Class A, AB

I too have Class AB amps that run in class A mode until two-thirds power then run in AB mode. I a not sure of the design principles used - perhaps a sliding bias or the final ouput stage only goes to work at the higher operating range....

The definition of Class AB is: Class A to some given power, it might be 5% or 75% (doesn't matter), and then 'B' operation after that

Surely an amp that is capable of operation to 75% of power in Class A is much more expensive to build than one that runs only Class A to 5% of same rated power.

Why does it not matter or am I missing something?

Shadorne, I used the example of 75% to make a point- I don't think there are any amps out there that operate with that much 'enrichment' in AB mode.

Atmasphere,

Most of the Chord line of power amps are 'A' weighted to two thirds power or 67% - not far off 75%.

All,

While we are on the discussion of Class A, remember that the reasons for these amps, with massive heat sinks and huge power consumption, are to avoid crossover distortion affecting 3rd, 5th, 7th and 9th and much higher odd-order harmonics which can work its way up from a room pounding mid/bass frequencies to be audible, even in tiny amounts, in the 2 Khz to 8 Khz range(where your hearing is naturally very sensitive and where the harmonics may bear little in relation to anything musical).

A trick that works:

Rather than use only a super massive Class A (very expensive) to drive your entire speaker. The cheap trick is to bi-amp your speakers and use Class A for the tweeter (only very modest power needs) and use AB for your woofer - this way you keep crossover distortion completely away from your tweeter AND you have a massive powerful punchy bass without having so much heat and huge expense. Anyway just a thought...and although it may be a cheap option it will still sound sweet.