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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.
A magnetic pickup consists of a permanent magnet wrapped with a coil of a few thousand turns of fine copper wire. The pickup is most often mounted on the body of the instrument, but can be attached to the 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 (or by radio transmission). 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 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 resistively-damped 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.
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.
Both men developed essentially the same concept: A humbucking pickup, shown in the image at the top of this page, generally comprises two standard pickups wired together with identical coils of opposite polarity and with identical magnets also opposed in polarity. Any ambient magnetic noise, such as hum from power-supply transformers, will induce in each coil a voltage of equal magnitude but opposite sign or direction, so that these two voltages (-V + V) cancel each other when combined -- the pickup "bucks hum." However, the vibrating steel strings disturb both magnetic fields, and because the fields point in opposite directions, they induce opposite voltages in the two coils. These two coils, as previously stated, are also "out of phase," so the signal induced by the magnet of opposite polarity in the coil of opposite polarity is "in phase" with the signal from the other magnet and coil: (-1)(-1) = 1.
A 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 resonant frequency and attenuates the higher frequencies, giving a fatter and less trebly tone than either of the two component single-coil pickups would give singly. This is the essence of the "humbucker tone."
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.
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. Popular pickup configurations include:
- S-S (Fender Telecaster)
- S-S-S (Fender Stratocaster)
- H-H (Gibson Les Paul)
- S-S-H (Fender Fat Stratocaster)
- H-S-H (Superstrats)
Less frequently found configurations are:
- S (Fender Esquire)
- H (Gibson Les Paul Junior)
- H-H-H (Some Gibson Les Paul Gold Tops and Customs)
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 hearing-aid loops.
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) 
- 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 
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 an 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 pick-ups
Pick-ups 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 pick-ups 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 pick-ups are very convenient as they require no power source to operate. They are the most popular and widely used pick-up type on electric guitars, and their frequency response curve is unique to the type and manufacturer.
Active pick-ups 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 (in comparison to passive) and much flatter frequency response curve. They also are less affected in tone by varying lengths of amplifier lead, and amplifier input characteristics. Magnetic pick-ups 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 pick-ups 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 pick-up. Most piezoelectric and all optical pick-ups are active and include some sort of preamp.
- Single coil
- Magnetic circuit
- List of electronics topics
- Nominal impedance
- Instrument amplifier
- Pre amplifier
- Lollar pickups
- UltimateGuitar.com Pickup FAQ
- Constructing an Under Saddle Transducer - Instructions for building a piezo cable guitar transducer. An article on the Liutaio Mottola Lutherie Information Website.
- Neophyzix Sound Research/Q-tuner neodymium powered guitar & bass pickups. Downloadable 24 Bit/96 kHz audio wav demo.