"Tim Martin" wrote in message


So; suppose we have a sound reinforcement system, with
an ordinary class AB amplifier, whose manufacturer says
it can deliver say 250 watts per channel or more at some
distortion level. And we are using it for a pub band
rock performance, and towards the end of the evening,
it's at or close to full power, with speakers putting out
120dB or so.

Ah, the real world. ;-)

Os this amplifier working in Class A some of the time and
Class B some of the time, or what?

A typical power amp is working Class B (only half the output
stage conducting at any one time) almost all the time, with
a short trip though Class A (both halves conducting) in a
small region around each zero-crossing, related to current
passing through the output stage.

It's usually not a big thing that this zero-crossing is
often a different point than a zero crossing in the input
signal or the output voltage.