How to Identify and Eradicate the Different Hum Gremlins that Haunt Your Studio
By Jon Chappell and Craig Anderton
Figure 1: Ground loop schematic.
Ground loops produce that noisy, low-pitched dirt that plagues your audio signals. You find them whenever your audio hookups start to get complicated—such as multiple pieces of plugged in gear all linked by audio cables. You’re even more vulnerable if these multiple units are plugged into different wall outlets around your studio. But ground loops can happen in a setup as simple as your laptop computer and powered speakers. More than a few people have reported that their AC-powered multimedia speakers buzz when the computer is plugged in, but are quiet as mice when the computer is unplugged and running on its internal battery.
THE LOWDOWN ON LOOPS
Ground loops are easy to eliminate, once you understand them. And really, you don’t even have to understand ground loops if you just memorize the steps to fixing them should they occur. But bear with me a moment to see why your music will get a buzz on (in a bad way). Any device should have only one AC path to ground. A ground loop occurs when a signal finds more than one ground path available. In Figure 1 above, one path goes from device A to ground via the AC power cord’s ground terminal (in red), but A also sees a path to ground through the shielded audio ground cable (also red) via AC ground of device B. The reason that path exists is twofold: 1) both the AC and audio grounds are connected via the device's metal chassis; and 2) the voltage is not 0, on one or both of the lines.
Because ground wires have some resistance, a potential difference in voltage between the two ground lines can exist, and that causes current to flow through the ground paths.
This signal may get induced into the hot conductor. The loop can also act like an antenna for hum and radio frequencies (called the loop antenna effect). Furthermore, many components in a circuit connect to ground. If that ground is “dirty,” this noise might get picked up by the circuit. Ground loops cause the most problems with high-gain circuits, since massive amplification of even a couple millivolts of noise can be objectionable.
There are two main fixes: 1) break the loop by interrupting the audio ground; or 2) break it by interrupting the AC ground line. The preferred method depends on the nature of the problem, so let's look at various options.
Ground Lifters. Some musicians simply “lift” the AC ground by plugging a 3-wire cord into a 3-to-2 adapter. This is definitely not recommended since it eliminates the safety protection afforded by a grounded chassis. In other words, you risk getting shocked. But if you're on a gig, and it's an emergency and temporary, and you’re really careful, you can try this method as a quick fix.
Solution #1: The Single Plug. You can solve many ground loop problems by plugging all the equipment into the same grounded AC source, such as a power strip that feeds an AC outlet through a short cord, as this attaches all ground leads to a single ground point. However, it is crucial that the AC source is not overloaded and is properly rated to handle the gear plugged into it.
Solution #2: The Broken Shield. A solution for some stubborn ground loop problems is to isolate the piece of gear causing the problem and disconnect the ground shield at one end or more of the audio patch cords between it and other devices. The inner conductor is still protected from hum by a shield connected to ground, yet there is no completed ground path between the two devices, except for AC ground.
Sometimes a ground loop shows up as objectionable only if the grounded metal chassis of a piece of rackmount gear contacts the metal rail of rack cabinet. There’s an easy fix: Humfrees, from Dana B. Goods, are little plastic strips attached to your device’s rack ears that insulate the device from the rack (see Figure 2). They can be particularly effective with rackmount computer peripherals that dump a lot of garbage to ground.
Figure 2: Humfrees isolate the metal chassis of your device from the metal rails of your rack.
Solution #3: Audio Isolation Transformer. Using a 1:1 audio isolation transformer is much more elegant than simply breaking the shield, but delivers the same benefit: it interrupts the ground connection while carrying the signal. Although a cord with a broken shield is less expensive, the transformer offers some advantages. If necessary, it can also change impedance or levels if you choose a transformer with different impedances for the primary and secondary windings. For example, use the transformer to boost the level of a device with a fairly low output; this produces less noise than turning up the mixer’s preamp gain.
For a commercial solution, check out Ebtech’s rackmount Hum Eliminator. This consists of audio transformers in a rackmount case, and uses TRS (tip-ring-sleeve) phone jacks that work with balanced or unbalanced lines. To “break” an audio ground line, just use one of the transformers in the Ebtech instead. Ebtech also makes a model that converts back and forth between +4 and -10 signal levels—see Figure 3.
Figure 3: Devices like the Ebtech Hum Eliminator isolate the audio ground (note the 1/4" connectors at the bottom of the unit).
Solution #4: AC Isolation Transformer. Many times you can also break a loop by removing the direct connection from a piece of gear to AC ground through an isolation transformer (see Figure 4). This doesn't always work because the ground loop may not involve the AC line but various ground-to-ground connections; however, loops involving the AC line generally seem to be more problematic and common.
Breaking the audio connection is a simpler, lower-powered solution (and can also minimize computer-generated “hash), but an AC isolation transformer provides ancillary benefits. In summary, an AC isolation transformer can clean up the AC line, reduce spikes and transients, and provide performance almost equal to that of a separate AC line. One such device is made specifically for musicians: MIDI Motor’s Hum Buster, which has a large transformer with 10 isolated AC outlets.
Figure 4: Furman IT-1220 includes an isolation transformer, which works on the AC ground, not the audio ground.
So which is better, breaking the audio connection or the AC connection? It depends. If you have a lot of microprocessor-controlled gear and less than ideal AC, adding isolation transformers can solve various AC-related problems and get rid of ground loops. If you have just a simple ground loop problem, then patching in an audio isolation transformer may be all you need.
Jon Chappell is a guitarist and Associate #Editor at Harmony Central. He has contributed numerous musical pieces to film and TV, including Northern Exposure, Walker, Texas Ranger, All My Children, and the feature film Bleeding Hearts, directed by actor-dancer Gregory Hines. He is the author of The Recording Guitarist: A Guide for Home and Studio (Hal Leonard), Essential Scales & Modes (Backbeat Books), and Build Your Own PC Recording Studio (McGraw-Hill), and has written six books in the popular For Dummies series (Wiley Publishing).
Craig Anderton is Editor Emeritus of Harmony Central. He has played on, mixed, or produced over 20 major label releases (as well as mastered over a hundred tracks for various musicians), and written over a thousand articles for magazines like Guitar Player, Keyboard, Sound on Sound (UK), and Sound + Recording (Germany). He has also lectured on technology and the arts in 38 states, 10 countries, and three languages.