The Doepfer A-104 is a trautonium formant filter module for the Eurorack A-100 system. Made up of four aprallel resonant filters the A-104 can be used to make soe very interesting harmonc textures. Each filter can be switched into lowpass, bandpass or of and has its own resonance control. There is an input attenuation knob for controlling the amunt of level that feeds the filter for getting soft or driven tones, and the end result is something that sounds very throaty and vocal.