Guitar pick-up

A pickup device is a transducer (specifically a variable reluctance sensor) that captures or senses mechanical vibrations produced by musical instruments, particularly stringed instruments such as the electric guitar, electric bass guitar, Chapman Stick, or electric violin, and converts them to an electrical signal that is amplified using an instrument amplifier (such as a guitar amplifier) to produce musical sounds through a loudspeaker in a speaker enclosure. The signal from a pickup can also be recorded directly, using a DI box (a common practice with the electric bass) or broadcast on the radio or television. Most electric guitars and electric basses use magnetic pickups. Acoustic guitars, upright basses and fiddles often use a piezoelectric pickup.

A magnetic pickup consists of a permanent magnet with a core of material such as alnico or ferrite, wrapped with a coil of several thousand turns of fine enameled copper wire. The pickup is most often mounted on the body of the instrument, but can be attached to the bridge, neck or pickguard, as on many electro-acoustic archtop jazz guitars. Magnetic pickups used with string basses can be attached to the bridge. The permanent magnet creates a magnetic field; the motion of the vibrating steel strings disturbs the field, changing magnetic flux and inducing an electric current through the coil. The pickup is then connected with a patch cable to an amplifier which amplifies the signal to a sufficient magnitude of power to drive a loudspeaker. A pickup can also be connected to recording equipment via a patch cable. There may also be an internal preamplifier device mounted in an acoustic guitar or in an external box. When a preamp is used in this way, it is between the pickup and cable and can significantly reduce the equivalent impedance of the pickup coil.

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Wikipedia