Also, regarding "how would it know a transient is coming to switch it to a higher mode?," while I don't quite understand the reference by Mr. Colloms to "high-speed current-feedback circuitry," presumably the delay from amp input to adjustment of output stage bias is less than the propagation delay through the signal path from amp input to the input of the output stage.
There have been a few designs that have appeared over the years from other manufacturers which also changed output stage operating conditions in anticipation of signal requirements, except that what was being changed were the DC voltage rails supplied to the output stage. In those cases that I am aware of the purpose was to reduce power consumption and heat, and consequently cabinet size, weight, and cost, rather than to maintain what might be considered to be class A operation. Some of Bob Carver's older "magnetic field power amplifiers" were one such example.
Best regards,
-- Al
There have been a few designs that have appeared over the years from other manufacturers which also changed output stage operating conditions in anticipation of signal requirements, except that what was being changed were the DC voltage rails supplied to the output stage. In those cases that I am aware of the purpose was to reduce power consumption and heat, and consequently cabinet size, weight, and cost, rather than to maintain what might be considered to be class A operation. Some of Bob Carver's older "magnetic field power amplifiers" were one such example.
Best regards,
-- Al