Aphex Systems 320A owner manual Appendix B

320A Appendix B

Compellor

is cancelled out by the di￿erential ampli￿er. 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 signi￿cant voltage di￿erence between the wires, hence the noise (or hum) will not be picked up by the di￿erential input stage.

It can be seen that the signal generator driving the twisted pair will cause a di￿erence between the wires, and that signal will be readily picked up by the di￿erential 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 glori - ous. 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 ￿nd 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 di￿erently. 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￿es a di￿erence signal, but this time it is the di￿erence between two non-symmetrical conduc - tors. 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 “grounding di￿erence” that can cause current to ￿ow through the shield conductor. Grounding di￿erence is a serious problem in studios, because often the equipment grounds are connected to power outlet grounds, and there can be a sig - ni￿cant di￿erence of ac voltage between alternating wall outlet grounds. For this reason, unbalanced sys -

tems 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 o￿ the same distri - bution 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 ￿ow from the ac power system. It is sometimes very di￿cult 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 320D 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 stu￿! 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 di￿erences that exist between di￿erent wall outlets that return to opposite phases at the power distribution panel. Sec - ondarily, however, many pieces of equipment contain line ￿lters 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 time. That illustrated the ground loop e￿ect - - straight through you! No matter what you do, you may not be able to prevent some of your equipment from generating ground currents. The most likely culprits are digital products because they use switch - ing power supplies that require heavy line ￿lters to prevent conducted EMI from going out of the box.

Filters so employed very often take the ground leak - age current right up to the UL safety limits. Although it won’t kill you, that is a lot of ground loop current for audio cables to handle.

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