MOSFET

There is no perfect device. Mosfets output stages are:

- more robust than transistors
- have higher bandwidth
- are more tolerant of difficult loads

but are:

- less efficient since have lower voltage swing from the same supply voltage
- are more nonlinear than transistors around cutoff region
- require more gain in preceding stages to eliminate crossover distortions
- limited ability to apply local feedback (low gain)

My experience with different types of amps is very limited but judging by reviews and opinions there are good and bad amps in each category.

The only area where Mosfets win hands down are class D amps but this is different type of Mosfets (vertical vs lateral) specifically designed for switching.

"no need to go onto MHz ranges I believe"

In general, to keep things stable output stage should be as fast as possible (little phase shift)and input stage as slow as possible (large phase margin). Power output transistors of modern amplifiers have hundreds of MHz bandwidth (even though slower devices are available). Also smalls signal bandwidth is different (much higher) from large signal bandwidth. We need to keep total amp's bandwidth as high as possible to avoid phase shift in the pass-band. Jeff Rowland uses 180MHz opamps (OPA1632) in Capri preamp. I have impression, so far, that he knows what he is doing.

"Lower voltage swing from the same voltage supply compared to the BJT implies lower gain" - No it has nothing to do with gain but with characteristic.

"Lower gain (for the same amount of bias current) compared to the BJT implies lower bandwidth". - No it doesn't. Even between bipolar transistors current gain has nothing to do with max bandwidth. It is just matter of design. Mosfets are generally much faster.

"I also don't know if there is any substantial evidence of BJTs being less robust than MOSFETs" - sure they are more robust since Mosfets don't suffer secondary breakdown (hot spots of current on the wafer with higher voltages resulting in thermal avalanche - very nasty complication) and are thermally stable (BJT's Vbe drops with temperature increasing collector current hence increasing temperature - thermal runaway). Mosfet drain-source resistance has a positive temperature coefficient, so they tend to be self protective.

As for tubey sound - Mosfets have softer clipping than BJts but are more nonlinear and require much higher gain before feedback to correct it - in result behave pretty much the same as BJts. Higher gain results in design more prone to TIM and therefore higher order odd harmonics.

Just think about it - if one type of devices would have clear advantage nobody would use other devices. Some manufacturers (Krell, Levinson etc) use bipolar designs.

Magfan - Isn't HexFet a vertical Mosfet designed for switching (low resistance)?

I'm just saying that there is no perfect device. Mosfets have some advantages as well as BJts and tubes. I use class D Rowland amp where Mosfets are a must.

Bombaywalla - This is a very strange application since IGBTs are used mostly for very fast switching (motor control, switching power supplies etc.). Do you know by any chance why they were used in class A or AB audio amps?