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Helmuth Lemme is a German electronics expert and author. His excellent book on guitar electronics is available in German and English. Diesen Artikel und noch mehr gibt es in deutscher Sprache unter gitarrenelektronik.
An electric bass or guitar's sound depends greatly on its pickups. There are lengthy discussions between musicians about the advantages and disadvantages of different models, and for someone who has no knowledge of electronics the subject may seem to be very complicated.
Electrically, though, pickups are fairly easy to understand - so this article will examine the connection between electrical characteristics and sound. I am sorry to say that most pickup manufacturers spread misleading information on their products, in order to make more money and to agitate their competitors.
So some corrections of facts will be necessary. I am not affiliated with any manufacturer. There are two basic pickup types, magnetic pickups and piezoelectric pickups. The latter type work with all kinds of strings steel, nylon, or gut.
Magnetic pickups work only with steel strings, and consist of magnets and coils. Singlecoil pickups are sensitive to magnetic fields generated by transformers, fluorescent lamps, and other sources of interference, and are prone to pick up hum and noise from these sources. Dual coil or "humbucking" pickups use two specially configured coils to minimize this interference. Because these coils are electrically out of phase, common-mode signals i.
The arrangement of the magnets is different for different pickups. Some types have rod or bar magnets inserted directly in the coils, while others have magnets below the coils, and cores of soft iron in the coils. In many cases these cores are screws, so level differences between strings can be evened out by screwing the core further in or out.
Some pickups have a metal cover for shielding and protection of the coils, others have a plastic cover that does not shield against electromagnetic interference, and still others have only isolating tape for protecting the wire. The magnetic field lines flow through the coil s and a short section of the strings. With the strings at rest, the magnetic flux through the coil s is constant. Pluck a string and the flux changes, which will induce an electric voltage in the coil.
A vibrating string induces an alternating voltage at the frequency of vibration, where the voltage is proportional to the velocity of the strings motion not its amplitude. Furthermore, the voltage depends on the string's thickness and magnetic permeability, the magnetic field, and the distance between the magnetic pole and the string. There are so many pickups on the market that it is difficult to get a comprehensive overview.
In addition to the pickups that come with an instrument, replacement pickups - many of them built by companies that do not build guitars - are also available. Every pickup produces its own sound; one may have a piercing metallic quality, and another a warm and mellow sound.
To be precise: A pickup does not "have" a sound, it only has a "transfer characteristic". It transfers the sound material that it gets from the strings and alters it, every model in its own fashion. And the best pickup is useless when you have a poor guitar body with poor strings. The basic rule is always: garbage in - garbage out! Replacement pickups allow the guitarist to change sounds without buying another instrument within the limitations of body and strings, of course.
Different pickups also have different output voltages. High output models can make it easier to overdrive amplifiers to produce a dirty sound, while low output models tend to produce a more clean sound. Unlike other transducers that have moving parts microphones, speakers, record player pickups etc.
So evaluating pickups is much easier than with other transducers. Although the frequency responses of nearly all available magnetic pickups are nonlinear which creates the differences in sound , they don't have quite as many adjacent peaks and notches in their frequency response as for example a speaker.
In fact, the frequency response can be smooth and simple enough to be easily described with a mathematical formula. A real coil can be described electrically as an ideal inductance L in series with an Ohmic resistance R, and parallel to both a winding capacitance C. This replacement circuit can be used as a first approximation. It is a bit simplified compared to the reality but quite useful for the beginning. The finer details are explained later. For a humbucker, two of these circuits have to be connected in series.
Since both coils with precise manufacturing have practically identical properties, you may use the same simple replacement circuit for the electrical examination. You then have to use twice the values for the inductance and the resistance and half of the value for the capacitance as compared to one coil. Many people measure only the resistance and think they know something about a pickup.
But this is a fundamental error. By far the most important quantity is the inductance, measured in Henries. It depends on the number of turns, the magnetic material in the coil, the winding density and the overall geometry of the coil.
When the strings are moving, an AC voltage is induced in the coil. So the pickup acts like an AC source with some attached electric components Fig. The external load consists of resistance the volume and tone potentiometer in the guitar, and any resistance to ground at the amplifier input and capacitance due to the capacitance between the hot lead and shield in the guitar cable.
The cable capacitance is significant and must not be neglected. This arrangement of passive components forms a so-called second-order low-pass filter Fig. A pickup plus real external load pots, cable, and amp input resistance. Thus, like any other similar filter, it has a cut-off frequency fg; this is where the response is down 3 dB which means half power.
Above fg, the response rolls off at a 12 dB per octave rate, and far below fg, the attenuation is zero. There is no low frequency rolloff; however, a little bit below fg there is an electrical resonance between the inductance of the pickup coil and the capacitance of the guitar cable. This frequency, called fmax, exhibits an amplitude peak. The passive low-pass filter works as a voltage amplifier here but doesn't amplify power because the output current becomes correspondingly low, as with a transformer.
Fundamental frequency response of a magnetic pickup. Position and height of the peak vary from type to type. If you know the resonant frequency and height of the resonant peak, you know about 90 percent of a pickup's transfer characteristics; these two parameters are the key to the "secret" of a pickup's sound some other effects cannot be described using this model, but their influence is less important.
What all this means is that overtones in the range around the resonant frequency are amplified, overtones above the resonant frequency are progressively reduced, and the fundamental vibration and the overtones far below the resonant frequency are reproduced without alteration.
The resonant frequency of most available pickups in combination with normal guitar cables lies between 2, and 5, Hz. This is the range where the human ear has its highest sensitivity. A quick subjective correlation of frequency to sound is that at 2, Hz the sound is warm and mellow, at 3, Hz brilliant or present, at 4, Hz piercing, and at 5, Hz or more brittle and thin.
The sound also depends on the height of the peak, of course. A high peak produces a powerful, characteristic sound; a low peak produces a weaker sound, especially with solid body guitars that have no acoustic body resonance. The height of the peak of most available pickups ranges between 1 and 4 0 to 12 dB , it is dependent on the magnetic material in the coil, on the external resistive load , and on the metal case without casing it is higher; many guitarists prefer this.
The resonant frequency depends on both the inductance L with most available pickups, between 1 and 10 Henries and the capacitance C.
C is the sum of the winding capacitance of the coil usually about 80 - pF and the cable capacitance about - 1, pF. Since different guitar cables have different amounts of capacitance, it is clear that using different guitar cables with an unbuffered pickup will change the resonant frequency and hence the overall sound.
There are some books that deal especially with electric guitar pickups. They pay much attention to the resistance and the magnet materials. But the resistance is the least interesting magnitude of all. And statements like "Alnico 5 sounds like this, Alnico 2 sounds like that" are completely misleading.
Many "pickup experts" have never heard the term "inductance". What you find in those books is an obsolete "geocentric" view on pickups that will never work. The integral "heliocentric" view on pickups: Pickup, pots in the guitar, cable capacitance, and amp input impedance are an interactive system that must not be split up into its parts. If you analyze the properties of the parts separately you will never understand how the system works as a whole.
The sound material a pickup receives from the strings is not flavoured by the pickup alone but by the complete system. This includes the guitar cable. Another cable, another sound! This is a shame but it is true. You can easily check it up. A few pickup manufacturers know that fact but they conceal it.
The majority seems to be totally ignorant. As mentioned earlier, this overview has been simplified to make it easier to understand. Up to this point, it has not taken into account the influence of eddy currents in metal parts. Such currents appear wherever an alternating magnetic field flows through electrically conductive parts. Strong eddy currents can also occur in metal covers; these currents vanish when the covers are removed.
There are thousands of iron and steel core types, whose properties can differ widely, resulting in variable frequency transmission characteristics.
Plastic covers are not conductive. To a lesser extent, eddy currents can also occur in base plates as well as in metal magnets located underneath the coils. This slope is inversely proportional to the threefold power of the frequency. Thirdly, they cause the frequency transmission curve to drop slightly below the resonant frequency, as shown in Fig. There have been attempts to measure eddy currents by attaching resistors to the replacement circuit, in parallel to the coil or to the terminals.
This method has not been successful, however, for although it does reduce resonance superelevation, it fails to achieve the other two above-mentioned results.
Gitarren-elektronik by Helmuth Lemme
The Secrets of Electric Guitar Pickups