By Bruce Bartlett • October 14, 2009 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. Figure 1: A typical electric-guitar circuit. 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. 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. Hum Sources 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. Also, the power wiring and pickup act as two plates of a capacitor. The varying electric fields from the power wiring couple capacitively to the pickup and guitar wiring. Another hum source is radio-frequency fields from computers, motors, and TV transmitters (vertical sync, blanking and vertical component video). This RFI can be detected by the guitar or audio equipment. A major cause of hum is the ground loop. It is the circuit loop that is formed when two pieces of audio gear are connected to each other through a cable shield and also through the AC safety ground. If the two chassis are at different ground potential, a 60 Hz current can flow on the cable shield connecting them, causing audible hum. Figure 2: A ground loop. Figure 2 shows a ground loop. Two equipment chassis (guitar amp and mixer) are connected to two separate safety grounds by their AC cords. Also, the equipment chassis are connected together by the shield of the audio cable coming from the direct box. The shield and safety-ground wires form a ground loop. Causes & Cures Let’s look at several examples of causes and cures of hum related to electric guitars. Magnetic Hum Fields: As noted, 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. Electric Hum Fields: The power wiring and pickup act as two plates of a capacitor. The varying electric fields from the power wiring couple capacitively to the pickup and guitar wiring. Cure: A grounded Faraday shield. In the guitar body are cutouts that house the electronics, wiring and pickups. These cutouts should be lined with conductive foil (such as copper foil) that is soldered together to form one continuous piece. This shield must be connected to the guitar jack’s ground terminal so that the hum fields are bypassed to ground (ideally, to the mixer chassis). The guitar cord should also be well shielded. Use only high-quality cords with plugs having a metal jacket, which acts as a shield to the wires inside it. Some guitar amps are painted on the inside with conductive paint that acts as a shield, but this paint coating can crack when the amp cabinet is jostled, breaking the shield connection. It’s best to run a ground strap between all panels of the amp head cabinet. RFI (Radio Frequency Interference): A strong TV signal can be rectified and demodulated by some electronic components or a bad solder joint. Cure: This is a subject in itself (see the references at the end of this article). But for a quick fix, install ferrite beads and .001 microfarad capacitors on mic inputs. Install RFI chokes on guitar cords or mic cables. Check solder joints. Ground Loop: 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. There still may be a slight voltage difference between components because their power supplies reflect different voltages onto their chassis. A balanced AC power supply can eliminate this problem. 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: Suppose you’re recording a guitar with a direct box, and the guitar is NOT plugged into a guitar amp. If the ground is lifted on the direct box, the guitar is not grounded, so you hear a loud buzz. Figure 3: A wire between the guitar ground and the player’s body can stop hum.. Or if the shield connection is broken in the guitar cord or mic cable, the guitar is not grounded. Cure: 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. (Incidentally, the same thing happens if you replace the player’s body with a sheet of aluminum foil). Cure: Run a wire between a ground point on the guitar and the player’s skin. Figure 3 shows a ground wire (highlighted in yellow) between the guitar-jack ground and the player’s big toe! This grounds the player’s body, so that it acts as a partial shield for the guitar, rather than a capacitor. Note: For a technical discussion of body grounding, please see the sidebar at right. A body close to the guitar increases hum, and connecting the body to the guitar ground stops the hum. The body is not a ground for the guitar. Rather, the guitar ground is a ground for the body. Sidebar: Technical Discussion Of Body Grounding by Jim Brown, Audio Systems Group The human body is a conductor with relatively high resistivity, and it is a fairly large conductor. This means that when it makes contact with an electrical circuit it can act as an antenna, and it can also act as one “plate” of a capacitor. The other “plate” of that capacitor might be a noise source like a power line, a noisy electric light, or computer wiring. The noise might be base band (that is, audio frequency), or it might be modulated RF, or it might be both. The body will react differently to those noise sources depending on what they are—their frequency content, their internal impedance, their orientation with respect to the body, etc. And the body will interact with the circuit of the audio equipment and its wiring. The various effects of the body in any given circuit will add algebraically—that is, they may be varying degrees of in phase, and they may be in or out of polarity, and they will be at various relative levels with respect to each other, so in any given field condition they will be different. Some examples of the guitar problem. Let’s say that the body touches the “hot” conductor of a guitar cord plugged into an amplifier. The body can act as both a capacitor, coupling both audio and RF into the equipment, and it can act as an antenna. What’s the difference? The word “antenna” implies reception or transmission of an electromagnetic field—that is, the simultaneous existence in space of an electric field and magnetic field at right angles to each other, and in which energy is traded back and forth between electic and magnetic fields. An antenna has both current flow along it and a potential difference along it that either is caused by the field (reception) or generates the field (transmission). So when the body is acting capacitively, it is NOT acting as an antenna, it is not “receiving noise” and coupling it to the equipment. It has become part of the equipment’s wiring and is an element in the equivalent circuit. Now, the body may be acting as a capacitor to one noise source (or in one frequency range) and as an antenna to another, and may be doing so simultaneously! Another way that the body can get into the act is by causing current flow on the shield of an unbalanced cable. That current can couple noise in at least two ways. First is the IR (or IZ) drop in the shield, which is added to the signal. Second is via a pin 1 problem. When the orientation of the guitar is important, there are three mechanisms I can think of that can be at play. First and most obvious is the null that occurs when the circuit that is inductively coupled to a magnetic field is at right angles to that field. Second is the movement of the body and the guitar so that it is physically closer to the noise source, and thus has a higher capacitance to the noise source. Third is the directivity of the antenna that it is part of. Suggested References Radio Frequency Susceptibility of Capacitor Microphones (Brown and Josephson). AES Preprint #5720 Common Mode to Differential Mode Conversion in Shielded Twisted Pair Cables (Brown and Whitlock). AES Preprint #5747 Testing for Radio-Frequency Common Impedance Coupling (the “Pin 1 Problem”) in Microphones and Other Audio Equipment (Brown). AES Preprint #5897 A Novel Method of Testing for Susceptibility of Audio Equipment to Interference from Medium and High Freqeuency Radio Transmitters (Brown). AES Preprint #5898 Noise Susceptibility in Analog and Digital Signal Processing Systems (Muncy). JAES June 1995 So now we know why some heavy-metal guitarists play without a shirt. They’re removing the dielectric between their skin and the guitar. (Thanks to Chris Vice of Crown for that insight). Caution Do NOT use this ground wire in a concert situation if the guitar is plugged into a guitar amp. There might be a shock hazard if the player touches a mic. That can happen if the mic, which is grounded to the FOH mixer, is at a different ground potential than the guitar amp onstage. To reduce the potential between mixer and guitar amp, power the mixer through a thick extension cord plugged into the AC distro outlet that the guitar amp is plugged into.In other words, do NOT make a permanent connection between the player’s skin and the guitar ground in this situation. You might ask the player to keep their hands on the strings whenever possible. Shock Hazard This is not about hum, but is an important related issue. In concerts, electric-guitar players can receive a shock when they touch their guitar strings and a mic simultaneously. This occurs when the guitar amp is plugged into an electrical outlet on stage, and the mixing console (to which the mics are grounded) is plugged into a separate outlet across the room. As previously stated, these two power points may be at widely different ground voltages, so a current can flow between the grounded mic housing and the player touching the grounded guitar strings. Electric guitar shock is especially dangerous when the guitar amp and the console are on different phases of the AC mains. The cure is to power all instrument amps and audio gear from the same AC distribution outlets. That is, run a heavy extension cord from a stage outlet back to the mixing console (or vice versa). Plug all the power-cord ground pins into grounded outlets. That way, you prevent shocks and hum at the same time. Using a neon tester or voltmeter, measure the voltage between the electric-guitar strings and the metal grille of the microphones. If there is a voltage, flip the polarity switch on the amp. Use foam windscreens for additional protection against shocks. Quick Tips When you hear hum or buzz from an electric guitar, try these solutions: Turn up the guitar’s volume and treble controls so that the guitar signal overrides hum and noise picked up by the guitar cable and guitar amp. Ask the guitarist to move around, or rotate, to find a spot in the room where hum disappears. Flip the polarity switch on the guitar amp to the lowest-hum position. To remove buzzes between guitar notes, try a noise gate. If the hum stops when the player touches the guitar strings, ask the player to keep his or her hands on the strings, or run a wire between the player’s skin and a ground point on the guitar (such as the strings or the jack ground.) Set the direct-box ground lift switch to the position where you monitor the least hum. Replace or repair guitar cords that have broken shields. Use only high-quality cords with metal-jacket plugs. Power the guitar amp off the mixer’s outlet strip. Use guitars with humbucking pickups, or install modern humbuckers in older guitars. Line cutouts in the guitar body with copper foil wired to the guitar jack ground. If you suspect RFI, install ferrite beads, capacitors and chokes. Also see the references below. Replace any defective tubes in the guitar amp. If the power-supply filter capacitors in the guitar amp are corroded, replace them. This replacement should be done by an authorized technician. Use a quieter amplifier. Don’t use a noisy amp. Instead, record the guitar direct, then process its track with a guitar-amp modeling plug-in or processor. Don’t use SCR lighting dimmers because they add noise and hash to the AC power. Instead, use multiway incandescent bulbs to vary the studio lighting levels. If you must use a SCR dimmer, rotate its knob to find a position with the least hum (maybe the “off” position!). Run the studio off its own breaker, not shared with noisy loads such as air conditioning, power tools, etc. Don’t ground the neutral at more than one point (have an electrician check this). Use an AC line isolation transformer between the AC power and the studio equipment. If you follow these suggestions, the only buzz you get should be from the guitar player’s solo! Good luck. Acknowledgement: Many thanks to these SynAudCon members for their helpful discussions: Jim Brown, Rick Kamlet, Bob Hagenbach, Mike Miles, Pat Brown, Steve Roth, and Peter Patrick. About Bruce Bruce Bartlett Recording Engineer AES and SynAudCon member Bruce Bartlett is a recording engineer, audio journalist, and microphone engineer. His latest books are “Practical Recording Techniques 5th Ed.” and “Recording Music On Location.” http://www.bartlettaudio.com Comments Leave a Reply Cancel reply Your email address will not be published. Required fields are marked *Comment Name * Email * Website soundscope says I have an issue with the following: “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.” Just because two outlets are fed from different circuit breakers DOES NOT necessarily mean that the outlets are inherently at different ground potentials. Wilbert Lichtenberg Jr. (Pancho) says Mr. Bartlett I have been reading your articles here at Pro Sound Web and have always enjoyed them , but I think you may have got some of your facts wrong about how a guitar player can get shocked, please let me explain. I have been working on guitar amplfiers for 36 years. I mostly work on Tube amplfier, which a lot of them are quite old, like old Fender tube amps. Most of these amps would come with a 2 way AC cord, no ground pin and the two contacts on the ac cord were not polarized, you could plug them in either way in the outlet in the wall. They also had a polarity switch which would connect the metal case of the amp, throught a capasitor, to the Hot wire or the neutral wire of the ac cord. So with a older amp with a 2 prong ac cord you had a 50/50 change of turning the case of your amp into a 120 volt live circuit. So since your guitar cord is connected to the chassic of the amp and to your guitar ground through the ground of your guitar cord ,which was also connected to your strings, now your string would be 120 volts hot also. but if you were not touching anything that was grounded you would not get shocked till you touched something grounded. Now if you were standing , barefooted on the basement concrete floor you would get shocked, but if you had your tennis shoes on you would be insulated till you touched something with any part of your body. Now lets say you went up to a microphone that was connected up to a PA system which would normally be newer then your guitar amp and have a 3 way ac plug on it and say that it was plugged into a correctly wired outlet, them the mic case, windscreen, would be grounded. So when your lips touched the mic and you were touching your guitar strings you would have completed the circuit and could have 120 volt shock, but sometimes the shock was not so bad , just tingling whick was due to just how well you were in contact with the mic,or guitar, dry worn skin, woking mans hands, would not be as conductive as lets say a wet hand . I think most guitars player of a certain age(over 50) could tell you all about getting bit by there mic while holding there electric guitar. Now if you really want to get bit have the same older guitar amp with a older PA head that someone broke off the grounding pin on the ac cord and have each of the units (amp and PA system) on different legs of the AC power and have the polarity switches in the wrong position, now you can get shocked with 240 volts. I think that is how guitar or bass player could get shocked ,not the way you were talking about in the article.not to say it couldnt happen the way you were talking about just not too likely. Now a days most every guitar and bass amp has 3 way ac cord which if plugged into a correctly wired ac outlet and the PA has a 3 way ac cord and is plugged into a correctly wired outlet I would think that it would be next to impossible to get shocked . Any questions please email me. Please don’t take this in the wrong way i just think you may have gotten your facts wrong this time. Jim says Years ago I made an adjustable grounding bracelet from some old metal jewelry that the musician could wear. I simply clipped a jumper from the guitars input shield to that bracelet. It worked most the time if the musician was not grounded. Don’t get fancy with the bracelet as occasionally the player would want to wear it home. Michael Edinger says Despite having no hard facts or statistics, I am still surprised by the seemingly much more lethal 120V/60Hz mains in the USA, compared to EU voltages of 230V/50Hz- logically, it should be the other way around, right? the answer could be in the specification of power components in a device. When servicing, repairing or modifying US made equipment I often find substantial capacitive leaks from 120V mains input to chassis, enabling a hum provoking current flow to ground. Using a magnetic (coil) pickup as a sensing element, I regularly find very strong 60HZ magnetic leakage fields around the mains transformers built into US music equipment, suggesting that the build quality of these is questionable. A modification of US gear to european voltages and mains security standards often has a remarkable reduction in hum and noise, stray fields and leakage as an added benefit. Using toroid transformers when feasible, or C core or E-I using quality steel core laminated materials, with a grounded static screen between primary and secondary windings, and keeping magnetic core flux well below saturation is the way to go. John Maher says This is not so much hum, although we have the issue and have not found a solution, as it is about shock. During practice our electric guitar player was shocked when he contacted the mic and was touching his guitar strings. I experienced the same thing and for the performance connected his Line 6 pod (powered by 9-volt separate power supply) via direct box from the 1/4” output instead of using the XLR output. I later used a voltmeter and found no AC voltage from the stings to the mic case, yet I got shocked. Switching to DC, I found 47.6 volts. After a moment I realized our digital patch bay’s phantom power was causing the shock. We are now re-patching our stage to get only have phantom power where needed (it is switchable for groups of 8 inputs). I never thought 48v phantom power would be, well . . . shocking, but it was. Please note there is no disparagement intended against Line 6 products, but it is interesting that there is no protection in case of accidental phantom power. Interesting learning experience. Bruce Bartlett says You’re right, Soundscope. Outlets fed by different circuit breakers MAY be at different ground potentials. - Bruce Bruce Bartlett says If the PA system and guitar amp are grounded to different AC circuits (rather than to a common AC distribution system), then there still can be a voltage difference between the two grounds. Grounds in different locations in a venue are not always equal voltage relative to each other. You are right; before guitar amps had grounded AC plugs the shock hazard was much greater. Jak Florek says Any ideas PLEASE to solve this VENUE problem: Lighting rig on it’s own = no buzz. Guitar amps on their own = no buzz. Amps + lights on = horrendous buzz. BUT - p.a. power amps don’t buzz. Guitar amps buzz with guitars plugged and some do even without a guitar plugged in. The building has been rewired - separate clean circuit for sound. Makes absolutely no difference which sockets we use for anything… buzzzzzzzzes. And sometimes… it disappears! Then it’s back next day. Electricians have run out of ideas… seems there’s no faults in wiring + no faults in Lighting Rig (new, not on same circuit as sound). Bruce Bartlett says Thanks for the kind words, Alton. Tagged with: Guitars Power Techniques · all topics Subscribe to Live Sound International Subscribe to Live Sound International magazine. Stay up-to-date, get the latest pro audio news, products and resources each month with Live Sound.