Aphex Channel Instruction Manual download pdf (Page 19)

230

instruction Manual

Aphex Systems Ltd. Model 230Page 18

230

master voice channel

Page 19Aphex Systems Ltd. Model 230

Your balanced input stage looks at the two wires

and detects only the potential (voltage) difference

between them. Anything that is the same on the two

wires (for all practical purposes as seen measuring

from ground) is called a common mode signal and

is cancelled out by the differential amplifier. Figure

1 illustrates how the hum is induced into both wires

equally and therefore is cancelled out.

Since the balanced line has wires that are twisted

together, each wire tends to pick up the same amount

of induction from external sources. Induction will

create no significant voltage difference between the

wires, hence the noise (or hum) will not be picked up

by the differential input stage.

It can be seen that the signal generator driving the

twisted pair will cause a difference between the wires,

and that signal will be readily picked up by the dif-

ferential input stage. One of the beauties of the bal-

anced line is that it is completely independent from

ground. Nothing is connected to ground at all, nor

does it care about ground. Nevertheless, most pro-

fessional cable has an overall shield wrap that is

intended to be connected somehow to ground. You

may well ask why, and the answer is less than glo-

rious. Simply, nothing is perfect, not even balanced

cable. Under some circumstances the shield can over-

come extreme interference problems that can’t be

adequately rejected by the twisted pair alone. Things

like 2-way radios, television transmitters, and light

dimmers can induce very heavy interference that may

be reduced by shielding. You are going to find virtu-

ally all balanced cables include a shield so you need

to deal with it, even if it is not actually needed. That

subject will be addressed a little later.

Unbalancing

Unbalanced wiring works a little differently. Figure 2

shows the basic plan. In this case, the wires are not

twisted, they are coaxial. The unbalanced input stage

is somewhat like the balanced input stage because

amplifies a difference signal, but this time it is the dif-

ference between two non-symmetrical conductors.

To make things even less symmetrical, the outer con-

ductor is connected to ground at both ends. The

principle is that the outer shield conductor shields

the inner conductor from induced noises. This can

only work well if the cable is relatively short and

the ground at each end of the cable is somewhat

Appendices

equal, i.e., there is no “grounding difference” that

can cause current to flow through the shield con-

ductor. Grounding difference is a serious problem in

studios, because often the equipment grounds are

connected to power outlet grounds, and there can be

a significant difference of ac voltage between alter-

nating wall outlet grounds. For this reason, unbal-

anced systems can sometimes never be made hum

free, and just changing one piece of equipment in

a studio can cause hum to appear somewhere else.

When you are using unbalanced gear, it is a very

good procedure to power all your equipment from

one large power isolation transformer. At the very

least, make sure all equipment is powered together

off the same distribution panel circuit (same circuit

breaker).

Appendix B: Dealing With Grounds and

Hum

Ground Loops

Many people equate this term with hum, and that’s

just about the bottom line of it. If you have a ground

sensitive system, like unbalanced audio equipment

for example, then hum will result from ground

currents that flow from the ac power system. It is

sometimes very difficult to isolate and stop ground

currents between unbalanced equipment, but it is

quite easy to clean up balanced gear. That’s why pro

gear is always balanced! The cost of balancing is that

of more expensive connectors, cable, and electronics

but the cost is worth it when you depend on your

audio quality. That’s why the Model 230 is equipped

with a fully balanced I/O. Now that we’ve sold you

on only using really expensive pro gear, lets show you

how to get away with the really cheap stuff! At least

from the standpoint of killing ground hum.

A ground loop is an ac current that has become

routed through your audio ground system. The cur-

rent comes mainly from ground potential differences

that exist between different wall outlets that return

to opposite phases at the power distribution panel.

Secondarily, however, many pieces of equipment con-

tain line filters and transformers that leak a small

amount of ac power into the chassis and ground

return.

You may once have had the experience of getting

zapped by touching two pieces of gear at the same

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