The FUZZ DUO lets you switch between low-gain germanium & high-gain silicon transistors in the critical input gain stage to create your own signature fuzz tone. The BIAS control lets you dial in that perfect sweet spot, and the revised FILTER control balances between separate bass & treble filters. The FUZZ DUO requires the use hand-selected NPN germanium & silicon transistors, and works great when daisy-chained on a power supply.