The electronics of a bass guitar describe how it makes sound, or how information is sent out from the guitar to the amplifier. The pickups underneath the strings of a bass guitar are one of the biggest influences on its sound, and the type of pickups is what makes a bass guitar active or passive.
How Pickups Work
Pickups are aptly named because they pick up the sound from vibrating strings. Pickups contain magnets that create a magnetic field around the strings. When you pluck a string, the pickups obtain the vibrations from the strings, which is a disturbance in the magnetic field. This disturbance produces an electronic signal, which is then transmitted to the amplifier.
Passive Bass Guitars
Passive pickups use larger magnets and are able to grasp more noises than active pickups. Passive pickups tend to have a fatter and rounder tone than active pickups, and their sound can also be described as warm and full. Passive basses do not have as much ability for the player to control the tone, and you only have the ability to lower the bass and treble frequencies.
Active Bass Guitars
Active pickups employ the use of a battery-operated preamplifier. The preamp allows the player to both raise and lower the treble and bass frequencies, which gives you more control over the tone. Some basses with preamplifiers also have controls for mid-level frequencies, enabling you even more possibilities with the overall tone. Active basses tend to have a brighter and snappier tone. They also use smaller magnets than passive bass guitars, so there will be less ambient noise picked up.
Since active basses utilise a battery, you will have to keep some extra batteries on hand in case the one in your bass dies. Try out as many basses as possible when you are thinking of purchasing one. Do not simply generalise that active basses can only sound bright. The type of pickup that is used matters, but so does the quality of the pickup. If you buy a bass and decide you want to change the pickups, they can be replaced or exchanged for a different set.