FiiO E02i Rocky Headphone Amplifier for iPhone 3GS, 4, 4S, 5 Get enhanced audio output from an iPhone, along with switchable bass boost. Lossless bass boost circuit design realizes natural sounding and strong bass effect. The E02i provides all of the line controls for music and phone calls on the iPhone, including play, pause, change tracks, and answering calls The built in lithium ion battery lasts for up to 10 hours of normal use. In case the battery runs out, the bypass feature allows the E02i to continue function normally without the amplifier feature. An aluminum shell not only looks fashionable and is durable, but also fully shield external interference to prevent noise. Use any headphones to make calls on an iPhone with the E02i's microphone. Located on top of the E02i, the high sensitivity microphone is designed for clear conversation and low noise.