SoundCraft Spirit Monitor manual Balanced and Unbalanced

Balanced and Unbalanced

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 in- terference picked up on these wires is cancelled out. This is because, since both wires are in close proximity, the same in- terference 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 the source is unbalanced source, the signal should connect to +ve and the -ve pin of the input should be shorted to ground.

Note: many modern audio/musical instruments have elec- tronically balanced outputs which should not be unbalanced by shorting one wire to ground. Always use your inputs bal- anced where possible.

All of the outputs are ground compensated a technique which provides a very effective way of optimising noise immunity, without the cost and complexity of balanced outputs. These outputs employ ground compensation 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.

Polarity	You will probably be familiar with the concept of polarity in
	electrical signals and this is of particular importance to bal-
	anced audio signals. Just as a balanced signal is highly
	effective at cancelling out unwanted interference, so two mi-
	crophones 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 and Shielding

For optimum performance it is vital that all signals are refer- enced 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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