Planned DAC upgrades

Upgrades:

- V/I transformers
- USB in (TOSLINK in?)
- Black Gates on 6922 board (2x 470uF)
- Mundorf's SIlver/Oil decouplung caps (2x 0,470uF)
- other tubes (Amperex 7309?)

Improvements/fixes:
- equalize TP1 == TP2, higher TP3
- equalize filament voltages (redesign filament PS?)
- cap before filament VRs (100nF)
- cap before gretz (cca 10nF)
- PS (C/10 + C + R + C+ C/10)

TODO:
- smart SLA charger
- caps on SLAs

Interesting AN DAC upgrade.