The pickups are magnetic devices that sit below the strings. As the strings vibrate, an electrical signal is produced which is sent via an instrument cable to an amplifier. Pickups are either "passive", which means they send the signal with no additional amplification, or "active", which process the signal through a pre-amplifier. Some basses have pickups that can be set to passive or active mode. Basses that have two pickups (generally referred to as bridge and neck pickups) often have a switch that lets the player select neck-only, bridge-only, or a blended signal. Each pickup may have different tonal qualities that results from its placement relative to the bridge and its components.