My brother made me this mini-amp because I have trouble bigtime hearing myself play in orchestra. This little amp runs on an AAA battery. It has a clip on the back so I can clip it to my pocket, shirt or belt. There is a line out to earbuds, and the pickup is a clamp that clips onto the bridge. The amp has an on/off switch (little blue light goes on when it's on) and volume control. I can choose my volume level and adjust as necessary, and hear everybody else too, as the earbuds are loose. What a lifesaver. I am lucky to have a trophy brother!