Active pickups still use coils of wire, like their passive counterparts. However, they use far fewer coils. Instead, their circuitry incorporates an active preamp (usually powered by a 9V battery) to boost the signal level, filters and EQ.
Active pickups have similar basic construction to passive pickups, but the circuitry uses an active preamp (usually powered by a 9-volt battery) to boost the signal, allowing low-impedance designs (high current, potentially low voltage) incorporating fewer wraps of coil wire and less powerful magnets.
There are several benefits to this type of design: reduced noise (particularly with single-coil designs), higher overall output, consistent tone when turning down the volume control, no high-frequency loss when using long cables, consistent clarity and note-to-note definition whether using clean or high-gain distortion amp settings, and the ability to use active tone controls to boost or cut frequencies (whereas passive tone controls are really just low-pass filters that can only roll off treble frequencies to make the tone darker). The downside of active pickups is that the tone can cover a wider and flatter frequency range than many players are accustomed to, which some players find too cold or sterile, and the output is generally more consistent, which some players consider less dynamically responsive.
As always (at least when it comes to pickups), there are plenty of exceptions to these general descriptions. Some new active pickup models are designed to provide warm tones similar to vintage passive pickups, and some modern passive pickup designs have expanded treble and bass to provide a wider frequency range without sacrificing dynamic response.
The lower number of coils on an active pickup means that they have a lower natural output (i.e. before the pre-amp), are less susceptible to background noise, and are naturally much quieter in this regard. However, the active pre-amp means that these pickups generally have a far higher output gain than passive models, too. Add in the ability to EQ the tone beyond a simple tone control, and you have a pretty impressive pickup.
So, why aren’t all pickups active? Well, passive pickups, despite their drawbacks, have a greater dynamic range. If you’re the kind of player that likes to be able to move from whisper quiet to a screaming wail, then a passive set-up is likely to suit your playing style best. Plus, these types tend to lose high-frequency detail, and enhance lower frequencies, giving them a warmer tone.
Conversely, active pickups have backs of sonic detail, but a lower dynamic range. Tonally, they’re sometimes described as ‘sterile’ or ‘cold’. This is, perhaps, a bit unfair, as active pickups can certainly be used to create sounds that pretty explosive.
Their increased output before feedback has seen them become incredibly popular among guitarists in heavier, rock genres. Plus, their detailed sound lends itself to articulate passages, such as shred guitar lines, or even jazz.