Digizoid ZO 2 Personal Subwoofer and Headphone Amplifier Headphones with an impedance between 16 and 300 ohm are suitable for use with ZO Built-in, USB-rechargeable Lithium-polymer battery with a 1 hour charge time and approx. 17 hours of playback Standard 3.5mm stereo input and output Memory feature automatically recalls last volume and contour settings 32 contour levels allows you to adjust bass intensity and characteristics