Hmm, I'm rethinking if I need one
I was playing my Duo for a good 45 minutes to an hour before I went to change discs, when I did I noticed the Duo was getting pretty warm around the Duo logo area. After trying to swap discs, everything I put in would get read by the audio CD player.
I am currently letting it cool down but it seriously is getting too warm for me to think it's safe to use with the 100V adapter in a 120V outlet.
EDIT: I let it cool down for about an hour and it worked perfectly fine again
So I'm thinking I need to do something about the power supply situation? I know PC Engines, finicky as they are, are usually just fine with 120v power, but the non-R/RX Duos are very very picky!