AGPtEK Roar Headphone Amplifier Portable Amps with Bass Turbo for Phones, MP3s, Digital Players and Computers(Black) Shocking sound: Pre-Fever grade op-amp buffer with professional amp chip and separate bass switch, makes your earphone an unbelievable subwoofer Use Japan's Sanyo solid electrolyte to make power filter, which means the charge and discharge rate go more fast and the dynamic sound is better Designed with the SMD SMT technology to ensure small interference but greater sound quality Small size, big sound in your pocket, Size of a credit card Charge: Built-in lithium battery comes with protective circuit, it usually takes 2 hours for the charge but 48 hours playback