Class AB amps are more efficient than class A. But they can still be quite inefficient. The reason for that is that they need to be designed for their maximum power output. That means much higher power supply voltages than needed for most music listening. Class H amps are designed to improve their efficiency by switching between two (or more) voltage levels for the amps. By lowering the voltage, efficiency is improved when output level is below maximum.
This is a very nice teardown of the QSC amps which use class H. In there, there is also very good explanation of switching power supplies and general amplifier design. It is HTML 5 and benefits from watching at 1.5X speed:
This is a very nice teardown of the QSC amps which use class H. In there, there is also very good explanation of switching power supplies and general amplifier design. It is HTML 5 and benefits from watching at 1.5X speed: