What are Magnetic Pickups?
In 1825, William Sturgeon demonstrated that putting an iron core in a coil would increase the inductance. The increase of inductance depends on the permeability and the physical dimensions of the core. A ferrous core will also cause eddy currents which increase the effective resistance of a coil. Eddy currents are frequency dependant.
Coils without a core are called air coils. Perfectly wound coils are virtually free of eddy currents, and the effective resistance is about the same as the Dc resistance. However, a piece of metal in the vicinity of a coil, like a pickup cover, as well as a minute short in a coil, will cause eddy currents. Nickel silver or stainless steel will cause lesser eddy currents than copper, brass, soft iron or aluminum. Eddy currents interfere with the induced current in a pickup and alter its performance.
The function of a pickup can be easily explained by the laws of electrodynamics. When a ferrous string vibrates over a pickup, the changing magnetic field induces an AC current in the coil - the signal. Unfortunately, the coil also functions as a polarized antenna for electromagnetic radiation. Electromagnetic radiation from nearby transformers is the cause of the 60 HZ hum. Because the electromagnetic polarization of a coil is independent from its ferromagnetic polarization, we can make dual coil pickups that cancel the hum but reinforce the signal. Hum-canceling pickups were first introduced in the thirties, but at the low volume levels used at that time, hum was not yet a problem. Therefore, guitars were equipped with simple single coil pickups. This all changed in the fifties when the volume levels of electric guitars were dramatically increased.
A pickup transduces the mechanical energy of a vibrating string into an electrical signal. The intensity of the signal depends on numerous factors:
1. The momentum of the vibrating string is the product of its frequency, amplitude and mass, and depends on the force a player applies to the string ( the attack ) and the amount of energy absorbed by the guitar which explains that the output of a pickup varies in different guitars.
2. The efficiency of a pickup is the result of many factors.
number of turns per square.
Remember, a pickup is NOT a power generator, and the magnitudes of volts, amps and watts are extremely small; but, regardless how many turns you put on a pickup, you will NEVER get 100 watts out of a 50 watt amp.
3. The distance between pickup and string is a very important factor for output and sound.
As a general rule for the bridge pickup - put a nickel on top of the pickup under the high E string and play the highest note on that string. Adjust the height on that side of the pickup till the string touches the nickel. Repeat the same with the low E string, but use two nickels on top of each other. If this gives you too much output, you can reduce the height slightly. Don't forget that twice the distance will reduce the output by about 60%, and the sound will lose some lows. NOW, you can adjust the neck pickup to match the output of the bridge pickup. For the sound test, use stage volume.
4. The position of a pickup is the key for the tonal response between lows and highs.
One of the main flaws of multi-pickup guitars is the difference in sound between bridge and neck pickup. If you adjust your amp to get the right sound from the bridge pickup, your neck pickup might sound too bassy; or it's the other way around when you adjust the amp that the neck pickup sounds right. Also, a pickup in the neck position generates a much stronger signal than the same pickup in the bridge position. This is caused by the amplitude of a vibrating string which reaches, normally, its maximum at the center of its speaking length.
5. The position and the angle of the attack is where You, the player, takes over.
Experienced players can create all kinds of sounds, just with their right hand technique. About a year ago, I was playing at a blues jam, and during the break, a young player asked me, "How do you switch so fast from the bridge pickup to the neck pickup, sometimes in the middle of a fast phrase?" I told him, " I don't", and showed him my guitar. He couldn't believe it, my guitar had only one pickup - the neck pickup. I told him all about the hot and the sweet spots on a string.
Try for yourself -- take your guitar and switch to the neck pickup and set your amp on low volume. Pluck your string with a medium thick pick at a right angle with medium force. Start to pluck the string at the end of the fingerboard, and, slowly, go up towards the bridge. You will recognize a steady change in output and tone, and somewhere close to the center between bridge and neck, you will find that beautiful sweet spot. Now, change to a more forceful attack, and also change the angle of attack to 45 degrees; you'll be surprised of all the sounds you can get! Then, repeat the whole process with different pickup selections. Single coil pickups are generally more sensitive to these changes than large humbuckers. It requires a lot of practice before you can master this technique, but with practice, you can duplicate the effects of a wah wah pedal just by changing the force, the position and the angle of the attack.
The nature of sound is well documented in the laws of acoustics, but let me explain to you, in a very simple way, what sound quality means. The characteristics of a certain sound depend on the harmonic spectrum of a note. If all instruments would reproduce a perfect harmonic spectrum, we would live in a boring world. For example, a soprano saxophone would sound like a flute or a clarinet, but none of them would sound the same as they do now. Sound means imperfection, and this imperfection is the beauty of musical instruments.
A vibrating string does not produce a pure tone that can be explained with the single curve of a sine wave. The tone of a vibrating string consists of several different sine waves, resulting in a complex waveform that can be found by adding the ordinates of all its component sine waves. The resulting waveform is influenced by the resonances of the instrument's components; the relation between the diameter and the speaking length of a vibrating string; the force applied by the player along with the position and the angle of the attack.
Power and sound are the resultants of vectors, and most data can be calculated with some simple equations. If inductance, Q-Factor, capacitance and frequency is known, we can calculate, with this basic data, the inductive and capacitive reactance of an AC circuit, as well as impedance, phase angle and power factor in a complex circuit where we are dealing with series and parallel combinations of inductors, capacitors and resistors. Although this is all very valuable information, it is, by far, not enough to determine sound and performance of a pickup.
In an AC circuit, we are mostly dealing with a fixed frequency; but a vibrating string provides a multi-frequency signal of the full harmonic spectrum of a note, and each frequency of the harmonic spectrum is subject to a different reactance, impedance, phase angle and power factor. A well-designed pickup must be capable to capture each minute vibration of a string and respond to each minor adjustment of the amp. This requires that a pickup can reproduce a strong, clear fundamental, followed by a well-dosed amount of lower midrange and a well-balanced high end.
1959 -- when I had the idea of replacement pickups -- I had
a clear vision for a small, non-confusing assortment of pickups
- to meet the increasing demands of many players for a richer, fuller
tone and a close to zero noise level. Many adopted my idea of replacement
pickups, and by 1980, it had developed into a pickup circus!
I still follow my original idea and offer a small assortment of pickups to enhance the performance of your guitar, at realistic and affordable prices - made in our workshop in Corona, Caifornia.
Copyright © 1996-2010 Bill Lawrence. All Rights Reserved.