I am currently working on a class A audio amplifier designed to work with line in voltage (about -400mv to +400mv). I plan on using an IRFP240 for the mosfet but am undecided on the transistor stage. resistor values will need changing of course, but I would like to think I have the arrangement of this circuit correct? I would love to know any improvements, as I am a beginner at audio amplifier design and the design goals are to be a simple, but high quality audio amplifier.
MOSFETs have very large input capacitance (several thousand pF).
To drive this effectively you will need to run the driver stage at higher current with a lower collector resistor.
Although feeding the collector resistor from the drain of the FET gives DC feedback to stabilize the operating point it will have the disadvantage of limiting the output swing as it goes towards ground. A different arrangement with separate DC feedback would be preferable.
Using a resistor feed for the drain does limit the overall efficiency to ~15% (depending upon the actual value). A constant current source can achieve 25%. An active source turns the stage into push-pull with a 50% efficiency limit.
One of the classic class-A audio amplifiers was designed by John Linsey Hood fifty years ago, it is worth reading about.