FiiO E17 Alpen Portable Headphone Amplifier USB DAC Standard mini USB interface: Connect to your PC for digital audio transfer and/or charge of the 1500 mAh internal battery 3.5 mm stereo headphone output: Suitable for 16-150 ohm headphones; MAX output current &gt; 80 mA; THD &lt;0.003%@1KHz 3.5mm SPDIF input (optical, coaxial) digital connector to receive PCM signals up to 24-bit/192kHz 3.5mm Analog Line-In for use with other audio sources, 18-Pin multi-functional interface to connect FiiO's L7 and E9 docks Digital Bass, Treble, Gain, Volume Controls