The A-104 is a fourfold formant filter as used in the Mixtur Trautonium. It uses four parallel resonance filters (12dB/oct), each filter can be switched to low pass or band pass or off. Frequency, resonance and level are controlled for each filter separately (no voltage control). The frequency range for the filters is about 50Hz ~ 5kHz. The filter audio inputs are very sensitive so that distortion may intentionally be used to create new sounds - if desired.
The A-104 can be used to create resonances (e.g. the vocal-like effect known from the Trautonium). In combination with the subharmonic generator A-113, the Trautonium Manual A-198 and some other A-100 modules a Trautonium replica is achieved).