An NPN transistor or an enhancement P-channel MOSFET offer the advantage of shifting the input voltage more positive at the emitter or source, which works well with positive voltage requirements of the enhancement N-channel MOSFET used in the output stage, as its gate must be at some 1-4 volts positive voltage for its source to at 0 volts. The transistor offers a smaller, but predictable voltage shift, whereas the MOSFET offers a larger, but much less predictable voltage shift, as would a N-channel FET or, to a lesser degree, a vacuum tube.

     All in all, a very seductive circuit, except for a few major problems. The top potentiometer sets the output stage's idle current and the bottom potentiometer sets the DC offset. Unfortunately, these potentiometers do not work independently of each other, as a change in one setting will necessitate a change the other.
     A more serious problem with the circuit is that it is not very safe. What happens when the tube fails or is pulled or jiggled? The answer is that the top MOSFET will be completely turned off and the bottom MOSFET will be completely turned on, which will slam the output to the bottom rail voltage and destroy the woofer's voice coil. A coupling capacitor and a DC servo-loop, when used in an alloyed solid-state buffer, eliminates the need to worry about the potential DC offsets at the output, but it only partially alleviates our worry when used with a hybrid buffer, for example when the tube is removed or jiggled while the buffer is in use. A DC servo can try to adjust all it wants, but if there is no grid for it to work into or if the heater opens, it cannot control the DC offset at the output.
     One solution is to use coupling capacitors in between the tube and the MOSFETs. This modification allows the DC servo to directly control the output stage's DC offset, with or without the triode present in the circuit.

     In the circuit above, reader Tony has designed a hybrid buffer that uses a triode to drive the output MOSFET's high input capacitance. The circuit differs from the last in that it need not be run in strict Class-A, as it can be run in Class-AB or even Class-B.
     The fall from Class-A into Class-B will increase the output distortion, but greatly lessen the idle dissipation, allowing higher power supply voltages, with a consequent increase in output power. As an added feature, the input is DC coupled from into to output.

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