13
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 frequency 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 Standards Committee (SC-05-05)
working on this subject. He tirelessly tours the world giving seminars and dis-
pensing information on how to successfully hook-up pro audio equipment
2
.
He makes the simple point that it is absurd that you cannot go out and buy
pro audio equipment from several different manufacturers, buy standard off-
the-shelf cable assemblies, come home, hook it all up and have it work hum
and noise free.
Plug and play.
Sadly, almost never is this the case, despite the
science and rules of noise-free interconnect known and documented for over
60 years
(see References for complete information).
It all boils down to using balanced lines, only balanced lines, and nothing
but balanced lines. This is why they were developed. Further, that
you
tie the
shield to the chassis, at the point it enters the chassis, and at both ends of the cable
(more on ‘both ends’ later).
Since standard XLR cables come with their shields tied to pin 1 at each
end (the shells are not tied, nor need be), this means equipment using 3-pin,
XLR-type connectors
must tie pin 1 to the chassis
(usually called chassis
ground) — not the audio signal ground as is most common.
Not using
signal ground
is the most radical departure from common pro-
audio practice. Not that there is any argument about its validity. There isn’t.
This is the right way to do it
. So why doesn’t audio equipment come wired
this way? Well, some does, and since 1993, more of it does. That’s when Rane
started manufacturing some of its products with balanced inputs and outputs
tying pin 1 to chassis. So why doesn’t everyone do it this way? Because life is
messy, some things are hard to change, and there will always be equipment in
use that was made before proper grounding practices were in effect.
Unbalanced equipment is another problem: it is everwhere, easily available
and inexpensive. All those RCA and ¼" TS connectors found on consumer
equipment; effect-loops and insert-points on consoles; signal processing boxes;
digital and analog tape recorders; computer cards; mixing consoles; et cetera.
The next several pages give tips on how to successfully address hooking
up unbalanced equipment. Unbalanced equipment when “blindly” connected
with fully balanced units starts a pattern of hum and undesirable operation,
requiring extra measures to correct the situation.
