This is a simple active Direct Inject Box with some very nice features. It is similar to the active box featured in the Boscorelli Book The Stompbox Cookbook. The active circuit provides for some gain (volume) control on the output to the XLR jack. The circuit has a charge pump in order to provide more headroom for the input signal. It features a gain control, polarity switch and ground lift switch. If built in a metal enclosure (recommended) you should use a XLR jack with a plastic casing so that the ground lift to work properly.

Description from General Guitar Gadgets.