I use 12v from a running computer PSU molex (yes, the horror!) But it is actually not that noisy, barely noticeable on the waterfall. It is a high quality PSU though. I also tried a standard 12V 5A?adapter and that is ok too. A pack of old 18650 batteries is of course totally noise free, but if they are not new and high quality, voltage may drop once you start transmitting. My advice is no need to go crazy with fancy noise free adapters for QDX, as its filtering capabilities are amazing.
Vojislav [YU4HAK]