I just posted schematics in the Yamaha Technical Documents. The circuit is similar to API kind of. The input switch is a multi deck pad network switch to allow the mic/line input transformer to do double duties. Where API used a direct to DOA scheme in line mode and a 1:10 mic transformer in mic mode.
With the PM2K, and miking something moderate level, you turn the input switch up past 12:00 or so and it kicks in the first DOA, a Yammy 80200 (but with a lower ratio input transformer than API) After this point it's API ish if you pretend all the chip IC's are 2520's. The very first IC in the "line" mode signal path (IC2 I believe) is a prime candidate for replacement with Pierre Paulo Abatte's APP922 DOA that I believe he has a version in the SIP format like the Hitachi chips. After that there are two more IC's used for low cuts, which I don't generally use, but from ear they seem handy and to not effect things too much when used. They are switchable in/out. The signal then goes into an eq section that even when switched "off" forces this signal through two IC's. I believe this is the same with an API console where you're forced through two 2520's unless you physically bypass the eq section with a patch cable. Anyway, you can hard bypass the eq on the PM2000 as well, but I think (from memory) you lose a little gain in the channel. I haven't tried it yet, but I'm imagining that adjusting the 2.2K resistor off of the 80100 (IC10) negative input will get a bit more juice out of the opamp to take care of that discrepancy.
Anyway, as to it's "discreteness" in the input strips, there are at minimum 3 IC chip and 1 DOA that the signal goes through. Sounds pretty good as is though (the input strips) Which could be reduced to 1 IC chip and 1DOA with the eq bypass, or possibly 2 DOA's (with one of those specialty APP992's) I don't know enough about this stuff to figure the filtering and stability of changing from an IC Hitachi chip to a DOA, but I wonder if less capacitance could be used on the rails there and less bypass caps.
The master section is whole different discussion. I guess the coolest thing might be to put an API ACA bus as the master with the option of using different opamps that are +/-24V capable (or build a complete different power supply to power 2520's) My faves are Pierre Paulo's APP992 and Yamaha's own 80100. 990C's are good too imo, but I like the APP992's a bit better. Just my opinion so far.
I installed the ACA in mine, didn't understand at the time how to vary the gain of the opamps and then took it out frustrated with it not putting out enough gain for the master. Silly me. I then installed two 1272's and like them, but I think I want to try the CAPI ACA bus again, now that I understand better. Hmmmm. Grass is always greener I guess.
I actually turned two input channels into a fully API 536 circuit (using APP992 and 80100 DOA's at +/-24V's) Not sure I like them better than my other channels yet though. I need to do more listening. They work fine doing all the busing functions and everything.
Killer console.
