XD-02 is a portable headphone amplifier that consists of USB DAC and AMP.  USB accepts up to 24Bit/192KHz high resolution audio signal.  It has a built-in OP+BUF architecture, high slew-rate, OCL (output capacitor-less) with split power supply.  It also has 1 way coaxial signal output, 1 way Line Out/Line in, one 2-Setting Gain Switch, one Bass boost On/Off switch.  Its power supply is a built-in 4.2V li-polymer rechargeable battery.