Active and Passive Pickups
Pickups can be either active or passive. Pickups, apart from optical types, are inherently passive transducers. "Active" pickups incorporate electronic circuitry to modify the signal. "Passive" pickups are usually wire wound around a magnet, and are the most common type used. They can generate electric potential without need for external power, though their output is relatively low, and the harmonic content of output depends greatly on the winding.
Active pickups require an electrical source of energy (usually one or two 9V batteries) to operate and include an electronic preamp, active filters, active EQ and other sound-shaping features. They can sometimes give much higher possible output. They also are less affected in tone by varying lengths of the electric cable connecting the guitar to the amplifier, and amplifier input characteristics. Magnetic pickups used with 'active' circuitry usually feature a lower inductance (and initially lower output) winding that tends to give a flatter frequency response curve.
The disadvantages of active pickup systems (pickups->preamp) are the power source (usually either a battery or phantom power) and higher cost. They are more popular on electric bass, because of their solid tone and improved clarity; most high-end basses feature active pickup systems. Most piezoelectric and all optical pickups are active and include some sort of preamp.
The main advantages of active bass pickup systems is their cleaner, clearer more "Hi Fi" sound. Many players, notably Stanley Clarke, Flea, Victor Wooten, Abraham Laboriel and Doug Wimbish have used active bass pickups to produce their characteristic bass tones.
They also allow more "headroom" and dynamic range. Good quality active systems produce less noise and hum compared to their passive counterparts.
Read more about this topic: Guitar Pickups
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