A power supply and interface circuit assembly is used with a portable media player (PMP) to relay signals between the PMP and a peripheral device(s), such as a head-mounted display. A power supply in or attached to the assembly provides power to the circuitry, the PMP, and the peripheral device. The assembly is able to manage the charging and discharging of power to the PMP and the peripheral device and to manage multi-media signals between the PMP and the peripheral device to provide a complete, mobile interface assembly.