March 2008: I've started playing with the conversion of an old PC into a music server to investigate the potential benefits of using a PC's hard drive as a transport, with a dedicated USB DAC. I started with a well-regarded non-oversampling USB DAC kit from Scott Nixon called the USBdk:
Although typically powered by a wall wart supply, I built mine in a cast aluminum chassis with a pre-regulated 12VDC supply. I upgraded a few passive parts on the PCB, but otherwise its stock.
Above: Front view, click to enlarge. The two small LEDs indicate power to the DAC and a live USB connection to the PC.
Above: Rear view, click to enlarge. Although I built a 12VDC power supply into the unit, I left the DC power connector accessible to experiment with battery power in the future.
Above: Interior view (from the bottom of the chassis), click to enlarge. The DAC is the blue PCB on the top left, the small vertical PCB at the top center is the 12V regulator, and the toroidal power transformer occupies the right side of this photo (which is actually the front of the unit).
Above: DAC PCB, click to enlarge. I built the kit mostly as stock, but I replaced most of the resistors with Resista's, and replaced most of the electrolytic caps with Nichicon Muse. The output couplers are the silver (no plastic wrap) electrolytics between the two large Sonicaps, and the Sonicaps bypass those output couplers. I used Vampire RCA jacks.
This unit has worked perfectly since I first powered it up, and was quick and easy to assemble. Scott ships the kits with the SMD parts pre-mounted on the board making it that much easier.
Listening results to follow...