My Celtic metal band's violinist wanted to amplify his acoustic violin without needing to stay put in front of a microphone stand. I came up with this simple and effective solution : a piezo connected directly to a jack plug. The piezo his held on the bridge with a small cloth peg. The sound quality is not the best but does the job : we could hear clearly the violin above the drums, bass, guitar, recorder and vocals, and he could play without thinking about the mic placement.