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18
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 TM300) 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 device is
probably the culprit.
We also recommend that you use balanced audio cabling and connectors wherever possible. The TM300 provides
electronically balanced inputs for all channel line inputs. The wiring diagram in the “Connecting The TM300” section
of this manual (page 10) shows how 1/4" TRS (Tip/Ring/Sleeve) and XLR connectors should be wired for use with
these inputs and outputs.
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 TM300 in a fixed location such as a rehearsal 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 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).
ENGLISH