Aphex 228 - Manual (Page 16)

Aphex Systems Ltd. Model 228 Page 14
228
instruction Manual
Page 15 Aphex Systems Ltd. Model 228
228
8-channel audio interface
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 terminating 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 products audio ground and chassis ground are one and the same. This is just good,
common sense engineering practice (which is what you would expect from us, course). Unfortunately, many
products are designed so that the noisy currents from the shield drain into signal ground instead of chassis
ground. This practice creates a real problem for end-users. The appropriate overall grounding scheme of an
audio system would be a lot easier to predict without this problem
1 .
The standard balanced line wiring recommendation from Aphex Engineering is this: In the majority of cases
maximum noise rejection occurs when the shield is connected to the input ground only (especially in loca-
tions 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 con-
necting a fairly simple audio system it may be fine as is.
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 impedance 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 keyboard synthesizers. It is generally ideal to drive any audio line from a low impedance and receive
into a high impedance (generally at a minimum of a 1: 10 ratio - this is called a “bridging” impedance). This
has in fact become modern practice and all balanced inputs are normally running 10K ohms or higher imped-
TABLE 1 - BALANCED & UNBALANCED CONNECTOR WIRING STANDARDS
3-Pin XLR 1/4” TRS Phone Standard Wiring Convention (Balanced)
Pin-1 Sleeve Ground/Shield (Earth, Screen)
Pin-2 Tip Positive (Signal, High, Hot)
Pin-3 Ring Negative (Signal Reference, Return, Low, Common)
1/4” TS Phone RCA Standard Wiring Convention (Unbalanced)
Tip Center Pin Positive (Signal)
Sleeve Shell Ground/Shield (Signal Reference/Return)
A word on optional shield connections: Connecting the cable shield of a balanced line at both ends creates
unnecessary ground loops which may carry noise and hum currents that can be amplified. 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” (mentioned 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 interference 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.
Appendices