Basic Electric Guitar Circuits 1: Pickups
Part 1: Pickups
Passive (i.e. battery-free) electric guitar circuits are relatively simple and the possibilities for customization are endless. A basic understanding of pickups, potentiometers, capacitors and switches is all you need to get creative and take more control of your instrument's voice on an electronic level.
Where does the electric guitar signal come from?
Pickups are transducers that convert the mechanical energy of a vibrating guitar string into electrical energy by way of electromagnetic induction. It is a fundamental concept studied in physics and electronics that a changing magnetic field will generate a current through a coil of wire. The electric guitar pickup uses permanent magnets and pole pieces to form a steady magnetic field in the vicinity of each individual guitar string. An opposite magnetic polarity is induced in the metallic (steel core) guitar string when mounted above its respective pole piece and when the string moves, the otherwise steady magnetic field changes accordingly. Wire is wrapped around the poles thousands of times to form a coil within the magnetic field to pick up an induced current and voltage.
Instantaneous Voltage Measurements Induced by the Moving Steel Core Guitar String
~I~ = current, ~V~ = voltage
~N~ = magnetic north, ~S~ = magnetic south
~B~ = magnetic field, ~\Delta B~ = net change in magnetic flux
The output signal from the pickups is AC (alternating current) because the direction of the current alternates, producing a positive voltage when the string moves in one direction and a negative voltage when the string moves in the opposite direction.
The previous drawing illustrates the electrical and magnetic function of a single-coil pickup. Some pickups might use six permanent magnets in place of the six pole pieces to create the magnetic field, but the idea is the same: create a steady magnetic field around a coil in proximity to the guitar string. The name "single-coil" pickup becomes more significant when compared to the humbucker or "dual-coil" pickup.
Pickups: Single-Coil vs. Humbucker
The first successful guitar pickup was developed in the early 1930s by Rickenbacker® to help amplify Hawaiian lap steel guitars which were popular at the time. The first pickups were single-coils and while they do a good job of picking up the guitar signal they are also susceptible to picking up interference from nearby electrical devices. The Gibson® humbucker (US Patent 2896491) was developed in the 1950s to eliminate the "hum noises" resulting from electromagnetic interference. The humbucker uses two coils and a pair of pole pieces (having opposite magnetic polarities of each other) for each string. The coils are wound and connected to each other in such a way that the current produced by the moving guitar string in the two coils adds up (in-phase), while the current produced by electromagnetic interference in the two coils cancels (out-of-phase). Not only does the humbucker drastically reduce noise from interference, but it also has a different characteristic sound. The single-coil pick up is commonly considered to have a thin, clear and bright (more treble) sound, while the humbucker is known to have a full, but dark (less treble) sound with more overall signal output.
Connecting Multiple Pickups
When connecting more than one pickup, it is important to follow the manufacturer color codes and wiring diagrams so that the phase relationship is correct. The phase relationship of a pickup is determined by the winding direction of the coil and the polarity of the magnets.
Same Winding Direction / Same Magnetic Polarity
The two coils of the traditional humbucker are connected in series with the phase relationship shown below.
Opposite Winding Direction / Opposite Magnetic Polarity
Most modern Stratocaster® style guitars with three single-coil pickups are supplied with a reverse wound/reverse polarity middle pickup for a parallel hum canceling effect when the guitar is switched to a two pickup position (e.g. neck & middle pickup together) as shown below.
Most replacement electric guitar pickups have limited electrical specifications given on the packaging or on-line which can give you a basic idea of the relative output level and how bright or dark a similar pickup will sound.
- DC Resistance: This can be measured directly with an ohm meter and gives you an idea of how many turns of wire the coil has. If the same gauge of wire was used for two pickups, then the pickup with fewer turns to the coil will have a lower resistance which, in general, makes for a lower output level and a brighter sound.
- Inductance: Inductance is the ability of an inductor (or coil) to store energy in a magnetic field. A higher inductance makes for a higher output level and a darker sound.
- Peak Frequency: This is the frequency beyond which the output level begins to fall dramatically. A higher peak frequency would make for a brighter pickup.
Variety is the spice of tone
Guitar pickups are a vital component of your tone and replacing them is something that most guitarists can learn to do themselves. Using high quality pickups can go a long way to bringing new life and excitement to your playing experience. There are hundreds of pickup manufacturers and thousands of pickups to choose from. Whether you are looking for a hotter pickup, trying to capture a beloved vintage tone or seeking single-coil sound in a noiseless package, brands like DiMarzio®, Seymour Duncan®, Lace®, Porter®, Fender®, Gibson® and many others offer a solution.
In Part 2, we will discuss how the volume and tone controls, called potentiometers, function in an electric guitar circuit.