Three magnetic pickups on an electric guitar. From left to right they are a humbucker and two single coils.

A piezoelectric pickup on a classical guitar.

EMG 85 — one of the most famous active pickups in the world

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Pickups

A pickup device acts as a transducer that captures mechanical vibrations (usually from suitably equipped stringed instruments such as the electric guitar, electric bass guitar or electric violin) and converts them to an electronic signal which can be amplified and recorded. more...

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Magnetic pickups

Electromagnetic pickups use the principle of variable magnetic reluctance. The pickup consists of a permanent magnet wrapped with many turns of fine copper wire. The pickup is mounted on the body of the instrument, close to the strings. When the instrument's strings vibrate in the magnetic field of the permanent magnet, they alter the reluctance of the magnetic path seen by the magnet. This changes the flux in the magnetic circuit which in turn induces a voltage in the winding. The signal created is then carried to amplification or recording equipment via a cable (or by radio transmission).

Output

The output voltage of pickups varies between 100 mV rms to over 1 V rms for some of the higher output types. Some high output pickups achieve this by employing very strong magnets thus creating more flux and thereby more output. These can be detrimental to the final sound because the magnets can tend to attract the strings and damp them thus ruining sustain. Other high output pickups use instead a much larger number of turns of wire to increase the voltage generated by the string's movement. This however gives the pickup a larger output resistance/impedance and this can affect the high frequencies if the pickup is not isolated by a buffer amplifier.

Pickup sound

The turns of wire in close proximity to each other have an equivalent self-capacitance which, when added to any cable capacitance present, resonates with the inductance of the winding. This resonance can accentuate certain frequencies, giving the pickup a characteristic tonal quality. The more turns of wire in the winding, the higher the output voltage but the lower this resonant frequency. The inductive source impedance inherent in this type of transducer makes it less linear than other forms of pickups, such as piezo-electric or optical. The tonal quality produced by this nonlinearity is, however, subject to taste, and may therefore also be considered by some to be aesthetically superior to that of a more linear transducer.

The external load usually consists of resistance (the volume and tone potentiometer in the guitar, and any resistance to ground at the amplifier input) and capacitance between the hot lead and shield in the guitar cable. The cable capacitance has a large effect and must not be neglected. This arrangement of passive components forms a second-order low-pass filter. Electromagnetic pickups are usually designed to feed a high input impedance, typically a megaohm or more and a low impedance load will reduce the high-frequency response of the pickup because of the filtering effect of the inductance.

Humbucking

One problem with electromagnetic pickups is that — along with the musical signal — they also pick up mains hum. Mains hum consists of a fundamental signal at a nominal 50 or 60 Hz, depending on local alternating current frequency, and usually some harmonic content. The changing magnetic flux caused by the mains current links with the windings of the pickup, inducing a voltage by transformer action.