Pickups convert the mechanical energy of a vibrating string to an electrical signal, allowing it to be amplified, processed and reproduced.
[1] Pickups vary greatly in construction, size, types of materials used, as well as various electrical properties, but are generally divided into two categories – single-coil and double-coil (also known as humbucker).
The Varitone is actually a variable notch filter consisting of one of several capacitors (selected with a rotary switch) in series with an inductor, forming an LC circuit.
Some manufacturers have used a pot to vary the amount of signal shorted to ground from one coil, thus producing a range of tones between a humbucker and a single-coil.
This modification requires the start and end of both coils to be exposed, which is more commonly available on aftermarket than stock pickups.
In case of a humbucker this results in a sound that is so weak as to be almost unusable, as well as the loss of the pickup's hum-cancelling properties (due to the coils being magnetically out-of-phase, but electrically in-phase with each other).
Compared to coil split the sound is usually a bit fuller and the pickup's hum-cancelling properties are retained.
The operation is the same as in a balance control found in stereo equipment – in the middle position (often marked with a detent) both pickups supply their full output, and turning the pot in either direction gradually attenuates one of the pickups while leaving the other at full output.
While the modifications described above have all been passive (i.e. they don't require an external power source), active electronics considerably increase the number of possible wiring options.
These can range from simple preamps that offer a volume boost and buffer the instrument's signal (to prevent loss of higher frequencies in longer cable runs), to multi-band equalisers and more.
[30][31] Enterprising guitarists have even built entire effects processors into guitars, such as the Korg Kaoss Pad.