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 electrical signal, which can be amplified and recorded.
A magnetic pickup consists of a permanent magnet wrapped with a coil of a few 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 and/or pickguard, as on many electro-acoustic archtop jazz guitars. The vibration of the nearby soft-magnetic strings modulates the magnetic flux linking the coil, thereby inducing an alternating voltage through the coil of wire. This signal is then carried to amplification or recording equipment via a cable. There may also be an internal preamplifier stage between the pickup and cable. More generally, the pickup operation can be described using the concept of a magnetic circuit. In this description, the motion of the string varies the magnetic reluctance in the circuit created by the permanent magnet.
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, greatly reducing sustain. Other high-output pickups have more turns of wire to increase the voltage generated by the string's movement. However, this also increases the pickup's output resistance/impedance, which can affect high frequencies if the pickup is not isolated by a buffer amplifier.
The turns of wire in 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 resonance 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 resistively-damped second-order low-pass filter. Electromagnetic pickups are usually designed to feed a high input impedance, typically a megohm or more, and a low impedance load will reduce the high-frequency response of the pickup because of the filtering effect of the inductance.
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.
To overcome this effect, the humbucking pickup was developed, concurrently and independently by Seth Lover of Gibson and Ray Butts, working for Gretsch. Who developed it first is a matter of some debate, but Seth Lover was awarded the first patent (Template:US patent). Ultimately, both men developed essentially the same concept.
A humbucking pickup, shown in the image on the right, generally comprises two standard pickups wired together with identical coils bathed in fields of opposite magnetic polarity. Ambient hum from power-supply transformers, radio frequencies, or electrical devices reaches the coils as common-mode noise, inducing an electrical current of equal magnitude in each coil. With the coils wired together in "buck" series, the induced currents sum and cancel each other out. However, the signal present from the vibrations of the guitar strings sums and adds together, doubling output.
One side-effect of this technique is that, when wired in series, as is most common, the overall inductance of the pickup is increased, which lowers its resonance frequency and attenuates the higher frequencies, giving a fatter and less trebly tone than either of the two component single-coil pickups would give alone. A second side-effect of the technique is that, because the two coils are wired in series, the resulting signal that is output by the pickup is larger in amplitude, thus more able to overdrive the early stages of the amplifier. This is the essence of the "humbucker tone."
An alternative wiring places the coils in buck parallel. The equal common-mode mains hum interference cancels, while the string variation signal sums. This method has a more neutral effect on resonant frequency: mutual capacitance is doubled (which if inductance were constant would result in a lowering of resonant frequency), and inductance is halved (which would raise the resonant frequency without the capacitance change). The net is NO change in resonant frequency. This pickup wiring is rare, as guitarists have come to expect that humbucking 'has a sound', and is not neutral. On fine jazz guitars, the parallel wiring will produce significantly cleaner sound however, as the lowered source impedance will drive capacitive cable with lower high frequency attenuation.
With a notable exception of rail pickups, pickups have magnetic polepieces — one or two for each string. These polepiece centers should be perfectly aligned with strings, or else sound will be suboptimal as pickup would capture only a part of vibrational energy.
String spacing is not even on most guitars: it starts with minimal spacing at nut and ends with maximal at bridge. So, bridge, neck and middle pickups should have a different polepiece spacing on the same guitar.
There are several standards on pickup sizes and string spacing between the poles. Spacing is measured either as a distance between 1st to 6th polepieces' centers (this is also called "E-to-E" spacing), or as a distance between adjacent polepieces' centers.
(Vintage Gibson guitars)
(Most Fender guitars, modern Gibson, Floyd Rose bridges)
|Very close to bridge, extra pickup
(Roland guitar synth hex pickups)
(Fender Telecaster guitars)
Usually an electric guitar has more than one magnetic pickup. A combination of pickups is called a pickup configuration. It is usually notated by just writing out the pickup types, using "S" for single-coil and "H" for humbucker, in order from neck pickup to bridge pickup. This order matches left to right enumeration from a perspective of right-handed guitarist playing the guitar, although reverse order (right to left) could be rarely used too. Popular pickup configurations include:
- S-S (Fender Telecaster)
- S-S-S (Fender Stratocaster)
- H-H (Gibson Les Paul, Fender Double Fat Stratocaster)
- S-S-H (Fender Fat Stratocaster)
- H-S-H (Superstrats)
Less frequently found configurations are:
- S (Fender Esquire, Gibson Les Paul Junior, Gibson Melody Maker, some telecasters)
- H (minimalistic rock/metal guitars like Kramer Baretta, hollow body jazz guitars)
- S-H (minimalistic superstrats like Hamer Californian Deluxe, entry-level guitars like Squier '51)
Examples of rare configurations that only a few particular models use include:
- H-H-H (some Gibson Les Paul Gold Tops and Customs)
- H-S-S-H (Music Man Steve Morse Signature)
More recently, many semi-acoustic and acoustic guitars, and some electric guitars and basses, have been fitted with piezoelectric pickups instead of, or in addition to, magnetic pickups. These have a very different sound which some prefer, and also have the advantage of not picking up unwanted magnetic fields, such as mains hum and feedback from monitoring loops. The advantage of such systems allow for switching between magnetic pickup and piezo sounds, or simultaneously blending the output.
Piezoelectric pickups have a very high output impedance and appear as a capacitance in series with a voltage source. They must therefore have an instrument-mounted buffer amplifier fitted if the sound is to retain its full frequency response. Piezo pickups are usually mounted under the bridge and sometimes form part of the bridge assembly itself.
The piezo pickup gives a very wide frequency range output compared to the magnetic types and can give large amplitude signals from the strings. For this reason, it is usually necessary to run the buffer amplifier from relatively high voltage rails (about ±9 V) to avoid distortion due to clipping. Some musicians prefer a preamp that isn't as linear (like a single-FET amplifier) so that the clipping is "softer", although such an amplifier starts to distort sooner, this makes the distortion less "buzzy" and less audible than a more linear, but less forgiving op-amp . However, at least one study  indicates that most people can't tell the difference between FET and op-amp circuits in blind listening comparisons of electric instrument preamps, a finding which correlates with results of formal studies done in other types of audio devices. Sometimes, piezoelectric pickups are used in conjunction with magnetic types to give a wider range of available sounds.
For early pick-up devices using the piezoelectric effect, see phonograph.
Hexaphonic pickups (also called divided pickups and polyphonic pickups) have a separate output for each string (Hexaphonic assumes six strings, as on a guitar). This allows for separate processing and amplification for each string. It also allows a converter to sense the pitch coming from individual string signals for producing note commands, typically according to the MIDI (musical instrument digital interface) protocol. A hexaphonic pickup and a converter are usually components of a guitar/synthesizer.
Such pickups are relatively uncommon (compared to normal ones), and only a few notable models exists. Hexaphonic pickups can be either electromagnetic or piezoelectric.
- Roland GK-2 (single coil) and GK-2a (humbucking) are one of the most famous models, factory-installed on many guitars. Compatible with popular Roland GR series of guitar synthesizers.
- Copeland Hex (by Rick Copeland). 
- Shadow GTM-6
- AXON AIX-101
- Zeta Strados (violin) 
- Barbera transducers (violin, cello, bass etc.) 
- RMC Acoustic Gold pickup saddles & Poly-Drive II remote preamp (for acoustic guitar) 
- RMC Acoustic Gold pickup saddles & Poly-Drive IV onboard preamp (for acoustic guitar) 
- RMC Pow'r Bridge G pickup saddles (tune-o-matic) & Poly-Drive 1 onboard preamp (for electric guitar) 
- RMC Pow'r Bridge ST pickup saddles (strat-style) & Poly-Drive 1 onboard preamp (for electric guitar) 
- RMC Pow'r Bridge W pickup saddles (wilkinson) & Poly-Drive 1 onboard preamp (for electric guitar) 
- Ghost Bass Bridge 
- Graphtech Ghost Pickup System 
Optical pickups are a fairly recent development that work by sensing the interruption of a light beam by the string. The light source is usually a LED, and the detector is a photodiode or phototransistor. These pickups have complete insensitivity to magnetic or electric interference and also have a very wide and flat frequency response unlike magnetic pickups.
Optical pickup guitars were first shown at the 1969 NAMM in Chicago, by Ron Hoag 
Active and passive pickups
Pickups can be either active or passive. Pickups, apart from optical types, are inherently passive transducers. So-called active pickups incorporate electronic circuitry to modify the signal. Passive pickups are usually wire wound around a magnet. They can generate electric potential without need for external power, though their output is relatively low, and the harmonic content of output depends greatly on the winding.
Passive pickups are very convenient as they require no power source to operate. They are the most popular and widely used pickup type on electric guitars, and their frequency response curve is unique to the type and manufacturer.
Active pickups require an electrical source of energy to operate and include an electronic preamp, active filters, active EQ and other sound-shaping features. They can sometimes give much higher possible output. They also are less affected in tone by varying lengths of amplifier lead, and amplifier input characteristics. Magnetic pickups used with 'active' circuitry usually feature a lower inductance (and initially lower output) winding that tends to give a flatter frequency response curve.
The disadvantages of active pickups are the power source (usually either a battery or phantom power), cost, and less defined unique tonal signature. They are more popular on bass guitars, because of their solid tone; most high-end bass guitars feature an active pickup. Most piezoelectric and all optical pickups are active and include some sort of preamp.
The main advantages of active pickups are that they are hum and noise free, at least compared to their passive single coil counterparts, as they require no ground wiring to connect to ground, which also may isolate the player from being electrocuted, should the wiring elsewhere, in other equipment such as a guitar amp, have become hazardous in any way.