The Rolls PM55P with built in Optical Limiter is designed for performers who need to monitor their own signal, such as their vocal, with the main mix. The unit has become very popular with drummers who need to mix a click with a monitor signal. And many sound technicians use the PM55P as a quick line tester to troubleshoot audio systems. Operates on either one 9 volt battery or the included PS27s power supply. The PM55P also has an included belt clip.