Passive and Active

Active pickups have a built-in preamp. A couple of examples are EMG's (the "real" ones, not the Selects or HZ's), and Seymour Duncan Lightnin' Rods. They don't necessarily look any different; in fact, that's the point of Lightnin' Rods. They look like "traditional" pickups, but are active.

Active pickups require a power source usually provided by a battery or batteries.

When people talk about an active bass , however, they may be talking about a bass which has a passive pickup and an active preamp, such as a Music Man Stingray or Deluxe Active Jazz Bass, etc.

 

From the RMMB FAQ -

http://www.wheatdesign.com/rmmbfaq.html

"A bass pickup works by detecting the changes made by the moving string in the magnetic field of the pickup. A normal bass pickup consists of one or more magnets wrapped in coils of thin wire. The vibration of the metal strings changes the magnetic field of the pickup, and the changing magnetic field produces a voltage across the two ends of the wire wrap. This voltage can be detected and amplified by a bass amp, and then converted back to sound by a speaker.

This signal needs to be amplified in order to be audible. Passive pickups do not amplify the signal at all: they require the bass amp to do all of the amplification. Active pickups contain a small amplifier inside the pickup housing. This amp boosts the pickups signal to a higher level which is then sent down the instrument cable into the amplifier. (Good amplifiers have two input jacks, one for active basses and one for passive basses, to reflect this difference.) You can also get basses which have onboard preamps which take the pickup signal and pass into into an amplifier which is on the bass but not part of the pickup. These onboard preamps can also contain tone controls and other electronics; a few expensive active pickups also contain tone controls inside the pickup housing."