The HR-2 is supposed to have AD797 in it. I assume that the HR-2 at the meet was unmodified, but I don't know. My modded SDS Labs amp was running a AD8620AR at the meet (even though I briefly rolled a AD8066AR in there for comparison, but switched back to the 8620).
For the new amp that Morsel and I are designing, we have not firmed up the plan on which output MOSFETs to support, but *if* we must do only one version of the PCB, it's probably going to be the IRFs. It's unfortunate that the Hitachi/Renesas are not pin-compatible with the IRFs. While the Hitachi/Renesas 2SK213/2SJ76 series have nice low gate capacitance and low gate turn-on threshold, they also have low transconductance and limited current capability (only 0.5A, contrast that to 17A/19A for the IRFZ24N/IRF9Z34N). I would like to have an output stage that has real chutzpah (to be able to *drive* difficult lo-Z loads with ease even at high volumes), and using those Hitachis would mean needing to parallel multiple pairs, negating some of the low capacitance advantage and adding complexity, PCB and heat sink real-estate and cost.
The IRFs are not pigs as far as gate capacitance go, considering their high current capability (e.g., lower than the Hitachi 2SK1056/2SJ160, which are rated at 7A). I achieved very good high frequency performance with the IRFs on my modified SDS amp. Frequency response is -3dB at well over 1MHz with all the good, fast opamps (see the measured specs and square wave performance on my web page for it). Even the 200KHz square wave response is beautifully steep and clean with the right opamps.
I have tested the circuit on a breadboard with the Hitachi 2SK213/2SJ76 MOSFETs, and they do extend the high frequency even further, but we're talking about something so far out of the audio band it's really academic. Sonically, I don't think I could distinguish any difference between the Hitachis and the IRFs with my HD600s.
So, perhaps the gate capacitance issue is a moot point. What is important is that the IRFs are readily available from Digikey and Newark, and are less expensive than the Hitachis. Not to mention that only one pair per channel of the IRFs is needed.
Anyway, I think this is drifting off topic for this forum. Morsel and I will post on the DIY forum when we have something more concrete to discuss.
-Ti