SoundCraft Spirit Live 4 manual Ba lanced a nd Unba la nced, Pola rity

Ba lanced a nd Unba la nced

All channel inp u ts are balanced , i.e. there are sep arate +ve(hot) and -ve (cold ) w ires for each signal p lu s a grou nd . The d esign of the d ifferential inp u t am p lifiers is su ch that interference p icked u p on these w ires is cancelled ou t. This is becau se, since both w ires are in close p roxim ity, the same interference w ill be p icked u p on each w ire and balanced inp u t am p lifiers w ill only am p lify the difference betw een +ve(hot) and -ve(cold ). Any signal on both hot and cold (i.e. noise) w ill not be am p lified - this is know n as com m on m od e rejection (CMR). Balanced inp u ts shou ld alw ays have both +ve and -ve connected or if only an u nbalanced sou rce, the -ve p in shorted to grou nd .

N ote: m any m od ern au d io/ m u sical instru m ents have electronically balanced ou tp u ts w hich shou ld not be u nbalanced by shorting one w ire to grou nd . Alw ays u se you r inp u ts balanced w here p ossible.

The Mix L/ R and Mono ou tp u ts, Grou p and Au xiliary ou tp u ts are ground compensated and p rovid e a very effective w ay of op tim ising noise im m u nity, w ithou t the cost and com p lexity of balanced ou tp u ts. These ou tp u ts em p loy grou nd com p ensation techniqu es to cancel ou t the effects of variation in grou nd p otential betw een the m ixer and other equ ip m ent w hich w ou ld otherw ise show u p as hu m . If the ou tp u t is d riving a d evice or am p lifier that has an u nbalanced inp u t, connect the -ve(cold ) signal to the grou nd at the d estination, not at the ou tp u t of you r SPIRIT LIVE 4 console.

Pola rity	You w ill p robably be fam iliar w ith the concep t of p olarity in
	electrical signals and this is of		p articu lar im p ortance to
	balanced au d io signals.	Ju st as a	balanced signal is highly
	effective at cancelling	ou t u nw anted interference, so tw o
	m icrop hones p icking u p the sam e signal can cancel ou t, or
	cau se seriou s d egrad ation of the signal if one of the cables has
	the +ve and -ve w ires reversed . This phase reversal can be a
	real p roblem w hen m icrop hones are close together and you
	shou ld therefore take care alw ays		to connect p ins correctly
	w hen w iring au d io cables.

Grounding a nd Shielding For op tim u m p erform ance it is vital that all signals are referenced to a solid , noise-free earthing p oint and that all signal cables have their screens connected to grou nd . To avoid earth ‘loop s’, u se balanced connections w here p ossible and ensu re that all cable screens and other signal earths are connected to grou nd only at their sou rce and not at both end s.

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