Ba la n c ed a n d Unba l a nc e d
All chan nel inp uts ar e balance d, i.e. ther e are sep arate
+ve(hot) and -ve (cold) wires for each signal plus a ground.
The de sign of th e differen tial inp ut am plifiers is such th at
interference picked up on these wires is cancelled out. This is
because, since both wires are in close pr oximity, the same
interference will be picked up on each wire and balanced
inpu t amplifiers w ill only amp lify the difference between
+ve(hot) and -ve(cold). Any signal on both hot and cold (i.e.
noise) w ill n ot b e a m p lified - th is is k n o w n a s co m m on m od e
rejection (CM R). Balanced in pu ts sho uld alwa ys hav e both
+ve and -ve connected or if only an unbalanced source, the
-ve pin shorted to groun d.
Note: many modern audio/musical instruments have
electron ically bala nced ou tpu ts wh ich shou ld no t be
unbalanced by shorting one w ire to ground. Always use you r
inputs balan ced where p ossible.
The Mix L/R an d Mono outpu ts, Group an d Auxiliary
outputs ar e ground compensated and provide a very effective
way of optimising noise immun ity, without the cost and
complexity of balanced outputs. These outputs employ
ground comp ensation techniques to cancel out the effects of
variation in ground potential between the mixer and other
equipmen t which w ould otherw ise show u p as hu m. If the
output is driving a device or amplifier that has an unbalanced
input, connect the -ve(cold) signal to the ground at the
destination, not at the outpu t of your SPIRIT LIVE 4 conso le.
Pola r ity
You will probably be familiar with the concept of polarity in
electrical signals and this is of particular importance to
balanced audio signals. Just as a balanced signal is highly
effective at cancelling out unwanted interference, so two
micro pho nes p ickin g u p th e sam e sign al can can cel ou t, or
cause serious degradation of the signal if one of the cables has
the +ve and -ve w ires rev ersed . This phase reversal can be a
real problem when microphones are close together and you
should ther efore take care always to connect pins correctly
when wiring audio cables.
Groun ding a nd Shie lding
For optimum p erformance it is vital that all signals are
referenced to a solid, noise-free earthing point and that all
signal cables have their screens con nected to ground . To
avoid earth ‘loops’, use balanced conn ections wh ere p ossible
and ensure that all cable screens and other signal earths are
connected to ground on ly at their source and not at both ends.
Pag e 7