I'm considering an M-audio USB Duo for live recording. I need phantom power
and standalone ADC, which the Duo has.

The main problem is powering this batteries. It uses 9VAC!

Most people are using power inverters, right?

What about ripping it apart and finding what DC voltages it uses? Has anyone
done that? It seems it may be more efficient and less noise prone. BTW, even
if it needs something like +/- 18VDC inside, you can supply this easily from a
12V battery using inexpensive laptop adapters. These are DC-to-DC converters
with internal switching supplies. They may be noisy, but at least they don't
have 60Hz like standard inverters.

Thanks,
Richard