Shown above is the revised schematic. The amplifier now consists of four cascading emitter followers. Each is loaded by a constant current source and each operates in strict Class-A. The total dissipation at idle is about 214 watts (many square inches of heat sink are needed). Notice the half an ampere of idle current for the final driver stage, which equals 1 watt into an 8-ohm load. Also notice how the voltage references are placed in series with the transistor's collectors, which saves extra resistors and extra heat. Another feature worth looking into is the output stage's active constant current source. Notice how the NPN transistor has been replaced by a power MOSFET and how a transistor has replaced the voltage reference. The problem with the NPN transistor was that is required too much current into its base. The MOSFET, On the other hand, is voltage driven and easily controlled by the TIP31C. How was it possible to replace a 250-watt transistor with a 150-watt MOSFET? First of all, the current source, unlike the emitter follower, draws a constant current.

     Additionally, MOSFETs do not suffer from the secondary breakdown problems that power transistors do and they are in general much more robust in actual use. For example, I would have little or no fear of running a 25 watt tube at 25 watts of continuous dissipation, nor would I have much fear of running a 150 watt MOSFET at 90 watts of continuous dissipation, but I am a quite a bit nervous about running a 250 watt transistor at 90 watts of continuous dissipation. Not all specs are equal.
     So is this revised circuit perfect? No, of course not. One aspect that still troubles me is the output stage's emitter follower; I would prefer to use a MOSFET, but I realize this would ruin the low DC offset offered by the transistor. Of course, adding an input coupling capacitor and a DC servo loop would take care of any DC offsets, but for some, the DC from input to output aspect of this buffer is a key feature. (Here is one of those odd nonlinear behavior examples: I know many solid-state fans who would never dream of placing coupling

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