How Guitar Pickups Work
by Tom Watson
No matter how visually attractive a particular Stratocaster may be, the heart of the instrument will always be the nature and the quality of sound it produces. We don't admire the guitar greats for the beauty of the guitars they played. We admire them for the timeless textures of sound they were able to produce.
The Stratocaster is a superb marriage of art and science. The visual art of the Strat appeals to the eye while invisible science helps account for the tones which please the ear. But we can only say "helps" because the ability of the individual player contributes to the production of tone in significant and unquantifiable ways.
A number of scientific fields are called upon in the creation of a Fender Stratocaster, but probably none so central to the production of potentially artistic tone than the sciences involved in the construction of a Stratocaster's pickups. Here's a brief look at the science behind the magic.
All guitars produce sound when a string is plucked because the string vibrates and the vibrations create sound waves. However, in order for us to hear the sound waves produced they need to be made louder. Amplified. An acoustic guitar is its own amplifier. The acoustic's hollow body serves as a sound chamber enlarging (amplifying) the sound waves produced by the plucked string.
The drawback to acoustic amplification is the limitation on how much amplification can be naturally produced. Although the amount of amplification can be affected by such factors as the size of the acoustic's hollow body and the force by which the strings are plucked, the instrument has a natural upper limit that creates a handicap for musicians in situations where greater volume is needed.
This proved to be the dilema of many guitarists in the big band era of the 40s and early 50s. Even the large acoustic jazz boxes were unable to produce sufficient amplification to let the guitar to be heard at the right volume when competing with rhythm and horn sections.
Electronic amplification provided the solution.
The electronic amplification of a vibrating string requires two basic components, some sort of "microphone" or pickup to capture the string's vibration and translate it into an electric signal and a second component that can take that signal and amplify it into audible sound (the guitar amplifier). Our concern is with the mechanics of the pickup.
In one sentence
A vibrating steel (or other ferro-magnetic metal) string causes a flux in the magnetic field created by a pickup's coil, a flux that produces an altnernating (AC) voltage (a signal) that is transmitted to the electronic amplifier where it is enlarged then sent to the amplifier's speaker.
Electric current passing through wire wrapped around a magnetically conducive pole piece or block turns the pole piece into an electromagnet (a magnet created by the presence of electric current). The electromagnet generates a magentic field around itself. When a string vibrates inside the magnetic field of an electromagnet, a form of electrical energy is created, a vibrating (oscillating) electric current that can be transmitted to the other critical device, the electronic amplifier. The amplifier increases the strength of the current received from the pickup thereby enlarging or amplifying it.
Let's take a slightly closer look at the basic elements of a pickup. It must have an electromagnet to produce a magnetic field to interact with the motion of the vibrating string. The electromagnet used can be either a single bar of metal lying under all of the instrument's strings, or, it can consist of separate magnetically conducive pieces of metal lying beneath each string.
The pickups commonly used on the Fender Stratocaster are of the one-electromagnet-per-string variety, each electromagnet being what we call a "polepiece". The advantage of using one electromagnetic polepiece per string is that the distance between the electromagnet and the string can affect the strength of the resulting electric current, thus allowing the guitar player to adjust the strength of the signal created by each string by adjusting the height of the individual polepiece.
The individual polepieces are made electromagnetic by running an electric current through wire (called the "coil") wrapped around them. That's why pickups are "wound" with as many as 7,000 turns of fine wire. It is the electric current running through this wire that transforms the polepiece into an electromagnet.
The third basic component of the pickup is the permanent magnet, located at the base of the pickup. This magnetic element is not dependent upon an electric current for its magnetic properties. The permanent magnet serves as a sort of information center, taking the electronic signal received from the electromagnet and transmitting that signal through wires to what are called "resistors", the two most common of which are the guitar's volume and tone control. The volume control adjusts the strength of the signal sent to the amplifier and the tone control filters out the higher frequencies of the signal. After passing through the resistors, the signal then goes to the output jack that through contact with the guitar-to-amplifer cable sends the signal to the amplifer.
Here is a crude illustration of the components of a single coil pickup.
Single coil tone versus single coil hum
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A simple guitar pickup with one bobbin of wrapped wire is called a single coil pickup, the "coil" being the wrapped wire. Look at any original Stratocaster built between 1954 and 1979, and you will see three single coil pickups.
Single coil pickups are a good news, bad news proposition. The good news is that they produce a certain tone that many guitarists appreciate and enjoy, a tone that's impossible to describe in a few words but often refered to as "twangy". The single coil tone is a sound that has become closely identified with the Stratocaster. The bad news is that single coil pickups are subject to outside signal interference, resulting in what is commonly referred to as "hum", though technically, it's more accurate to say "noise" because the single coil will pick up a variety of signal interferences in addition to what's commonly referred to as 60 cycle hum caused by electromagnetic radiation from nearby transformers.
Most Gibson guitars, by way of comparison, employ a dual coil pickup, which are essentially two bobbins wired in opposite directions that "cancel", or in Gibson terminology, "buck" the single coil signal intereference propensity. The classic "humbucker".
Unfortunately, while the dual coil system prevents the single coil hum, it results in the loss of the single coil tone.
Actually, the classic Fender Stratocaster also employs its own variation of dual coil humbucking. When you place a five-way Stratocaster pickup selector in position 1,3, or 5, thereby selecting the bridge (position 1), middle (position 3) or neck (position 5) pickup alone, you hear hum. That's because each of the pickups is a simple single coil. However, when you move the selector to position 2 (a combination of the bridge and middle pickup) or position 4 (a combination of the middle and neck pickup) the hum disappears. That's because the middle pickup was wound in a direction that's the reverse of the neck and bridge pickups, so when combined with either the neck or bridge pickup the two act as a single humbucker, cancelling out the unwanted hum.
For years, Fender players wished they could have the best of both worlds, the single coil tone produced by the bridge, middle and neck pickups used alone without the singel coil hum. Several after-market pickup manufacturers, such as Bill Lawrence and Seymour Duncan, addressed this desire by producing pickups that promised single coil tone without the hum, and Fender itself offered two solutions on some of its factory made guitars: the Lace Sensor pickups introduced in 1987 and the Vintage Noiseless pickups introduced in 1998.
2004 saw the arrival of Fender's third single coil "noiseless" pickup solution, the Samarium Cobalt Noiseless (SCN) pickup, which promises to carry the classic Fender Stratocaster tones into the future.