Trying to reclaim the topic (I can't believe the discussion has diverted from a DIY low power Class A, no feedback tube/MOSFET hybrid amp intended for high-end reproduction to a commodity, Chinese made high power Class D amp board).
This is really a fairly simple and achievable project to accomplish, since it is highly modular. I believe I covered it a previous posting, but in summary, it consists of using the Pass Burning Amp complimentary source follower MOSFET output boards available at the DIYAudio.com Store, together with a Broskie (TubeCAD) Aikido front end, with the output configured as a White Cathode Follower. If you use the Broskie boards, there are provisions on the board to configure the output as such. The Aikido board is cap coupled to the MOSFET output board. The rest is power supply, and again, DIYAudio.com store has a nice bipolar universal power supply board, and Broskie has tube power supply boards of various configurations, regulated and non-regulated. Pick one that works for an Aikido preamp with LV heater supply. I did add some chokes instead of resistors so both the MOSFET supply and driver supply boards are CLC. Antek has decent toroidal transformers. I believe the biggest constraint on a project like this is the chassis, however I used a 4U Deluxe chassis from ModuShop 2000 (Italian made) sold through DIYAudio.com store that is shipped flat-pak in a few days. It is great working with this chassis, since the heatsinks are pre-drilled for the boards and MOSFETs, and the is a perforated steel base inside the enclosure for mounting standoffs and boards, etc., so minimal drilling is necessary.
If considering this, I would recommend reading through the BA3 build guide on the PASS forum on DIYAudio.com, keeping in mind that this hybrid version does not use the BA3 front end. However it provides documentation on how to build and setup the MOSFET output boards, as well as recommendations on power supply transformer selection. There is also a build guide for the DIYAudio.com universal power supply board, and the Deluxe 4U chassis should one choose this. Also, read the articles on TubeCAD.com on the Moskido, as well as
https://www.madscientist-audio.com/mosk ... ifier.html. The Aikido boards have documentation (unfortunately, Broskie makes it hard to download without buying the boards first). This will provide guidance on tube selection and operating points. I did not use the Aikido board (too big for my layout), but the info is useful.
This is easily built up as individual, testable, modules that are assembled in the amplifier. I have to admit the 4U chassis ends up being tight for a stereo build, a 5U chassis would provide more space and allows the MOSFETS to be run with more current (no additional power output but better drive), or use a higher voltage on the MOSFETs for more power beyond 25W. If the latter, you will probably need to have more voltage gain in the front-end circuit, so perhaps a 12AX7, 12AY7, 12AT7, 5751 (6SL7 if running octal). You need to have a beefy tube for the White Follower, if noval, a 6N6P, ECC99, 6N30, or similar in the two standard basings, or a 5687 if building point-to-point. If you are building octal, then something like a 6BX7 should work well for a White follower, with a 6SN7 or 6SL7 at the front (depending on your voltage gain needs, a 6SN7 should work for a 25W version of the amp).
I hope this helps provide background. Pete, I suggest you read through the information I pointed to so you have a better idea of what is behind my amp.
David