating region. In the case of a single-ended circuit, it puts the operation at the most linear portion of the output device's curve.