I use a 12 volt 500mA adaptor. You can go down to 9 volts or possibly lower, but current rating should probably be higher in that case (800mA or more should be OK). The switching regulator in the cradle has a wide input range 9 - 15 volts or so, but it's better to stay near the lower end of the range. As for the plug - yes - centre is positive. Size - outside diameter 5.5mm, inside diameter 2.1mm. You may get away with 5.0mm o/d or 2.5mm i/d variants, but contact will probably be less reliable.