Aphex 1788A Premium Eight Chan Mic Pre - Manual (Page 40)

Page 40
MICROPHONE PREAMPLIFIER
Page 41
Instruction Manual
APPENDIX D
CONNECTOR WIRING STANDARDS
The 3 pin XLR, 1/4” (63.5 mm) TS mono phone and
the 1/4” (63.5 mm) TRS stereo phone are the most
commonly used line level connectors in pro audio.
Less common is the use of the “RCA” phono jack,
which is essentially a consumer type connector. The
XLR and the TRS are three conductor and are used
for balanced connections. The TS and the RCA are
two conductor and are used for unbalanced connec-
tions.
In addition to the three main contacts on an XLR
there is also a grounding lug contact. This lug is con-
nected to the connector’s case (shell). In all Aphex
products audio ground and chassis ground are one
and the same. Aphex products that use XLR connec-
tors tie Pin I to the XLR case automatically. Therefore
it is not necessary to use the XLR case-ground lug.
This also makes possible the use of XLR ground drop
adapters (see Note 3).
TABLE 1: The wiring convention shown is now
standardized in 17 countries including the USA.
Please note that any equipment that still uses Pin 3
as positive on XLR connectors is not adhering to the
standard.
THE PIN 1 DILEMMA AND HOW IT AFFECTS
CABLE SHIELD CONNECTIONS
The three main contacts on an XLR (or TRS) and
the accepted wiring assignments shown above are
only part of the picture. The standard for terminat-
ing ground is Pin 1 (Sleeve). But which ground? It
could be connected to audio signal ground or chassis
ground depending on the method of grounding used
by the equipment manufacturer. In all Aphex prod-
ucts audio ground and chassis ground are one and
the same at all I/O jacks. This is just good, common
sense engineering practice (which is what you would
expect from us, course). Unfortunately, many prod-
ucts are designed so that the noisy currents from
the shield drain into signal ground instead of chas-
sis ground. This practice creates a real hum and
noise problem for end-users. The appropriate overall
grounding scheme of an audio system would be a lot
easier to predict without this problem1.
The standard balanced line wiring recommenda-
tion from Aphex Engineering is this: In the majority
of cases maximum noise rejection occurs when the
shield is connected to the input ground only (espe-
cially in locations with high levels of RFI). That means
the sending end shield should be left disconnected.
However, if you already have cables with the
shield connected at both ends, go ahead and
try them out. If you are connecting a fairly
simple audio system it may be fine as is.
A word on optional shield connections: Con-
necting the cable shield of a balanced line at both
ends creates unnecessary ground loops which may
carry noise and hum currents that can be ampli-
fied. Connecting the shield only at the sending
end (instead of the receiving end) may exaggerate
common mode noises at the receiving input stage. It
can actually increase RFI and noise more than having
no shield at all. Because of the “Pin I Dilemma” (men-
tioned above) you may be forced, in some situations,
to experiment with how the cable shield is connected
to ground to eliminate a pesky hum or radio interfer-
ence problem. It might be good to try XLR ground
drop adapters (see Note 3) as a method of trying
these conflicting methods out and being able to
change easily if necessary.
IMPEDANCE
Regardless of inaccuracies, it has become more or
less standard over the years to refer to balanced
lines as low impedance and unbalanced lines as high
impedance. The fact is, however, that both balanced
and unbalanced lines are operated at low imped-
ance in modern practice owing to the fact that all
output stages have become low impedance. A few
exceptions might be outputs from passive mixers,
instrument pickups, electric guitars and some key-
board synthesizers. It is generally ideal to drive any
audio line from a low impedance and receive into a
high impedance. Generally, a minimum 1: 10 ratio is
possible. This is called “bridging”. This has become
modern practice and all balanced inputs are normally
running 10K ohms or higher impedance. Because of
these developments, it is now no longer as critical to
consider impedance when dealing with interfacing
pro line level equipment (impedance “matching” is
mostly a requirement of the past).