The 2SK1530/2SJ201 are Tosiba Lateral MOSFETS. These are designed specifically for Audio. They have a negative temperature coefficient and do not suffer from secondary breakdown/thermal runnaway, thus not needing temperature compensation via mounting the Vbe multiplier transistor on the heatsink. If this IS done it might cause crossover distortion before or after the amp has warmed up depending on where the bias is set.
The IRF device are HEXFETs (D-MOS) and do suffer from secondary breakdown and it is adviseable to use temperature compensation with these. Not only is the capacitance different, but so is the Vgs ON, thus they need a higher voltage bias than the Toshiba/Hitachi devices.
I've seen an amplifier (Hybrid) that the designer gives referances to using both lateral or IRF devices and this design includes a Vbe mupltiplier but no mention of transistor mounting is included. However there is a suggestion for changing bias set ups (I.E. different R values) to account for the differing bias needs and to make ajustment easier. The OPS is a Source Follower.
These are the main differences I see between these two types of transistors aside for the price. The Lateral types are much more expensive, but more rugged and OPS designs for these can be more simple.
I would be wary of dropping IRF devices in place of lateral in any amplifier that doesn't have Temp. compensation. This could be disasterous (particularly with Source Follower configurations), with the simpler Japanese designs such as the MJ amp or similar. However if the OPS is a Complimentary Feedback type with driver transistors using a Vbe multiplier it is conceavable that you could swap the two and only have to tweak the bias.
-D.
