Aphex 228 - Manual (Page 13)

Aphex Systems Ltd. Model 228 Page 12
228
instruction Manual
Page 13 Aphex Systems Ltd. Model 228
228
8-channel audio interface
Balanced transmission works something like this. 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 practi-
cal purposes as seen measuring from ground) is called a common mode signal and is cancelled out by the dif-
ferential 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 differential input stage. One of the beauties of the balanced line is
that it is completely independent from ground. Nothing is connected to ground at all, nor does it care about
ground. Nevertheless, most professional 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 glorious. Simply, nothing is perfect,
not even balanced cable. Under some circumstances the shield can overcome 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
virtually 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 ampli-
fies a difference signal, but this time it is the difference between two non-symmetrical conductors. To make
things even less symmetrical, the outer conductor 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 equal, i.e., there is no “ground-
ing difference” that can cause current to flow through the shield conductor. 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 alternating 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 equip-
ment is powered together off the same distribution panel circuit (same circuit breaker).
Appendices
Figure 1 Balanced Line Model Figure 2 Unbalanced Line Model