It's amazing how little I actually know about the DAM DAC... There's just so much hidden in the electrical circuits and components that could affect the performance, completely hidden away from non-majors behind the pretty abstractions... Let alone the fact that we don't really know the full dam1021 schematics...
I found out after studying all 77 pages of
Soekris Dam Dac - Tir Na HiFi where Nigel and friends went for (and got VERY far) the ultimate dam1021, destroying a few boards now and then of course.
First of all, correcting my last post, the power regulation on the dam1021 is more likely:
AC/DC -> Bridge rectifier -> PWR A+/-
PWR A+ -> 3.3V Regulators-> Digitals (FPGA, oscillator and microprocessor)
PWR A+ -> 1.2V Regulator -> ?
PWR A+/- -> +/-5V Regulator -> +/- 5V DC -> Op-amps -> 4V vref -> shift registers
PWR A+/- -> Output buffer op-amps
[Probably still missing some but better than last...]
With this, now I can answer my own questions...:
(1) Digital/analog separation in the ground plane, or power in general, is probably not perfect. There is no conclusive evidence but the fact that Nigel and friends believe the vref battery mod was the most significant among all the mods which they A/B tested gives some hint. The mod basically provides independent power supply to the analog circuits (with output buffer already removed...).
(2) One could go all out on battery/externally regulated power supplies for all voltages/major components on-board, as a few did. And replace the 1.2V switch-mode regulator which seems to be noisy by an LDO counterpart... Here's my current idea of a reasonable and accessible mod, if the output buffer is not needed:
i. Supply PWR A+ with 5-7V DC, replace 3.3V regulator with Sparko discrete regulator, wire the VCC on the oscillator directly to the regulator for improved performance.
ii. Replace 1.2V switch-mode regulator with LDO.
iii. Supply ~4V (such as 3.3V with LiFePo4 battery) to shift registers, bypass op-amps. If done with batteries capacitors can be left out, but if done with (here I'm very much not sure...) regulators, supercapcitors might be needed. A possible regulated supply is perhaps Regulated DC -> 4* 4V Sparko Regulators. Sparkos are rated at 3uV ripple, 1.5% voltage precision, and 1A output, which should be enough for the shift registers (??).
iv. Supply future Soekris add-on buffer/headphone amp with the same regulated DC as the Sparkos for the shift registers. Or use an existing buffer kit.
v. (optional?) Swap out certain on-board ceramics for better caps / bridge more caps at places if there's room.
This should provide a higher level of digital/analog separation, better regulated voltage for critical components, and doesn't go as far as using battery supplies. Ideally, this should only require one transformer with two sets of secondaries for digital and analog respectively. one dual-rail linear regulator for analog and one single-rail linear regulator for digital. The enclosure would also have a quality buffer/head-amp built-in.
Does this sound reasonable? If so I might consider doing it next year when I have more time... also ideally after Soren releases the head-amp board so I can stack it with the balanced dam1021's....
This is just a naïve attempt at a decent mod... please offer critiques of any kind!
Happy New Year!