Appendix C
Appendix C: Proper Wiring Techniques
A true balanced line should be used wherever your equipment allows. Use “twisted pair” shielded cable. For unbalanced wiring you should use high grade, low capacitance shielded wire for best results. If you have an unbalanced output but have a balanced input, the
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 |
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there is also a grounding lug contact. This lug is con | - |
nected to the connector’s case (shell). In all Aphex |
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products audio ground and chassis ground are one |
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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
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
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 ne as is.
A word on optional shield connections: Connecting
the cable shield of a balanced line at both ends cre - ates unnecessary ground loops which may carry noise and hum currents that can be amplied. 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 conicting methods out and being able to change easily if nec - essary.
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
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