Krell KSA-250 power amplifier When is Class-A Class-A?
A contentious subject is exactly how much of the KSA-250's power delivery is in pure class-A, which is when both "upper" and "lower" sets of output transistors are turned on all the time. By contrast, class-B is when the upper transistors are turned on only during the time the output waveform lies on the positive side of ground potential and the lower transistors only during the time it lies on the negative side of ground. Class-A/B, which applies to the vast majority of solid-state amplifier designs, involves a small standing bias current to ensure that neither upper nor lower ouput transistors turn off for small signals, thus minimizing crossover distortion.
With amplifiers like the Krell and Threshold models reviewed this month, there will come a point when the output stage's operation changes from class-A to class-A/B, this proportional to the standing bias.
The Krell's rated output of 250W per channel into 8 ohms implies a standing bias current of 3.95A per channel—square root of 250W/(2 x 8 ohms)—if all this power is to be delivered with the output stage running in class-A. Assessing an amplifier's bias current is not straightforward, however, particularly when, like the Krell, it doesn't have a fuse in series with the rail voltages.
Looking inside the KSA-250, the emitters of the 12 pairs per channel of output-stage transistors appear to standing on series resistors of nominal 1 ohm value. The average voltage drop across these resistors was 110.5mV, implying a standing bias for each of 110.5mA; ie, a total of 1.33A. This will give a maximum power for true class-A operation into 8 ohms of 28.5W (14.5dBW) rather than 250W.
Does this matter? I don't think so, as the amplifier's output stage will still run in class-A into 8 ohm loads for all but the final 9.5dB of power delivery, implying that the virtues of class-A operation—constant power-supply stress and constant output-device temperature and transfer function, among other things—will still be there to a large extent.—John Atkinson