Bass pickups consist of an electromagnetic coil and a wide uniform magnetic field, giving them the accuracy, warmth and roundness that characterizes them.
Different copper alloys wire windings ensure full restitution of the sound spectrum, from the subwoofer (like a low B on a 5 strings) to the treble as well as respecting the dynamics of each string.
To eliminate any risk of feedback in low frequencies (due to their wide bandwidth), most of them have a copper shield, are molded in the resin and are wax potted.
All these pickups are passive and compatible with commercially available preamps.