The famous Whirlwind Selector is the only A/B A+B box that provides totally noiseless switching between channels. It can be used either as a 1 -in 2-out device or as a 2-in 1out device. Whichever way you use it, the Selector is the best channel-switching device your money can buy.The Selector can be powered by either a 9-volt battery or DC adapter (Whirlwind accessory T401-ND, not included). To insert a battery, simply remove the 4 screws from the bottom of the unit and clip a 9-volt alkaline type battery to the battery clip. Replace the bottom and screws.