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Extracts from this topic, and this page in particular, have been reprinted with my permission in AUSTRALIAN GUITAR MAGAZINE, Volume 16 (July 2000).
Here are some pictures of typical pickups ...
The job of a pickup is to convert the vibration of a string into an electrical signal. The basic design consists of a magnet and coil arrangement where the strings interact with the magnetic field to induce a voltage in the coil. Even with this simple concept, there are quite a few variables and trade-offs.
A common misunderstanding is that the pickups on your guitar sound different because they are made different. Maybe they are different, but there is also huge difference in sound caused by the pickup's position on the guitar. I'll explain this in more detail later.
Most electric guitars up to around the 70's (including Fenders and Gibsons) generally used the same pickups in all positions, so it was the pickup's position alone that caused different sounds. These days, it is more common to combine different pickup types, and use hotter pickups in the bridge position.
Stronger magnets (or placing pickups closer to the strings) gives a higher output but also damp string vibration by pulling the strings towards the pickup. In severe cases, this can cause "false harmonics" or "double notes".
Providing additional windings on the coil increases the output, particularly midrange. Manufacturers can only take this so far, because eventually the loss of highs gives a very muddy sound. Also the high output can overload the input of some preamps, making a "clean" sound impossible without reducing the guitar volume setting (this may be desirable by some players). Additional coil windings produce a higher impedance which pose problems with treble loss when used with long cables. There are similar options and compromises with coil wire gauge.
"Active" pickups typically have a lower number of coil windings, giving a low output, low impedance, and very clean and clear, uncoloured sound. The low output is boosted by an on-board active preamp which maintains the low impedance. Low impedance pickups (on their own, or with a preamp) can drive long cables without noticeable treble loss.
Probably the most obvious difference in pickup designs is the single pickup versus humbucking pickup. A humbucking pickup contains two single coils placed side by side, with a common magnet arrangement. The sound is typically "fatter" (more midrange) due partly to the larger number of coil windings and partly because the sound of the string is "read" over a longer portion of the string.
The shape of the magnetic field affects how much of the string is read. The pole-pieces you see on strat single coil pickups are individual magnets and give a very focused magnetic field, reading a small section of the string vibration. On the other hand, screws in a humbucking pickup are just screws, but conduct the magnetic field from a magnet placed underneath the pickup. There is another set of magnetically conductive slugs in the other coil, so the combined humbucker reads a larger length of string vibration.
I've had email from a reader who related a story of a player who has placed washers around the tops of the pole-pieces of his strat pickups (these protrude a little from the top of the coil). This was allegedly to allow him to move the pickups closer to the strings without the problem of excessive magnetic pull on the strings. I've never tried this myself, but it does illustrate how changing the magnetic field of a pickup changes the way a pickup works and sounds.
Magnetic material also affects tone. Popular opinion is that Alnico II produces a sweet, vintage sound, while Alnico V is a little stronger, and gives a brighter, more attacking tone (ideal for rock). Ceramic magnets have a slightly harder edge, and are favoured by metal players.
With so many variables it's easy to see why there are many different choices. There are a few established standards, notably the original Fender Stratocaster, Gibson PAF, along with some popular models from retrofit manufacturers like DiMarzio, EMG and Seymour Duncan.
Even before you plug your guitar in, its character is determined greatly by the type of guitar woods used, construction methods, string gauges, etc. It is this basic character that the pickup picks up, adding its own colouration.
All of the above describes magnetic pickups, which are by far the most common pickups used on electric guitars. They are designed to work with strings that interact with a magnetic field, so they are not normally subject to acoustic feedback (you can't talk into one!). These magnetic pickups CAN be microphonic if anything metallic in or around them is loose and vibrates with the sound. This is common in old and cheap pickups that have loose coil windings or fittings. It is rarely a problem with modern pickups that are wax potted to make sure everything is secure.
Magnetic pickups are not suitable for nylon acoustic guitars, because nylon strings cannot interact with a magnetic field. Even for steel-string acoustic guitars, magnetic pickups are rarely favoured, because of their limited frequency range, and the fact that they read only a small portion of the string length. For acoustic guitars, it is essential to capture the vibration of the top, and this is most commonly done with a piezo pickup mounted underneath the bridge saddle.
Piezo pickups use crystals to detect pressure changes, and provide a very wide frequency response. They also have a very high impedance (typically over 5 Meg), and therefore need to be buffered with a preamp. This is usually provided in the acoustic guitar itself, often with extra tone control options, such as bass, middle and treble.
Opinions vary, but to my ears, these piezo pickups convey about half of the sound of an acoustic guitar. Acoustic guitars are just about always recorded with one or more very high quality microphones, but this is difficult to do effectively in a live environment.
Many guitar makers, such as Parker (with their revolutionary Fly series), Godin, and others have added piezo bridges to solid body electric guitars. These guitars offer standard electric sounds, acoustic sounds, and even the ability to mix both! Their acoustic sounds can be quite convincing in a band setting, mainly because of the wide frequency response, and partly by the conditioning we listeners have had from piezo equipped acoustic guitars. Of course, these sound even less like a real acoustic guitar than the 50% rating I gave for piezo-acoustics.
Fishman, L R Baggs and other many manufacturers offer bridge piezo pickups that can be added to many guitars.
Fishman offer an interesting pickup for acoustic guitars presently, called the Rare Earth pickup. It is a slim magnetic pickup that clamps into the sound-hole. I have not used one, but the humbucking model in particular has had good reviews. Apparently, the rare earth magnetic material provides a wider frequency response than typical electric guitar magnets. Also, a "deluxe" model includes a microphone that you can bend underneath inside the guitar, providing you with a blended acoustic and magnetic sound.
On the leading edge of pickup design at the moment is, wait for it, Light Pickups! These pickups detect string movement by the interference in a beam of light. Presently, the string needs to pass through a small chamber to do this, but it's a fascinating idea. These pickups should be immune to both hum and acoustic feedback (although I'm not sure about mirror balls!)