You’re recording an electric guitar, or amplifying it through a P.A., and there it is: hum! This annoying sound is a common occurrence.
Acoustic guitars fitted with pickups can have the same problem.
Hum is an unwanted 60 Hz tone—50 Hz outside the U.S.—plus harmonics. If the harmonics are especially strong, the hum becomes an edgy buzz.
Let’s take a look at what’s going on and how to fix it. First we need to review how an electric guitar works.
Inside The Electric Guitar
Built into the guitar, under the strings, is a magnetic pickup: a transducer that converts the strings’ vibration into an electrical signal. The pickup is a bar magnet wrapped with thousands of turns of wire, forming a coil.
When the player plucks the steel strings, they vibrate next to the magnet, producing a similar vibration in the magnet’s magnetic field, which in turn causes a varying current in the coil.
Another type of pickup uses a separate magnet under each string. Some pickups have a screw on each magnet’s polepiece to adjust the distance between the polepiece and string, allowing level control of each string.
A humbucking pickup uses two coils wired in series but with opposite polarity so that they cancel common hum fields. One coil is mounted far from the strings.
The high-impedance signal from the pickup coil goes through a simple circuit (Figure 1) and comes out the unbalanced guitar jack.
Components in the circuit are usually connected by single wires. The sleeve (ground) terminal on the jack is connected to the pickup coil, the strings, and the shield around the circuit.
Figure 1: A typical electric-guitar circuit. (click to enlarge)
From the guitar jack, the signal travels through a guitar cord: an unbalanced shielded cable.
At the end of the cable, the signal can go to several destinations: a direct box, a guitar amp, a mixer’s high-Z input, or guitar stomp boxes/processors.
Most acoustic-guitar pickups are piezoelectric types installed under the bridge or saddle. Vibrations of the guitar body flex the pickup, which generates an electrical signal. It’s very high impedance, and often is run through a preamp built into the guitar which reduces the impedance.
Whether the guitar is electric or acoustic, any component in the signal chain is susceptible to picking up hum and buzz, especially because the entire circuit is high-impedance unbalanced.
Alternating current in a building’s power wiring generates electric and magnetic fields that oscillate at 60 Hz and its harmonics. Hum fields also radiate from lighting circuits and fluorescent lights.
The magnetic fields couple inductively to the guitar wiring. When the magnetic lines of force cut the conductors in the guitar and its pickup, the conductors generate a 60 Hz signal, which is amplified by the mixer or guitar amp.