Ground loops can be vexing, but try the following fixes before you go crazy
What was that buzz? That strange hum? The digital hash from your computer that’s showing up in the mic preamp? You may be a victim of ground loops, which can occur easily when using multiple AC-powered devices. But before we can solve the problem, we need to understand what causes it.
A ground loop means there is more than one ground path available to a device. In the illustration above, one path goes from device A to ground via the AC power cord’s ground terminal, but A also sees a path to ground through the shielded cable and AC ground of device B. Because ground wires have some resistance (the electronic equivalent of friction), there can be a voltage difference between the two ground lines, thus causing small amounts of current to flow through ground. This signal may get induced into the hot conductor. The loop can also act like an antenna for hum and radio frequencies. 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: break the loop by interrupting the audio ground, or break the loop by interrupting the AC ground line. The preferred method depends on the nature of the problem, so let’s look at various options.
BUT FIRST, A NON-SOLUTION: GROUND LIFTERS
Some musicians who are unconcerned about leading a long and productive life simply “lift” the AC ground by plugging a 3-wire cord into a 3-to-2 adapter. However, this is definitely not recommended since it obviates the safety protection afforded by a grounded chassis. Rather then spend the next page explaining why you shouldn’t do this, just don’t do it, okay?
SOLUTION #1: THE SINGLE-PLUG THEORY
Many ground loop problems can be solved by plugging all equipment into the same grounded AC source, which attaches all ground leads to a single ground point (for example, a barrier strip that feeds an AC outlet through a short cord). However, it is important to make sure that the AC source is not overloaded and is properly rated to handle the gear plugged into it.
SOLUTION #2: THE BROKEN SHIELD EFFECT
A solution for some stubborn ground loop problems is to isolate the piece of gear causing the problem, and disconnect the ground lead (shield) at one end only of one 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’s no completed ground path between the two devices except for AC ground.
If you make your own cables, it’s worth wiring up a few special ground loop-buster cords with a disconnected shield at one end. Mark them plainly; if used as conventional cords, you’ll encounter hum, loss of level, and other problems.
Sometimes a ground loop becomes objectionable only if the grounded metal chassis of a piece of rack mount gear contacts the metal rail of a rack cabinet. Use insulating washers, non-conductive rack rails, or “HumFrees” (from Dana B. Goods; these are little plastic strips that attach to your device’s rack ears and insulate the device from the rack). I’ve used HumFrees with rack mount computer peripherals that dump a lot of garbage to ground, and they seem to have made a difference.
SOLUTION #3: AUDIO ISOLATION TRANSFORMER
Using a 1:1 audio isolation transformer is much more elegant than simply breaking the shield, but accomplishes the same thing: interrupt the ground connection while carrying the signal. Although a cord with a broken shield is less expensive, the transformer offers some advantages. If needed, you can change impedance or levels simultaneously by choosing a transformer with different impedances for the primary and secondary windings (e.g., use the transformer to boost the level of a device with a fairly low output; this gives less noise than turning up the mixer’s preamp gain).
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. 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 audio is a simpler, lower power solution (that can also minimize computer-generated
“hash”), but an AC isolation transformer provides ancillary benefits. In short, 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.
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 just have a simple ground loop problem, then patching in audio isolation transformers may be all you need.
SOLUTION #5: DON’T WORRY, BE HAPPY!
No one with technical training would recommend the following approach, so don’t bother adding a comment saying this is all wrong. I know it’s all wrong—but I can’t help it if it works in some instances!
Here’s the deal: don’t worry about ground loops. In fact, make as many ground connections as possible — connect rack units to the metal rack case, run wires between the ground connections on various rack frames, run more wires from the rack frame to the metal case of barrier strips, and so on. Your system ends up with so many ground lines that the overall resistance to ground drops to just about nothing (putting resistors in parallel lowers the resistance). Essentially, you’re creating what’s called a ground plane.
I discovered this technique years ago when I was having some serious ground loop problems in my unbalanced, -10 studio accompanied by a looming deadline. I’ve since gone back and wired things “properly,” but I pass this incorrect technique along to those who have ground loop problems, looming deadlines, and don’t have the patience to rewire their studio.
Happy humbusting, y’all!