Doesn't matter, as long as you apply the proper DC voltage that is listed on the machine's jack. Seeing as it takes 6 AA batteries, I'd say around 7-9VDC is fine (7 is listed on the Turbo Express).
Really, it might be easier if you just buy a bunch of high-quality NiMH rechargables instead. I've been using them for years, and the life is excellent on this machine.