The MXR M133 adds a preset amount of gain, using a single control. With a guitar, this lets you boost your signal for lead work, adjust between two different guitars with unmatched output (i.e., humbucker to single coils), or it can supply a permanent boost in a long effects chain where signal drop off is a problem. Powered by a single 9 volt battery or Dunlop's ECB-003 AC adapter.