ive seen them in guitars and i think basses too, they use a battery.
anyone know?
Chief’s Domain - http://www.seas.ucla.edu/~ravi
I had a Peavey bass years ago with active pickups. From what I understand, they have a mini onboard preamp, run from the battery, which allows for such things as more bass/treble/midrange control, and maybe a “boost” feature, which basically does the same thing your reg amp does, but with more onboard control. I recall, i could change aforesaid settings w/o touching my amp, and it was pretty cool.
Klaatu has it, basically. An active circuit uses an electronic amplifier and filter system to boost or retard selected frequencies and generally increase the output of the pickups, resulting in a much higher signal to noise ratio and transmission of harmonics and partials that might normally be lost to the impedance of your guitar cable. Your amplifier receives a much hotter signal and you have much more control over tone. Preamps today are sometimes built right into the pickup itself and if any additional circuitry or switching is required, special components would be used to facilitate the desired changes.
In a passive circuit the raw signal generated by the pickups is filtered through capacitors and variable resistors removing unwanted frequencies. When you roll the tone control counterclockwise you are removing a large chunk of the high frequencies that add to your overall sound (upper partials, harmonics, etc.). Sometimes a bleed capacitor used on the volume pot will help pass high frequencies at lower volumes, but the bottom line is a passive circuit works by filtering out unwanted frequencies.