so i bought two 5 volt wall wart power supplies
Have you measured the voltage? Some of them are really crap i.t.o delivering what they say, and the usb must be within 4.75 and 5.25VDC (from memory, don't quote me on the figures) so you might like to be sure.....
(I have a wall wart with a switch for various outputs which gives something like 5 when it says 3.5, and 8 at 5 and blah blah...)