In an ideal world, wiring up musical equipment would simply be a matter of 'plug and play', but the use of different grounding schemes, plus the need to mix balanced and unbalanced equipment means that ground loop hum problems are commonplace. Sound on Stage Editor PAUL WHITE explains the commonly used audio wiring systems.

Most musical performers or studio operators will have been plagued by stubborn hums or buzzes at one time or another, and although there is often no simple way to eliminate these completely, there are cabling protocols which, if followed, will minimise the problem. The quick and dirty means of attempting to cure a hum is to remove the mains earth from the offending piece of equipment, but this is the wrong thing to do. It can be extremely dangerous, and in some countries, it may actually be illegal. So even if it cures the hum, don't do it!

Hum can be traced back to a number of sources — faulty or badly designed equipment, transformers located too close to audio cables, or due to so-called ground loops. There is little you can do about faulty or badly designed equipment other than get it modified or replaced. However, the new CE standard for equipment sold in Europe should help reduce the amount of troublesome new equipment finding its way onto the market. Audio cables should be kept well away from equipment containing large transformers (such as power amplifiers or motors); lighting dimmers also have a propensity to cause interference, although this can nearly always be recognised by its 'buzzy', rather than hummy, character. Mic cables should also be kept away from mains wiring and shouldn't run parallel to it.

Straightforward hums can usually be blamed on ground loops, sometimes called earth loops, but what are they, and what wiring system should be used to avoid them? Each piece of equipment within a sound system (with the possible exception of some equipment that is unbalanced and powered from a 2-pin mains adaptor) should have a mains ground connecting the metal chassis to the mains supply earth. The screen pin of any audio connector should be bonded to the chassis ground at the point where the connector passes through the chassis. However, some balanced equipment is supplied with the XLR pin 1s connected to the signal ground rather than the chassis ground — or to both, which means that standard approaches to wiring may not be as standard as they should be. Similarly, unbalanced equipment may have the chassis ground and audio ground linked together directly, or they may be joined via a resistor in an attempt to provide a ground lift. If this sounds confusing, I mention it mainly to illustrate that despite having complete control over external wiring, you still can't be sure how the system will behave because of possible variations in grounding internal to individual pieces of equipment.

Ground loops occur when two or more pieces of equipment are linked in such a way that there are two or more different paths between them along which ground current may flow — for example, via the mains earth and via the signal screen. Any current flowing around such a closed ground loop will cause a small voltage to develop across the impedance of the ground wiring, and this voltage is effectively superimposed onto the audio signal. Because we use an AC mains powering system, ground currents are likely to have a strong 50Hz component (60Hz in the USA and some other parts of the world), which manifests itself as a deep, annoying hum.

To get around the problem of circulating ground currents, the balanced cabling system was developed. The idea behind this was that if equal amounts of signal contamination occur in the closely spaced 'hot' and 'cold' wires of a balanced cable, they will be completely cancelled out by the balancing circuitry or transformer in the receiving piece of equipment. What's more, in a properly balanced system, the signal circuit comprises the two inner cores of the cable only, leaving the screen to serve a purely protective role. Conversely, in an unbalanced system, the cable screen doubles as a signal return path, which is clearly less than ideal as any unwanted current flowing in the screen is added to the audio signal.

In theory then, using 3-pin audio cables (usually XLRs) to connect up a system comprising entirely balanced equipment will bring the level of ground loop induced hum to below nuisance level. I say 'in theory', because if the internal grounding system within individual pieces of equipment isn't properly implemented, balancing may not be fully effective. Figure 1a shows a standard balanced XLR link between two balanced pieces of equipment.

If the internal grounding of one or more pieces of equipment is incorrect, simply connecting a balanced XLR cable between the two may not result in a hum-free connection. In such cases, the usual solution is to disconnect the cable screen at one end of the lead. This prevents low frequency hum currents from flowing along the screen, but in the 'jam side down' way this universe seems to operate, it also increases the susceptibility of the link to high frequency or RF interference. This situation may be improved by wiring a small capacitor of between 10nF and 100nF inside the XLR connector, between pin 1 and the end of the screen you've just disconnected. This provides a high impedance path to hum frequencies, but ensures a low impedance path for high frequencies, thus preventing the screen cable acting as an aerial for RF interference. Figure 1b demonstrates this method.

UNBALANCED

Domestic hi-fi, and some semi-pro studio gear, uses an unbalanced wiring system. However, providing the cable lengths are kept as short as possible, the performance should be acceptable. Hum problems can and do arise in systems where balanced and unbalanced equipment are used together, and although there are various wiring schemes to help improve matters, the results will always be a compromise — balanced and unbalanced circuitry is fundamentally incompatible. This is obviously a sad state of affairs, because most mid-priced consoles, used both live and in the studio, feature balanced inputs and main outputs, yet invariably have unbalanced insert points. What's more, many budget signal processors are unbalanced, although those with 2-pin external power supplies tend to produce fewer ground loop type problems unless their casework is grounded by some other means, for example, by the rack metalwork. Unwanted metalwork grounding can usually be avoided by using plastic mounting bolts and plastic insulating washers.

Strictly speaking, balanced processors powered from a 2-pin mains adaptor should always have their chassis connected to the mains earth for correct operation. In practice, however, a more pragmatic approach usually has to be adopted — grounding the chassis may well exacerbate any hum problems.

COMPROMISE

Having stated that the only foolproof system is a fully balanced one in which all the various bits of hardware have proper internal grounding schemes, it is time to look at the real world. What do you do to minimise the problems caused by those unbalanced pieces of kit or by balanced boxes with non-standard grounding?

The best way to hook up a piece of unbalanced equipment to a balanced system is to isolate it using a transformer. Isolating transformers are available from most pro audio suppliers and are supplied as external boxes that connect with standard cabling. Figure 2 shows how a transformer isolation box works. Transformer DI boxes may also be used to serve the same purpose if high to low impedance matching is necessary at the same time.

Sadly, a good isolation transformer is not a cheap option, especially if you need lots of them, so you may decide to chance your luck with a standard balanced-to-unbalanced cable. These provide a means of getting signal from one piece of equipment to another, but they can't guarantee immunity from ground loop hum problems. Figure 3 shows the standard wiring schemes for linking balanced and unbalanced equipment. Note that different wiring systems are used depending on whether the balanced source has a conventional (non-floating), electronically balanced output stage or a floating balanced output stage (transformer or electronic). Non-floating output stages will deliver only half their signal level into an unbalanced load so a 6dB level loss is to be expected. A similar situation pertains when feeding some electronically balanced input stages from unbalanced sources. As a rule, the manual provided with the equipment will tell you which type of balanced-to-unbalanced wiring scheme to adopt.

If you're lucky, you'll get away with little or no audible hum, but occasionally, you'll come up against a stubborn hum that doesn't want to play ball. The best way forward is probably to buy an isolating transformer for just those pieces of gear that won't cooperate. In very desperate situations,however, I have used a special lead with a resistor and capacitor wired into the screen at the unbalanced end of the cable to increase the ground loop impedance at low frequencies. This costs very little, and though I can't guarantee it will work in all cases, it has proved successful on a number of occasions in the past. Figure 4 shows how to make up such a 'screen lift' cable, and the same system often works for cracking ground loop problems in unbalanced systems too.

Where several unbalanced audio cables are plugged into one device, such as a semi-pro multitrack recorder, it can sometimes help to disconnect one end of the cable screen on all the cables but one so as to reduce the number of simultaneous ground current paths.

SUMMARY

In an electronically hostile environment, such as in live sound where there are numerous possible interference sources, a fully balanced system is the only way to ensure a hum-free existence, and even then, you need to know that all your equipment has a properly designed internal grounding system. In some cases, you can get a non-standard unit modified so as to conform to currently recommended grounding practice, and if this is possible, it's probably worth doing. In a practical, small PA rig, the only unbalanced links that you may not be able to do away with are the processors connected to a console's unbalanced insert points. Usually these will work OK, but if you have persistent problems, buying transformer isolators for both inputs and outputs may be an idea.

Unbalanced systems are not recommended for serious live work of any kind — even budget mixing consoles and power amps tend to be balanced these days, so there's really no reason not to take the balanced approach. Providing you follow the guidelines set out here, and use ground-lifted DI boxes or isolation transformers when feeding unbalanced sources into your PA mixer, you should avoid the worst of the hum problems. Good luck!