Ba la nced 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.
Pa g e 7
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