Recently jumped onto the USB audio bandwagon by buying a used Monitor 02. Natually nothing is good enough stock and some mod has to be done (whether necessary or not ), especially when I saw the huge "empty/waste" space inside that nice aluminum enclosure waiting to be "stuffed". I know this is an old model, but it is still in production and used ones can be had for cheap, maybe somebody is still interested in mod related info. Here is what I am doing to mine....
See photo #1 for power voltage points on the board. The right side of the line is powered by +5V from USB, for digital section, current draw is about 45mA (with power relay disconnected). The left side of the line is powered by the +/-12 from the little on-board module, current draw at idel is about 50mA for the +12V line and 25mA for the -12V line. Since both the USB power and little power module power (it is gotta be a switching supply) tend to be noisy, I will replace them with voltages from a linear supply. THe USB +5V can be cut off at the USB socket. My plan is to pipe in +30V from an external supply/wallwart, and split/regulate it inside the case, to generate the +/-12v and the +5V. The current will be set to 150mA, that should be enough even with the headphone amp running at full volume.
the main purpose of getting this little DAC is actually USB-> SPDIF conversion, any analog output is just bonus, I don't plan to do much to the analog line-out, and won't do anything to the headphone amp section at all. The DAC chip on the board is PCM1793 with voltage outputs, the stock op-amp output stage is probably good enough. (if it was the higher performence 1794 with current outputs then maybe I can justify a tube output stage, but the lowly 1793 doesn't seem to be worth the effort). I do want to do some opamp-rolling, though, I will take out the stock SOIC opamp and add a DIP-8 socket for easy op-amp swapping.
Edited by AudioCats - 11/18/12 at 9:27pm