I am glad that hardly a half day has passed and this announcement already generated so much interest. This is going to be a very exciting project because it combines the essential qualities of the PPA with that of a very
robust class-A MOSFET output stage (I love the term "studly" that morsel used in the announcement
), and makes several significant enhancements over the SDS Labs MOSFET amplifier as well.
I built and tested several iterations of prototypes on breadboards, exploring some different possibilities. As Morsel said, we tried a diamond buffer output stage, except with MOSFET output devices. It worked well enough, but ultimately didn't measure as well as a simpler, directly-driven MOSFET topology. Just for kicks, here is a photo of the diamond buffer-based amp prototype:
As for the choice of MOSFETs, we had to consider several factors. Since this amp is going to operate in class A, we would like to choose high current devices for reliability. The Ciss should not be too high, and yet high current devices always have higher Ciss due to their larger internal junctions. Stable and assured availability is also an important factor in our consideration. Linearlity, performance and suitability for the application, and last but certainly not least, the cost is also considered because we want this amp to deliver a lot for the money.
The Hitachi 2SK213/2SJ76 series were considered. These are nice devices with low Ciss, but IMO just a bit "weak" with a Ids rating of 0.5A. The 2SK1058/2SJ162 are stronger, but the IRF devices we chose are even more robust yet with comparable linearlity, Ciss. Plus they are stocked in quantity at Digikey and Newark Electronics. The 2SK/2SJ devices are scant by comparison (at least in the US) and more expensive. The choice then became clear. The Renesas/Hitachi parts are not pin-compatible with the IRFs, so we had to choose one or the other for our PCB layout.
As for the op amps, both Morsel and I like the AD8610. However builders/users can substitute others like the OPA637/627 just like you could with the PPA.
We would definitely welcome more comments about the other open choices as listed in the announcement. In particular the power supply, casing, heat management, etc.