Looking at the schematic, the TA-A1ES switches the quiescent bias current from low to high depending on volume output. At low volume it uses low bias to save on heat output from the output transistors. Both halves (both NPN and PNP output transistors, ie, + and - halves) of the push-pull output stage are kept conducting in such an arrangement so crossover distortion is kept to a minimum. The output stage is still operated in class A at low volume, as the bias required for class A operation for high volume isn’t needed at low volume.
There is a slight advantage to distortion at low volume when the output transistors are operated in high-bias class A all the time, as in traditional, hot-running class A amplifiers.
Note that nowhere in the TA-A1ES literature does it claim to operate in class A, instead saying it has a Smart BIAS system. My Denon POA-4400 designed in 1989 has what’s called Optical Class A which operates in a very similar manner to the Sony.
There is a slight advantage to distortion at low volume when the output transistors are operated in high-bias class A all the time, as in traditional, hot-running class A amplifiers.
Note that nowhere in the TA-A1ES literature does it claim to operate in class A, instead saying it has a Smart BIAS system. My Denon POA-4400 designed in 1989 has what’s called Optical Class A which operates in a very similar manner to the Sony.