Recommend a portable USB DAC?

[martinbls]

Well, in this case you might consider the Super Pro 707 USB DAC.

Super Pro DAC707

You can read something about it's potential here:

So many DACs, which to choose

[EC8010]

I'm happy to use an external brick DAC, but it must have either its own battery or run off USB. External power is not allowed as this has to be portable (otherwise, I'd use my DACmagic). I had a look at the Musiland, but Pacific Valve have not quite worked out that there are other countries in the world other than the US of A and their ordering system can't cope.

[TheSeekerr]

The usual suspects (so far as DIY goes) are:

AlienDAC (datasheet PCM2702 implementation - note that PCM2702 is becoming unobtanium fast)
BantamDAC (same as a above, but on a tiny board)
AMB Gamma-1 (PCM2707 for conversion to I2S, then a decent wolfson DAC)
GrubDAC (new design similar to bantamDAC, but using a wolfson DAC with a built-in voltage inverter which allows for a much simpler board and eliminates output coupling capacitors)

I use a Gamma-1 as my everyday DAC driving a Pimeta headphone amp, the combination is excellent. I also use a BantamDAC for connecting the laptop to the stereo - because the board is so tiny, it can be built as a "cable-dac" - a small box with a USB lead running in one side and RCA straight out the other. I've got a GrubDAC PCB on order, too, because one can never have too many USB DAC's, and I already had a few PCM2707 around.

If you'd like something which can drive headphones, look into the "Carrie" design over on Head-Fi, it's a version of the Mini3 adapted for USB power, with board dimensions designed to fit with a BantamDAC or GrubDAC in a Hammond 1455C80x case.