Xotic AC Boost Comp
From the manufacturer:
The AC-COMP features improved tonal characteristics that have been modified for tighter, yet smooth overdrive sound. With three compression modes and internal dip switches, you can choose your level of compression, tonal character and make subtle adjustments to treble and presence levels.
True bypass for eliminating any signal interference when switched off. Can be used with AC adaptor (optional) or 9V Battery.