Bass guitars are either "passive" or "active". Passive instruments usually feature only a volume control and a simple tone control that attenuates high frequencies, although they do tend to be cheaper. Active guitars require a power source, usually one or two 9v batteries housed in the underside of the body. The active circuit may electronically balance the output of the bass (thereby reducing hum and interference) and/or provide an equalisation circuit. The Musicman Stingray was the first production bass to feature bass, mid and treble controls. Additional EQ such as this can be extremely useful, allowing you to adjust the sound while playing without having to stop to fiddle with the amplifier! Active bass guitars usually have a higher output than passive basses, which has the added benefit of improving the signal to noise ratio when using effect pedals. A drawback of active designs though is that batteries have a limited lifespan - if your bass doesn't have an active/passive switch, you may need to change your batteries, sometimes very quickly!
EMG J/P-Pickup Set