Sonic Impact Portable T-Amp Digital Audio Amplifier. This is a battery powered 15 watts per channel stereo amplifier. It costs about $30 new. It runs on 8 AA batteries. I'm using POWEREX 2300 mAh NiMH rechargeables from Thomas-Distributing. They cost a bit more but you only have to buy them once and they last forever. I've run the amp at full blast for 3-4 hours without running dry.