5. Active Pickups
As we have seen, standard passive pickups have a relatively weak output which can be prone to interference and noise, and fairly strong magnets must be used to produce even this output level, which in turn can have an audible side-effect as the magnet exerts a pull on the string, damping its vibration and reducing sustain.
Active pickup designs address both of these problems by incorporating a pre-amp into the guitars circuitry. This boosts the output signal to a level that is much less affected by electro-magnetic interference, and not degraded by long cables. As the desired level is achieved by amplification within the guitar, this also permits the use of weaker pickup magnets which exert little string-pull, resulting in noticeably better sustain. The use of an onboard pre-amp also allows for much more powerful tone control than that offered by the simple high frequency roll-off found on most guitars.
Active pickups are usually powered by a 9V battery which is easily replaceable if incorporated into the guitars original design, however replacing passive pickups with an active set normally involves finding a place for the battery under the scratchplate. In this case, its definitely worth using a long-life battery and remembering to unplug the guitar when not in use, as removing the strings and scratchplate to change a battery is not something you want to have to do in the middle of a gig!
Active pickup technology was pioneered by EMG still commonly regarded as the market leader and is popular with many players. Many guitar and particularly bass manufacturers offer active versions of their standard models, and a good proportion of bass amps feature separate inputs for active and passive instruments. Because of their different output levels and other electrical properties such as impedance, its not generally possible to combine active and passive pickups on the same instrument.