Basic workings of a Guitar Pickup
Parts
Permenant Magnet
Fine copper wire
The more turns of wire in the winding, the higher the output voltage but the lower this resonance frequency
Poles
Pickups are Magnetic
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.
Pickup 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. 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 .
Pickup sound
Single coil pickups, like in a Fender Stratocaster or Telecaster
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[who?] to be aesthetically superior to that of a more linear transducer[citation needed].
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.
