Interconnection-2
Ground Loops
Almost all cases of noise can be traced directly to ground
loops, grounding or lack thereof. It is important to under-
stand the mechanism that causes grounding noise in order
to effectively eliminate it. Each component of a sound
system produces its own ground internally. This ground is
usually called the audio
signal
ground. Connecting devices
together with the interconnecting 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
chassis 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 interconnecting 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 an-
noying is when the audio signal is affected. Unfortunately,
many manufacturers of balanced audio equipment design
the internal grounding system improperly, thus creating bal-
anced equipment that is not immune to the cabling’s noise
currents. This is one reason for the bad reputation some-
times given to balanced interconnect.
A second reason for balanced interconnect’s bad reputa-
tion comes from those who think connecting unbalanced
equipment into “superior” balanced equipment should
improve things. Sorry. Balanced interconnect is not compat-
ible 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 interconnec-
tion schemes.
The potential or voltage which pushes these noise cur-
rents through the circuit is developed between the indepen-
dent 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 thou-
sandths of an ohm. So trying to measure this stuff won’t
necessarily help you. We just thought we’d warn you.
The Absolute Best Right Way To Do It
The method specified by AES48 is to use balanced lines and
tie the cable shield to the metal chassis (right where it enters
the chassis) at both ends of the cable
.
A balanced line requires three separate conductors, two
of which are signal (+ and –) and one shield (see Figure 1a).
The shield serves to guard the sensitive audio lines from
interference. Only by using balanced line interconnects can
you
guarantee
(yes,
guarantee
) hum-free results. Always use
twisted pair cable. Chassis tying the shield at each end also
guarantees
the best possible protection from RFI [radio fre-
quency interference] and other noises [neon signs, lighting
dimmers].
Neil Muncy
1
, an electroacoustic consultant and seasoned
veteran of years of successful system design, chairs the AES
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
1
2
1
3
Содержание SM 26S
Страница 4: ......
