Rane MP26, Руководство оператора

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Interconnection-2
Ground Loops
Almost all cases of noise can be traced directly to
ground loops, grounding or lack thereof. It is important
to understand the mechanism that causes grounding
noise in order to effectively eliminate it. Each compo-
nent of a sound system produces its own ground in-
ternally. This ground is usually called the audio signal
ground. Connecting devices together with the inter-
connecting cables can tie the signal grounds of the two
units together in one place through the conductors in
the cable. Ground loops occur when the grounds of the
two units are also tied together in another place: via
the third wire in the line cord, by tying the metal chas-
sis together through the rack rails, etc. These situations
create a circuit through which current may flow in a
closed “loop” from one unit’s ground out to a second
unit and back to the first. It is not simply the presence
of this current that creates the hum—it is when this
current flows through a unit’s audio signal ground that
creates the hum. In fact, even without a ground loop, a
little noise current always flows through every inter-
connecting cable (i.e., it is impossible to eliminate these
currents entirely). The mere presence of this ground
loop current is no cause for alarm if your system uses
properly implemented and completely balanced inter-
connects, which are excellent at rejecting ground loop
and other noise currents. Balanced interconnect was
developed to be immune to these noise currents, which
can never be entirely eliminated. What makes a ground
loop current annoying is when the audio signal is af-
fected. Unfortunately, many manufacturers of balanced
audio equipment design the internal grounding system
improperly, thus creating balanced equipment that is
not immune to the cabling’s noise currents. This is one
reason for the bad reputation sometimes given to bal-
anced interconnect.
A second reason for balanced interconnect’s bad
reputation comes from those who think connecting
unbalanced equipment into “superior” balanced equip-
ment should improve things. Sorry. Balanced inter-
connect is not compatible with unbalanced. The small
physical nature and short cable runs of completely
unbalanced systems (home audio) also contain these
ground loop noise currents. However, the currents in
unbalanced systems never get large enough to affect
the audio to the point where it is a nuisance. Mixing
balanced and unbalanced equipment, however, is an
entirely different story, since balanced and unbalanced
interconnect are truly not compatible . The rest of this
note shows several recommended implementations for
all of these interconnection schemes.
The potential or voltage which pushes these noise
currents through the circuit is developed between the
independent grounds of the two or more units in the
system. The impedance of this circuit is low, and even
though the voltage is low, the current is high, thanks to
Mr. Ohm, without whose help we wouldn’t have these
problems. It would take a very high resolution ohm
meter to measure the impedance of the steel chassis or
the rack rails. We’re talking thousandths of an ohm. So
trying to measure this stuff won’t necessarily help you.
We just thought we’d warn you.
Figure 1a. The right way to do it.
+
–
G
T
R
S
RED
BLACK 2-CONDUCTOR SHIELDED CABLE
2-CONDUCTOR SHIELDED CABLE
2-CONDUCTOR SHIELDED CABLE
SHIELD
RED
BLACK
SHIELD
RED
BLACK
SHIELD
RED
BLACK
SHIELD
RED
BLACK
SHIELD
RED
BLACK
SHIELD
CHASSIS
GROUND
CHASSIS
GROUND
SIGNAL
GROUND
S
R
T
G
–
+
BALANCED OUTPUTS BALANCED INPUTS
MALE FEMALE MALE
FEMALE
1
2
3 C
1
3
2 2
C 3
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2
1
3