Poor studio wiring can lead to hums and buzzes, but you can avoid the worst of these problems by following a few simple rules.
Modern studio equipment uses surprisingly little power, but the sheer number of wall sockets it takes up seems to defy anyone's best efforts to provide enough, no matter how thorough their initial planning. Which is why few SOS readers have the will‑power to keep on walking when Woolworths have a special £2.99 offer on plugboards!
If you have the luxury of planning your studio from scratch, one of the first areas to think about after general construction and acoustic design is the electricity supply. In a bedroom studio with relatively little equipment you'll probably manage by plugging into the nearest wall socket, but there's always a risk of picking up electrical interference from elsewhere in the house, or even from your neighbours. If possible, having a separate ring main for your studio is often better, and you should have as many permanent wall sockets fitted as possible. Even a modest studio installation may include literally dozens of mains plugs and wall‑warts, all vying for the available sockets.
All modern houses in the UK employ a form of wiring known as a ring main, where the mains wiring forms a complete loop that starts and finishes at the fuse box — as shown in Figure 1. Wall sockets are then positioned around the loop up to the maximum rating of the cable. If you're planning a new mains wiring system, you should install a separate 'consumer unit' feeding two separate ring mains for your studio area — one for sensitive audio gear and computers, and the other for coffee machines, air conditioning, and so forth. For those not familiar with the intricacies of consumer units, they are the modern equivalent of fuse boxes, but instead of fuses they use small plug‑in circuit breakers that trip out when too much current is drawn.
You should also specify an RCB (Residual Current Breaker), which will cut off the supply when there's an imbalance between the live and neutral currents — in other words, when some of the current is escaping from the circuit due to a fault condition. RCBs are more sensitive than Earth Leakage Trips, which are their older counterparts, so are more likely to prevent injury in a potential (no pun intended) electric‑shock situation. In addition, they are reset with a simple switch, which is far easier than mending fuses. However, be warned that they can trip if a light‑bulb fails, so it's best not to have table lamps or similar fittings plugged into your equipment power supply.
If you have the luxury of planning your studio from scratch, one of the first areas to think about after general construction and acoustic design is the electricity supply.
One of the regular questions put to us at SOS is whether plugging multiple extension boards into the mains is safe, in terms of the amount of power used. Well, in a typical home studio, the lights often take more power than the equipment. A single 13A socket can safely run a three‑bar electric fire, and you need a lot of gear to draw that much power. In fact, it's not usually electrical loading that causes the problem, but the quality of the electrical contacts.
If you look inside a typical 4‑ or 6‑way plastic plugboard, you'll discover that the sockets comprise pieces of folded metal crimped onto rigid wire buss bars. If it's a really good plugboard it may have spot welds too, but don't count on it! When you continually use these sockets, plugging and unplugging gear, the little spring contacts become loose and the result is an intermittent contact that can cause sparking. Once sparking starts inside a mains plug it produces a burst of loud buzzing and spitting, audible over the monitors — and Sod's Law has it that as soon as you disturb anything, while trying to track down the problem, the buzz will stop until you're safely back in your chair.
If you must use plugboards (and you probably will), there are a couple of basic rules to observe:
- Don't cascade plug boards: feed them from the wall sockets, not from each other.
- Don't keep pulling plugs in and out: only use plugboards for semi‑permanent connections.
A spray of Deoxit contact cleaner on the contacts can help ensure a good connection, and if you do start to encounter problems you can usually mitigate them by dismantling the plugboard and squeezing the contacts together slightly with a pair of pliers. You may also find it helpful to solder the crimped joints, but you'll need a fairly hefty soldering iron to do this properly. While you're in there, snip out the power neon and throw it away, as these can generate interference. Whatever you do, make sure the plugboard is unplugged before dismantling it — otherwise you'll find out how effective your RCB trip really is!
A ring main dedicated to the studio provides a useful degree of interference rejection. Even so, if you still experience clicks and buzzes when the fridge or central heating system switch on and off, have an electrician check them over and fit a suppressor kit if necessary. (Also see the 'Filter Tips' box for details on mains filtering.)
Interference should also be reduced by low‑impedance wiring. If you're having the wiring installed from scratch, it's probably worth paying extra for the heaviest grade of cable the sockets can accommodate. This helps minimise the effect of the mains supply to one piece of gear being modulated by the mains current drawn by others — especially heavy‑duty items such as large power amplifiers.
Having a low‑impedance ground path is particularly important in keeping interference under control, so get an electrician to check your ground spike, and fit a better quality one if necessary. Ground spikes generally work best if the ground is slightly damp, so having it just outside the house may be more effective than having it in the basement. At least then you can water it in dry weather!
One of the regular questions put to us at SOS is whether plugging multiple extension boards into the mains is safe, in terms of the amount of power used.
Hum can often be traced to magnetic fields radiated by transformers and mains wiring — though once you start to connect equipment, hum caused by ground loops can also become a problem. To minimise these risks, all mains cables should be run together as far as is possible. It helps if they can be run around the top of the room, with drops to the various sockets, as this keeps them away from your vulnerable signal wiring.
Further benefit may be obtained from running the cable in grounded metal conduit rather than the more common plastic conduit. Always keep your audio cables away from mains cables and wall‑wart transformers, and where mains and signal cables must cross, arranging them to cross at right angles will minimise any interference problems.
To reduce the risk of ground‑loop hum in a small system, try to feed all your audio equipment from a single pair of mains sockets, each of which has a single plugboard (ideally 6‑way) plugged into it. If you need more outlets, use commercial‑grade, metal‑cased, switched mains sockets screwed to a suitably solid piece of wood and wired to a heavy‑duty junction box using 2.5mm twin‑and‑earth house wiring cable. The junction box then terminates in a trailing mains lead made from the heaviest flexible cable that you can fit into a 13A plug, as shown in Figure 2 (below). By mounting the distribution boards in the back of your equipment racks you can keep the amount of exposed cabling to a minimum.
If you're still concerned about the power consumed by your equipment, you can normally find the power requirements (expressed in Watts) in the specifications section of the user manual, or marked on the back of its rear panel. Add all these together and ensure that you're not connecting too much to any individual socket. A single socket can handle up to 3kW (3000 Watts), though you shouldn't assume that you can draw that much from every socket in the room, because the current trip will prevent you from taking more than the maximum the whole ring main is rated for. However, most home studios take well under 1kW altogether so, as I said earlier, overall power isn't usually a problem in project studios.
Computers and their peripherals need a very clean mains supply to operate, though for most of the time they will work perfectly happily from the same ring main as your audio. However, if you can install a separate mini‑ring purely for your computers, at the same time as the rest of the wiring is being installed, that can only be a good thing.
A proprietary computer mains filter is a wise investment: though the protection they provide is limited, they help filter out high‑frequency interference. A mains spike suppressor is also useful, to prevent equipment damage in the event that a brief high‑voltage transient should find its way onto your mains system, as it might during thunderstorms. (See the 'Uncontrollable Surges' box for further details.)
I know it's boring compared to making music, but you must also check your equipment every few months to make sure that the plugs are working properly, that the correct ratings of fuse are fitted, and that the wires haven't worked loose. I don't know the exact mechanism by which this happens, but a wire that's been tightly clamped inside a mains plug for a couple of years can end up being quite loose, even though it hasn't been moved. Aside from being very dangerous, loose wires cause interference.
Don't go around removing earth leads in an attempt to cure ground‑loop hum. It may solve the hum problem, but it's very dangerous.
It's also vital to check the cable clamps inside your plugs, especially if they're of the older type that have to be tightened with screws. The newer automatic plastic grips are much better and shouldn't work loose. Make sure the cable grip is clamped firmly on the outer sleeve of the cable, not onto the inner blue, brown and green wires. While you're checking the fuse rating, also make sure that the fuseholder grips the fuse properly. If it doesn't, close it up slightly using pliers and apply a spray of Deoxit for good measure. While you're at it, check that the screw‑in fuseholders on the backs of your various bits of gear are reasonably tight — but don't force them, or you may never get them undone again.
Finally, don't go around removing earth leads in an attempt to cure ground‑loop hum. It may solve the hum problem, but it's very dangerous. The earth system is there to ensure that, in the event of a fault, the current goes to earth via the earth wire and not via the person touching the equipment at the time. You should be able to solve ground‑loop problems either by using balanced connectors or by modifying your signal leads, as explained in the 'Ground Control Of Major Hum' article in SOS August 1994 issue.
For situations where the ultimate quiet mains supply is required, balanced mains units are available. These look like huge power amplifiers and regenerate a local, isolated and independent version of the mains supply. Industrial‑scale units can power a small town, but more practical devices are sufficient to power some modest audio equipment with a pure, balanced supply of the correct stable voltage and an appropriate frequency (handy for keeping an American 60Hz Hammond B3 organ in tune in a European studio!). Although these are seriously expensive bits of kit (the balanced mains unit as well as the B3), they sometimes provide the only solution in extreme circumstances.
A UPS does exactly what it says on the box — it provides a power supply which will not be interrupted even if the mains supply fails completely. In my part of the country, where the mains supplies are all on overhead wires, UPS units are de rigeur, as even tiny glitches can ruin the whole day! UPSs are also increasingly being used to protect other computer‑based equipment, including digital sound consoles, hard‑disk audio recorders, and the like.
The UPS charges internal batteries while mains power is available, and draws energy from those same batteries to regenerate the supply if it fails. Obviously, the amount of energy that can be stored is limited, but as long as it allows sufficient time to shut the equipment down safely and properly during a power cut, contentment and smugness will prevail!
Various power capacities are available, and should be carefully matched to the load presented by the attached equipment. Mains suppression and filtering is normally built into a UPS, and the more sophisticated units can even generate a report of mains‑supply voltage anomalies. Many modern UPS devices also incorporate a serial data output and suitable software to persuade an attached computer to save all work in progress and shut itself down without human intervention. In theory, the auto‑shutdown features should be usable with any PC or Mac‑based mixer or editing workstation too, although I haven't tried this myself.
Voltage spikes or surges are typically caused by switching heavy loads (drawn by all manner of industrial machinery, lift motors, welders, fluorescent lights, fridges, and so on) on or off. At a point in its sinusoidal cycle when the mains power‑supply voltage nears its peak (a nominal 230Vrms), the actual peak instantaneous voltage is 325V. Now consider a (not uncommon) brief voltage spike of several hundred volts on top of that sine‑wave crest and you can imagine how damaging it could be. The spike could also be in the opposite polarity to the mains cycle, reducing the instantaneous voltage instead of boosting it, which could cause just as many problems. Voltage spikes of up to 650V (twice the normal mains voltage) are not that uncommon, and should a lightning strike occur nearby the mains could easily carry brief voltage spikes in excess of 1000V! (Maplin sell a 'Surge Clock', for around £23, which registers any mains voltage peaks in excess of 700V for anyone interested in seeing just how many big spikes get into their studio.)
Although the mains output from power‑station generators is truly balanced, by the time the supply reaches a domestic dwelling it has become unbalanced because the electricity people like to nail the Neutral side to Earth at all the local sub‑stations (a practice not followed so enthusiastically in America). Therefore most of the spikes and glitches on the mains supply will be one‑sided. If a voltage spike is the same polarity on both Live and Neutral it will tend to cancel itself out at the mains transformer in the equipment, and should not cause any significant problems, but if the spike is single‑sided (only on the Live side) it will induce a great deal of stress in the equipment power supply, possibly causing a damaging over‑voltage condition. Most electronic components have a maximum working voltage, and once this is exceeded, the component usually dies — sometimes dramatically so!
The obvious solution is to incorporate a device to limit or clip the incoming voltage to a safe maximum. Such devices are called Voltage Suppressors or Anti‑Surge devices. They are wired between Live and Neutral (sometimes also between Live and Earth) and have no effect while the voltage across them remains below a certain limit. However, when that limit is passed they break down, to dissipate the unwanted energy. This act usually destroys the device, but better that than the far more expensive equipment it was protecting! However, this is not necessarily entirely positive, as the act of clipping the mains voltage can generate a lot of high‑frequency noise itself — which is another good reason for the mains filter. Clearly, if you choose separate surge protection and mains filtering, the surge protector should be the first device the mains supply sees, with the filter second and the equipment third.
All kinds of nasties can find their way into the mains power supply, and some are even put there deliberately. For example, the electricity companies use bursts of radio‑frequency signals on the mains to control many of their power‑distribution networks, as well as some kinds of consumer unit. Other, more nefarious, transient elements can be generated by industrial and domestic machinery such as welding equipment, lifts, lawnmowers, fridge‑freezers, fluorescent lights, and car or motorbike ignition systems. Mains filters exist to try to remove or reduce these unwanted high‑frequency signals, while surge suppressors are intended to limit the maximum instantaneous voltage reaching the equipment to safe levels. The goal is to provide as pure a mains supply as possible.
The first point to note about mains filters and surge suppressors is that they are essentially unidirectional. The majority of units are designed to be most efficient at stopping interference from entering an item of equipment, not to stop nasties generated internally from escaping — although they will usually reduce emissions to some extent. If you want to stop emissions from a specific piece of equipment, EMI Mains Inlet filters can be purchased as ready‑built units. Many suppliers also offer a wide range of ready‑built filters and surge protection devices, either as separate units or as combined devices with multiple mains outlets. However, although these multi‑output filters are very convenient, particularly sensitive equipment will be better served with its own device, and a lead that's as short as possible between filter and equipment.
What's involved in removing unwanted elements from the mains supply? Well, despatching radio‑frequency interference is relatively easy — a simple low‑pass filter is all that is required. Typically, the filter would consist of a large choke (a pair of high‑current coils wound on a common former a little bit like a transformer) in series with the supply, and some small‑value, high‑voltage capacitors in parallel with it. This filter would be tuned to roll off above a couple of kHz and would reduce RF signals 40 or 50dB below their original level. In fact, any device with a conventional linear power supply will provide reasonable RF attenuation anyway, and in practice RF noise rarely causes significant problems with most equipment.