16
Grounding Techniques
Hum and buzz are the biggest enemies you face when interconnecting a large number of different pieces of equipment
to a central audio mixer. This is because each piece of equipment may operate at a marginally different voltage (this
difference is called potential) and, when two devices at slightly different potential are physically connected with audio
cabling, the end result can be nasty, extraneous noise (mind you, connecting two devices at very different potential can
result in a major electrical shock!).
However, there are several steps you can take to avoid grounding problems. First, assuming you have an isolated
electrical circuit that can handle the electrical demands of your mixer and all connected audio equipment (these needs
will usually be modest), you should always plug your mixer and all connected equipment into the same circuit. If possible,
nothing else but this equipment should be connected to that circuit. If you can’t do this, at least avoid plugging your mixer
and audio equipment into the same circuit that is already powering things like heavy machinery, air conditioners, heaters,
refrigerators, washing machines, neon signs or fluorescent light fixtures. One particular culprit that will almost certainly
create problems is the standard light dimmer (the kind that uses silicon controlled rectifiers). Where low-level lighting is
desired, use incandescent fixtures with autotransformer-type dimmers (sometimes called Variacs) instead—these cost
considerably more than the standard dimmer you’ll find at your local hardware store, but are well worth the extra expense.
Three-prong plugs (such as the one used by the SD8) should always be used as is; don’t use adapters to lift the ground
(unless you’re using a “star ground network”—see below). If you hear hum or buzz from a device that uses a two-prong
plug (or an external two-prong AC/DC adapter), you can try reversing the plug in the socket. If that doesn’t work, you
may need to physically ground that device’s chassis by connecting a wire (called a strap) from it to a grounded piece of
metal. Some pieces of equipment have a screw-type ground post to which the strap can be connected; if not, you can
attach some kind of metallic binding post to the case itself. If you are using rack-mounted audio devices and are
experiencing hum or buzz, there’s a simple test to determine the source of the problem: while keeping all devices
powered on and connected with audio cabling, physically remove each device, one by one, from the rack. If the hum
disappears when a particular device is removed, you’ll know that that device is probably the culprit.
We also recommend that you use balanced audio cabling and connectors wherever possible. The SD8 provides
electronically balanced connectors for all mic and line channel inputs. The wiring diagram in the “Connecting The SD8”
section on page 11 of this manual shows how XLR and 1/4" TRS (Tip/Ring/Sleeve) connectors should be wired for use
with these inputs.
In addition, you can minimize possible interference by planning your audio, electrical, and computer cable runs so that
they are as far apart from one another as possible and so they don’t run parallel to one another. If they have to cross,
try to ensure that they do so at a 90° angle (that is, perpendicular to one another). In particular, try to keep audio cabling
away from external AC/DC adapters
If you’re using the SD8 or in a fixed location such as a recording studio, you may want to invest the time and money into
creating a star ground network. This is by far the best technique for avoiding grounding problems. It involves using a
formidable ground source such as a cold water pipe or a copper spike driven into the earth. A thick grounding cable is
connected to that source and is then brought to a central distribution point; from there, individual cables are connected to
each piece of equipment. This setup also requires that you lift the ground plug of all three-prong AC connectors, so there
is the possibility of danger if it is done incorrectly. We strongly recommend that you contract with a qualified professional
to carry out this or any kind of electrical work.
Another, less common problem you may encounter is that of oscillation (a ringing tone), which, apart from being
annoying, is potentially dangerous to your speakers. This is generally caused either by poor outside wiring or by
returning a signal out of phase (most commonly from an outboard signal processor). If audible oscillation occurs, try
isolating each input signal by turning down all other inputs. If one signal alone is causing the problem, you should be
able to eliminate the oscillation by reversing that signal’s phase (many signal processors have a switch that allows you
to do this).
