TDM Audio 32CL-2 v.2 - Manual (Page 23)

© 1996 TDM Design, Inc. Compressor/Limiter Owner’s Manual Page 23
and that when no input is present, the unreasonably high gain in the system is producing a lot of
noise.
If you check out all of these possible causes and you still can’t resolve the problem, contact your
vendor, or call TDM for support and/or service (see Contacting TDM).
60 Hertz Hum or Buzz
60 Hertz hum or buzz in a system can be extremely difficult to track down because it is usually
not a problem with any one piece of equipment. It is usually caused by how the entire system is
connected and grounded.
To fix a hum or buzz in a system, suspect any piece of equipment that gets a ground connection
from more than one place. These problems are called “Ground Loops” and the technical explana-
tion of why they cause problems is that there is actually a voltage difference between the two
different grounds. The problem is most often caused by a single piece of equipment grounded to
two different power sources that are located some distance apart. For example, a mixing console
is plugged into a grounded outlet at the back of an auditorium, and the power amplifiers are
plugged into a different outlet 100 feet away at the stage. The mixing console is connected by
shielded cable to the amplifiers and the shield is grounded. This causes both the mixer and the
amps to be individually grounded, and each gets another ground from the other through the
shielded cable.
A problem like this can be fixed in several different ways. The mixer ground could be lifted. This
is commonly done by plugging the mixer’s three-prong plug into a two-prong grounding adapter
(you can get these at any hardware store), and plugging that into the outlet. This effectively dis-
connects the mixer’s ground lead from the outlet so that the mixer is now grounded only to the
amplifiers. The ground could also be lifted at the amplifiers so that they are grounded only to the
mixer. If the cable connecting the mixer to the amplifiers is a balanced (3-wire) type, the ground
can be floated at either end of this cable by disconnecting the wire connected to pin 1 of the XLR
adapter at one end or the other (but not both). Sometimes, because of the particular setup, you
will have to try several of these options before finding one that works.
CAUTION: Check local codes and regulations for rules pertain-
ing to electrical grounding. It may be illegal in some places to lift
the ground of a piece of equipment— especially if this piece of
equipment is installed publicly.
Another common cause of ground loops is direct input (or DI) boxes. These let you plug an
instrument such as a guitar or bass directly into a microphone input. The problem is that the
person playing the guitar or bass might be using an amplifier or some other signal processing
equipment on the stage that is plugged into a grounded outlet. This creates a ground loop be-
tween their setup and the grounded outlet that the mixing console is plugged into. Fortunately,
many DI boxes have a ground lift switch that you can use to break this ground loop. Because they
are so convenient, ground lifts on DI boxes are often the first option tried when a hum or buzz
surfaces.
If you suspect that there is a problem with your TDM 32CL series compressor/limiter that is
causing a hum or buzz, try removing the unit from the system and plugging its output directly into