Installation - Yamaha MC1204 Operating Manual

INSTALLATION
AC Power Connection (for models with 3·conductor
power cable)
Mixers provided with a 3-wire power cable should be
PC grounded for safety and optimum shielding against
noise.
lit
a 3-wire PC outlet is not available, or there is
any chance the PC outlet may not be grounded, a
separate ground wire must be connected from the mix ­
er chassis to an earth ground . Cold water pipes
generally provide good grounds unless they are in ­
sulated by a length of PVC (plastic) pipe or a water
meter. Avoid hot water pipes and gas pipes. When a
convenient, confirmed ground is not available, use a
length of copper pipe driven into moist, salted earth to
a depth of at least 1.5 meters (5 feet). Alternately, use
a chemical type grounding rod.
Hook·Up Cables and Hum Avoidance
The mixer's primary inputs and outputs feature elec ­
tronically balanced circuits and connectors. When these
connectors are used with the appropriate 2-conductor
shielded cables (e .g. standard microphone cables),
these circuits provide optimum protection against hum
and noise pickup. The XLR type connectors are wired
with pin 2 as "audio high" and pin 3 as "audio low"
in accordance with DIN and JIS standards . In the bal ­
anced TRS connectors, the tip is audio high and the
ring is audio low. Pin 1 in the XLR type connectors, and
the sleeve in the TRS connectors are ground. Some
professional audio equipment and microphones are
wired with pins 2 and 3 (XLR) reversed. Generally, this
will cause no problem other than a polarity reversal.
However, if such a piece of equipment uses a balanced ­
type connector for an unbalanced input, or an adaptor
is used to match an unbalanced connector to a bal ­
anced input, the high side of the audio circuit could be
grounded. In this case, reverse the audio high and
audio low wiring at one end of the connecting cable,
or use a suitable polarity-reversal adaptor. Regardless
of connector polarity, if hum is encountered try cutting
the shield connection at one end of the cable.
All, unbalanced phone jacks are intended for use with
standard tip-sleeve 1/4" phone plugs and single­
conductor shielded cable. Do not attempt to reduce
hum by cutting the shield connection on these cables .
Rather, restrict unbalanced cables to about 10 feet (3
meters), and try to set up the system so that either (a)
the equipment involved is all connected to the same PC
circuit, or
(b)
the third-wire PC mains ground is used on
only one piece of equipment, typically the mixer. Break·
ing the ground path can create a SHOCK HAZARD.
When routing cables, especially unbalanced cables ,
avoid strong sources of electro-magnetic interference
or radio frequency interference generated by electric
motors, fluorescent lights, dimmer panels, etc . To avoid
crosstalk-induced feedback, never bundle microphone
input cables with mixer output cables: these cables
should cross at right angles where practical.
Grounding
Careful grounding procedures are essential for proper
operation, not only of the mixer, but of the entire audio
system. Many grounding techniques exist, and a
number of books have been written on the subject. The
following are good sources of grounding information:
THE AUDIO CYCLOPEDIA by Howard M. Tremaine
(Pub. Howard W. Sams)
SOUND SYSTEM ENGINEERING by Don and Carolyn
Davis (Pub. Howard
W.
Sams)
GROUNDING AND SHIELDING IN INSTRUMENTA­
TION by Ralph Morrison (Pub. John Wiley & Sons)
"Ground loops" are often caused by multiple paths
from the equipment grounds to the PC mains ground
(or earth ground). Ground loops are a major cause of
hum and noise in an audio system. In severe cases,
ground loops can even cause the equipment involved
to break into oscillation. This can cause distortion and
even damage to amplifiers and speakers. One way to
avoid ground loops is to make sure that there is only
one path to the PC ground for the entire audio system.
A popular method is to cut the shield ground of balanc·
ed cables at the input end of the cable. Another techni ­
que is to ground all shields at one piece of equipment,
typically the console, and cut the shields at the other
ends of the cables. (This is NOT possible with un ­
balanced cables).
Check Mains Voltages
Connect the mixer to the PC mains only after confirm­
ing that the line voltage and frequency are correct. A
simple check with a voltmeter can save your equipment
-and the show. It is also a good idea to check for pro·
per polarity at the PC outlet. The power switch on the
mixer should be OFF before connecting the the mixer
to the mains.
As
a further precaution, disconnect the
mixer from the mains while audio cables are being
installed.
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