Thank you muchly for the reply, NZtechfreak. I actually have a micro-USB to type A (female) dongle cable into which I put a standard mini USB cable to connect my S3 to my Ultra Micro DAC. Since the DAC is fully powered via the wall, I expected it to work. It doesn't, however.
This could be due to the fact that I'm running essentially AOSP, and it doesn't have the Samsung tweaks to make USB DAC output work. That would make me a sad panda since I vastly prefer CM10 over Samsung's "Touchwiz" thing.
The dongle I got via ebay was called "Mini-USB OTG On The Go Host Lead Cable Cable Convertor" (... except it's definitely micro--not mini--since it fits my S3). It was ultra cheap from Hong Kong... Maybe I should try another one before I decide the cable is fine.
Anyway, thanks again. I'm sure I'll figure this out eventually