Alembic pickups use ceramic magnets. The Series I pickups were wound low impedance, which means they had less turns of a heavier gauge wire for a cleaner tone, but they have to nbe used with a preamp.
Their later pickups are stacked humbuckers. They seem to be wound a bit hotter (the ones I measured were 8K), but they still need a preamp to work because they are low output.
Alembics were the original "active" pickups.
Alembics have low pass filters (the first one) but not high pass filters.
The electronics use low pass filters for tone controls. As you turn the tone knob it sweeps the frequency where the high end is removed. They also have a "Q" switch or knob, which boosts the resonance at the cutoff frequency. This makes the tone sharper, kind of like a wah pedal.
Here's an audio example of a low pass and then high pass filter.
Low Pass-High Pass
Because the pickups are low impedance, the resonant frequency is higher than on regular high impedance pickups, so the filter allows you to get a lot of different tones by lower the resonant peak, which makes them sound more like passive pickups.
If you want to know what an Alembic bass sounds like, listen to Stanley Clarke.
