Do you plan on using B&K microphones with you new M5 mic pre? Have you come to inherit an older M4? Then you absolutely need this option for the BT2 power supply that is included with your M5. Why? Well because certain microphones, like B&K and DPA mics, use 130 Volts for their phantom power, rather than the 48V the majority of mics use. Not sure if you mic requires this option? A simple phone call or email will sort that out quick.