DigiZoid ZO FS - Headphone Amplifier The only analog equalizer and portable headphone amplifier for Apple, Android and Windows devices Select from 33 bass and 33 treble levels for ultimate control over your sound High resolution volume control designed for use with sensitive IEMs and high impedance headphones Full-color OLED screen encased in an anodized aluminum enclosure, with a built-in rechargeable battery Audiophile performance and power in one of the smallest form factors available  all made in the USA