A pickup acts as a transducer that captures mechanical vibrations, usually from suitably-equipped stringed instruments such as the electric guitar and converts them to an electrical signal which can then be amplified, recorded and broadcast.
A magnetic pickup consists of a permanent magnet such as a AlNiCo, 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 and string basses. The vibration of the nearby soft-magnetic strings modulates the magnetic flux linking the coil, thereby inducing an alternating current 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 which 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. This can be detrimental to the final sound because the magnets' pull on the strings can cause problems with intonation as well as damp the strings and reduce 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 or a DI unit.
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[citation needed], 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[who?] 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 electric cable also has a capacitance which can be a significant portion of the overall system capacitance. This arrangement of passive components forms a resistively-damped second-order low-pass filter. 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.
Single coil pickups also act like an antenna and are prone to pick up mains hum (nuisance electromagnetic interference generated by electrical power cables, power transformers, and fluorescent light ballasts in the area) along with the musical signal. 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. The pickups also are sensitive to the electromagnetic field from nearby cathode ray tubes in video monitors or televisions.
To overcome this effect, the humbucking pickup was developed, concurrently and independently by Seth Lover of Gibson and Ray Butts who initially developed one on his own and later worked with Gretsch. Who developed it first is a matter of some debate, but Seth Lover was awarded the first patent (U.S. Patent 2,896,491). Ultimately, both men developed essentially the same concept. Another way to reduce nuisance hum when recording with humbuckers or single coils is to aim the instrument's neck in a different direction to find a location that minimizes the received noise signal.
A humbucking pickup is composed of two coils. Each coil is wound reverse to one another. However, the six magnetic poles are opposite in polarity in each winding. Since ambient hum from power-supply transformers, radio frequencies, or electrical devices reaches the coils as common-mode noise, it induces an electrical current of equal magnitude in each coil. Because the windings are reversed in each pickup coil, the electro-magnetic interference sine wave signals in each pickup are equal and 180 degrees out of phase to one another, resulting in them canceling each other. However, the signal from the guitar string is doubled, due to the phase reversal caused by the out of phase magnets. The magnets being out of phase in conjunction with the coil windings being out of phase put the guitar string signal from each pickup in phase with one another. When the two in-phase guitar string sine wave signals meet, the amplitude of the wave doubles, and as does the signal strength.
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 less trebly tone (i.e., "fatter") than either of the two component single-coil pickups would give alone. 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, as the lowered source impedance will drive capacitive cable with lower high frequency attenuation.
A side-by-side humbucking pickup senses a wider section of the string (has a wider aperture) than a single-coil pickup. This affects tone. By picking up a larger portion of the vibrating string more lower harmonics are present in the signal produced by the pickup resulting in a "fatter" tone. Stacked humbuckers have the narrower aperture of a single coil and sound closer to one.
Pickups have magnetic polepieces (with the notable exceptions of rail and lipstick tube pickups) — one or two for each string. These polepiece centers should be perfectly aligned with the strings, or else sound will be suboptimal as the pickup would capture only a part of the string's vibrational energy. An exception to this rule are the J- and P-style pickups (found on the Fender Jazz Bass and Precision Bass, respectively) where the two polepieces per string are positioned on either side of each string.
String spacing is not even on most guitars: it starts with minimal spacing at nut and ends with maximal at bridge. Thus, bridge, neck and middle pickups usually have 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.
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