You’re recording an electric guitar, and you hear a buzz or hum. Here are some causes and cures.
Magnetic Hum Fields
AC in a room’s power wiring generates electric and magnetic fields that oscillate at 60 Hz and its harmonics. 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.
Cure: The amount of hum generated depends on the angle between the pickup coil wires and the magnetic hum field. At certain angles, a lot of the hum goes away. So the player can rotate or move around to find a spot with minimum hum.
Suppose you’re recording a guitar direct, and the guitar is plugged into a guitar amp. The amp and your mixer have 3-prong power cords that connect to the safety ground. The amp is plugged into an AC outlet across the room, and your mixer is plugged into a nearby outlet. When you connect the amp ground to your mixer ground through the mic-cable shield, and monitor the signal, you hear hum.
Chances are that the outlets are fed from different circuit breakers, so the outlets are at different ground voltages. When you plug your amp and mixer into these separated outlets, and connect the equipment together with a mic cable from a direct box, the difference in ground voltages can make a 60-Hz hum current flow between the guitar amp and mixer. That’s a ground loop.
Cure: Flip the ground-lift switch on the direct box to break the loop. Also, it’s a good idea to power the mixer and guitar amp off the same outlet strip. That way, the ground voltage for all the equipment is about the same, so little or no hum current can flow between their chassis. Run a thick extension cord from the mixer’s outlet strip to the guitar amp, and plug the amp into the extension cord.
Before you plug in all those power cords, make sure that the sum of the equipment fuse ratings does not exceed the amperage rating for that circuit. In most cases, a single 20-amp breaker will handle a small studio.
Guitar Not Grounded
Cures: Flip the ground-lift switch to the grounded position when not using a guitar amp. Check inside the cable connectors to make sure the shield is soldered at both ends. Replace or repair guitar cords that have broken shields.
Player’s Body Not Grounded
When the guitar player touches the strings, does the hum stop? This indicates that the player’s body is acting as one plate of a capacitor. The capacitance between the body and power wiring adds to the capacitance between the guitar and power wiring, increasing the level of the hum transmitted from the power wiring to the guitar.
Cure: Run a wire between a ground point on the guitar and the player’s skin. This grounds the player’s body, so that it acts as a partial shield for the guitar, rather than a capacitor. (Caution: Don’t do that at a live concert because the guitarist might touch a mic with voltage on its chassis.) If the guitarist is singing into a mic in the studio, put a foam windscreeen on the mic, or a pop filter in front of it, to prevent possible shocks.
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Bruce Bartlett is a recording engineer, microphone engineer (www.bartlettaudio.com) and audio journalist. His latest books are “Practical Recording Techniques 6th Edition” and “Recording Music On Location 2nd Edition.”