Passive tone can give you some extra control over your voice. The circuit is quite simple – it is just a potentiometer and a capacitor that gets wired in parallel with your volume control. The capacitor acts as a voltage divider that increases with frequency, increasing the load on the pickup and decreasing the resonant frequency. This provides a very traditional tone on its own.
Including a passive tone with an on-board preamp lets you have that classic passive of a loaded pickup along with the isolation and power provided by the preamp. The passive tone control can be wired to work only in passive mode or also while in active mode. The installation is simple – only requiring the connection of 2 wires.