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Ba la n c e d a n d
Unba la nc e d
All channel inputs are balanced, i.e. there are separate
+ve(hot) and -ve (cold) wires for each signal plus a ground.
The design of the differential input amplifiers is such that
interference picked up on these wires is cancelled out. This is
because, since both wires are in close proximity, the same
interference will be picked up on each wire and balanced
input amplifiers will only amplify the difference between
+ve(hot) and -ve(cold). Any signal on both hot and cold (i.e.
noise) will not be amplified - this is known as common mode
rejection (CMR). Balanced inputs should always have both
+ve and -ve connected or if only an unbalanced source, the
-ve pin shorted to ground.
Note: many modern audio/ musical instruments have
electronically balanced outputs which should not be
unbalanced by shorting one wire to ground. Always use your
inputs balanced where possible.
The Mix L/ R and Mono outputs, Group and Auxiliary
outputs are ground compensated and provide a very effective
way of optimising noise immunity, without the cost and
complexity of balanced outputs. These outputs employ
ground compensation techniques to cancel out the effects of
variation in ground potential between the mixer and other
equipment which would otherwise show up as hum. 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 output of your SPIRIT LIVE 4 console.
Pola rity
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
microphones picking up the same signal can cancel out, or
cause serious degradation of the signal if one of the cables has
the +ve and -ve wires reversed. This phase reversal can be a
real problem when microphones are close together and you
should therefore take care always to connect pins correctly
when wiring audio cables.
Grounding a nd Shielding
For optimum performance it is vital that all signals are
referenced to a solid, noise-free earthing point and that all
signal cables have their screens connected to ground. To
avoid earth ‘loops’, use balanced connections where possible
and ensure that all cable screens and other signal earths are
connected to ground only at their source and not at both ends.
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