When the strings of an electric guitar are plucked, sound waves are generated in the air and electrical signals are also created in an associated electrical circuit. The vibrations of the strings, which are made of a magnetically permeable metallic material, induce signals in a device known as a pickup. The pickup is positioned underneath the strings of the instrument in the upper plate. The pickup operates on an electromagnetic principle and provides a signal to an output circuit.
The most essential components of a guitar pickup are a permanent magnet and a coil of wire. The magnet generates a magnetic field that passes through the pickup coil and also extends into a space occupied by at least one string of the instrument. Vibration of the string changes the reluctance of the magnetic path and creates disturbances in the magnetic field proportional to the string vibration. The changing magnetic field in the pickup coil in turn induces an electrical signal in the coil. From the output of the pickup a circuit connection is made to an amplifier and speaker system. Impedance matching in the electrical system may be required to transfer the music signal effectively.
The energy level of the string vibration is very small and design of the pickup is calculated to obtain maximum signal output. Pickups often include a pole piece in addition to the permanent magnet so as to concentrate the magnetic field where it is most needed and thus maximize the output signal.
In a musical performance the direct sound output of the electric guitar--i. e., its acoustical output--is so low that it does not add appreciably to sound output originating from a pickup or pickups that is disseminated through an amplifier and loudspeakers. In some instances an artist may wish to make a recording using only the sound output developed in the electrical system. Regardless of which way a performance or recording is to be handled, the fact remains that a high quality of musical sound output from each pickup is required.
The magnetic field of the pickup necessarily constitutes a load on an associated string, causing some damping or diminution in the string vibration. As a result, there may be some non-linearity with consequent distortion and loss of quality in both the acoustical sound output and the electrical sound output. Guitar musicians may differ, however, in their evaluation of musical quality, and in some instances a certain amount of distortion of a pure musical sound may be preferred.
As is well known, string vibrations occur at multiple frequencies--that is, a string that is vibrating at a certain fundamental frequency is concurrently vibrating at two, three, four, etc., times that fundamental frequency. At these higher overtone frequencies the length of each vibrating string segment becomes quite small. Thus, the vibrations of a string at different locations along its length will be out of phase with each other for some frequencies. If a dual pickup coil detects different portions of the string vibration that are out-of-phase with each other, the result will be a significant loss of signal strength at the corresponding frequencies.
The guitar is an instrument with six strings, each having its own assigned frequency. Theory might indicate that there should be a separate pickup for each string. However, practical cost considerations have resulted in the practice of using an elongated coil that can pick up vibration signals from all of the strings at once. The coil is made long enough to span all six of the strings and is positioned with its longitudinal axis essentially perpendicular to the length of the strings.
In every pickup coil a certain number of turns are needed in order to effectively respond to the changes in the magnetic field; i.e., to produce a useful output signal in response to the string vibration. Providing a coil with the desired number of turns necessarily results in inductance, resistance, and capacitance. For a given wire size the resistance of the coil is essentially proportional to the length of the wire and hence to the number of turns in the coil. The same is true for the capacitance. Changes in wire size and number of turns may result in a change in the musical character of the sound output that would not be acceptable to musicians using the instruments.
A well-known problem of the electric guitar is that the pickup coil, in addition to its desired function of picking up string vibrations, also tends to pick up electrical noise and interference signals from various extraneous sources, such as power circuits, radio and television equipment, and the like. It has long been well known to utilize a two-coil pickup in which the coils are interconnected in such a way as to balance out the extraneous signals. Such pickups are known in the trade as "humbuckers".
The operating principle of the humbucker is that the two coils are connected in opposite electrical polarities so that the noise signals which are electrically induced in them will cancel each other out. At the same time, the magnetic circuits of the humbucker's two coils are so arranged that the signals magnetically induced in them from the string vibrations will be added together rather than being cancelled. An important requirement for a humbucker is that the electrical impedances of the two coils must be substantially identical. Otherwise the noise signals induced in the two coils will be unequal and will not completely cancel out.