In Class A, the bias current is turned on to the transistors all of the time. Meaning that the transistors are always on and ready to reproduce the signal. In a Class AB amp, the signal triggers the current to the transistors, turning the transistors on when louder passages are coming and shutting bias off when signal quites down. The end result is that in a Class AB amp the current to the transistors is turned on and off, thereby sparing electricity. Those that enjoy Class A sound say that it's due to not hearing the byproduct of what they call switching disorder. That in essence is the sound of transistors turning on and off.
The down side to Class A is that the amp runs very hot when bias current is on all the time, which also increases the electric bill.
The down side to Class A is that the amp runs very hot when bias current is on all the time, which also increases the electric bill.