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.

"A class A amplifier has by definition 0 db of headroom."

I'm not so sure of that. Class A is often designed with bias current in order of 150% o max output current to guarantee that transistors won't become nonlinear - hence some headroom might be possible.

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.

Atmasphere, Your statement is simply not true. The class of operation has nothing to do with headroom. Headroom is the amplifiers ability to perform beyond it's rated power for a short amount of time, like during a dynamic peak in music. Class A operation eliminates switching distortion found in class AB amplifiers. Alternatives are A AB or AB1 which are high biased AB and run in Class A to a higher output rating than AB.
Also, your atatement of "headroom specs are a way to make cheaper amps look better" is also false. A much larger power supply is required to add headroom to an amplifier and the power supply is where the money is. The larger the storage capacitors the larger the tranformer has to be to keep the capacitors charged. This is where headroom comes from.
All of Krell's pure class A amplifiers have 3db of headroom. Classe Audio class A DR3B and DR3VHC have 8db of headroom. Bedini class A amplifiers have 3db of headroom. Just to mention a few.

Hmmm...I think someone who designs some of the best amps in the world knows what he is talking about.