Electric Guitar Pickups
To produce sound, an electric guitar senses the vibrations of the strings electronically and routes an electronic signal to an amplifier and speaker. The sensing occurs in a magnetic pickup mounted under the strings on the guitar's body. A simple magnetic pickup looks like this:
This pickup consists of a bar magnet wrapped with as many as 7,000 turns of fine wire. If you have read How Electromagnets Work, then you know that coils and magnets can turn electrical energy into motion. In the same way, they can turn motion into electrical energy. In the case of an electric guitar, the vibrating steel strings produce a corresponding vibration in the magnet's magnetic field and therefore a vibrating current in the coil.
There are many different types of pickups. For example, some pickups extend a single magnet bar under all six strings. Others have a separate polepiece for each string, like this:
Some pickups use screws for polepieces so that the height of each polepiece can be adjusted. The closer the polepiece is to the string, the stronger the signal.
The pickup's coil sends its signals through a very simple circuit on most guitars. The circuit looks something like this:
The upper variable resistor adjusts the tone. The resistor (typically 500 kilo-ohms max) and capacitor (0.02 microfarads) form a simple low-pass filter. The filter cuts out higher frequencies. By adjusting the resistor you control the frequencies that get cut out. The second resistor (typically 500 kilo-ohms max) controls the amplitude (volume) of the signal that reaches the jack. From the jack, the signal runs to an amplifier, which drives a speaker.
Many electric guitars have two or three different pickups located at different points on the body. Each pickup will have a distinctive sound, and multiple pickups can be paired, either in-phase or out, to produce additional variations.
Next, we'll look at the role of the amplifier.