Nimai

i recently used my nh900 to record a long recording. I had it connected to the mains for the whole period. when i heard my recordings i heard a buzzing sound in the background. Should this happen? If not then is it a fault of my player? Otherwise what would you reccomend i should do to increase battery life whilst recording? would the batteries from the MZN1 net MD work on a nh900? all suggestions would be appreciated. thanks