RapcoHorizon's i-JAM 3-n-1 is a must have for the performer using electric instruments. It provides the player with 3 interfacing solutions in one small, portable package. The i-Phone Instrument Interface is for use with applications such as amplifier models, recording software apps, or guitar instructional apps, just to name a few. The built-in amplifier allows continuous monitoring for the applications that do not provide this function internally. The Practice Amplifier needs no other interface. Just plug in your electric instrument, your headphones, and flip on the power, and play. The included 9 Volt battery is replaceable. The MP3 Play-along Device uses the built-in instrument amp simultaneously with any MP3 player. Just plug in any MP3 output device, your headphones, and your guitar to play along with your tracked music.