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Appendix c – Aphex Systems 320A User Manual

Page 29

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Page 29

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 “pseudo-balanced” configuration may

help deal with ground loop hum. This method and

others are illustrated in Table 2.

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 problem 1.

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:

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 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 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

Appendix C