Mackie SRM450 V2 User Manual page 15

Noise
• Make sure all connections to the active speakers are
good.
• Make sure none of the signal cables are routed near
AC cables, power transformers, or other EMI-inducing
devices.
• Is there a light dimmer or other SCR-based device on
the same AC circuit as the SRM450 v? Use an AC line
filter or plug the SRM450 v into a different AC circuit.
Hum
• Turn the LEVEL control all the way down. If the noise
disappears, it's coming from the signal source. If not,
try disconnecting the cable connected to the INPUT
jack. If the noise disappears, it could be a "ground loop,"
rather than a problem with the SRM450 v. Try some of
the following troubleshooting ideas:
• Use balanced connections throughout your system for
the best noise rejection.
• Whenever possible, plug all the audio equipment's
linecords into outlets which share a common ground
(see the diagram on page 1). The distance between
the outlets and the common ground should be as short
as possible.
Never remove the ground pin on the
power cord of the SRM450 v or any
other component. This is very dangerous.
• The hum may appear when using an unbalanced
source (consumer preamp, CD player, VCR, etc.). This
is caused by the unbalanced-to-balanced interface
between the devices (and exacerbated by the fact
that most consumer audio equipment have a two-wire
linecord, without the third-pin safety ground).
Use an interconnect cable wired as shown below. The
important point is that the shield and the wire from the
XLR pin  are joined at the RCA (source) end.
RCA
3-Conductor Cable
XLR
2 1
3
shield
• Disconnect any cables which come in from outside,
such as cable TV, satellite TV or roof top antennas. They
must be disconnected from every part of your system,
such as the TV, VCR and preamp. If the hum goes away,
you can add a "ground loop isolator" in your cable line.
This is an inexpensive device available from video or TV
dealers, or you can make your own from two TV baluns
(standard TV 75/00 ohm adapters):
join (+insulate)
Balun
The baluns are threaded at one end (75 ohm) to fit TV
coax cable and have two wires at the other end (00
ohm). They will not affect the video quality.
• If the hum persists, try removing components one at
a time from the back of the mixer or preamplifier, and
check for hum each time (turn off your equipment
before you undo any connections). It is fairly common
to find more than one problem.
• If your preamp or mixer are the only things connected
to the SRM450 vs and the hum is still there, try differ-
ent connection cables, or move the preamp/mixer to
another location.
• Pressing the LOW CUT FILTER may help reduce the
hum if you have trouble finding the cause of the prob-
lem. Do this anyway if you do not need to reproduce the
lower frequency range.
Balun
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