Thanks to TVs, energy saving thermostats, cheap fluorescent lights, and a host of motorised appliances, todays 240 volt mains is heavily polluted, resulting in a certain amount of noise interference. In the course of this new series, Ben Duncan suggests various cures that can be used to effect a complete healing of the mains.

Amongst the worst symptoms of feeding your gear on dirty juice are drowsy voltage, sharp stabbing pains in the power supply, loss of memory, confused routing, impulsive flatulence, speaking in tongues, and even damage to the vital organs, RAM and bridge-rectifium.

This first stage of the series deals with suppressing and filtering out audible mains interference, and provides the background material you need to go out and tackle one of audio's thornier problems on a pragmatic DIY basis.

Interference Analysed

Strictly, mains-borne interference is anything which makes the juice deviate from the rhythmic, sinusoidal 50 cycle pattern (figure 1). By definition, this includes voltage fluctuations per se, and mains failure, but these things are to be dealt with later. What we're concerned with here is the sort of interruption that creates noise hash, clicks and bangs on tape and monitors. These effects arise out of different mechanisms. Let's look into these in turn.

Impulsive Noise

This is the jargon term for clicks and bangs. The primary cause is sudden changes in current and voltage, the result of closing and opening (unsuppressed) switch contacts carrying any appreciable level of electrical power, be it AC or DC. In practice of course, we're looking at mains on/off switches on studio equipment, light and power switches, plus relays and thermostats hidden away inside domestic appliances. Another source of impulsive noise less prevalent today, is from old unsuppressed vehicles. Bike and Airframe magnetos are the worst.

Arc Hash

This is jargon for an irregular burst of 'total static' - a sort of Heavy Metal variant on pink noise. It's caused by arcing (sparking), usually the outcome of dubious mains adaptors, and dodgy high power switches or thermostats.

Thyristor Hash

This is again the result of switching, but unlike mechanical contacts, the speed of thyristors switches is more likely to result in very impulsive changes in current flow. Worse still, the thyristors forte - controlling big lamp and motor loads - involves lots of circuit capacitance and inductance respectively. For this reason, the interference effects of dimmer control gear are amongst the most tedious, thereby gaining the reputation of Bete Noire - if only in the soundmans' realm. Sonically, thyristor hash is a toned down variant on arc hash, more of a dulcet 'Bzzzzz' or Alto 'Burrrr'; this is the outcome of partial suppression. The early domestic light-dimmers had none at all, and the radiated hash could sound more like a spluttery arc.

Motor Hash

This comes from unsuppressed DC and universal AC/DC motors, the sort with commutator brushes. The sound varies, but it's familiar in any case, for lo and behold, the guilty appliances are all those labour-saving devices which need a high-torque motor such as vacuum cleaners, washing machines, driers, jigsaws, drill machines, angle grinders, sanders, saw benches, freezers etc. The aural effects range from a hail of spitting clicks, to a shrill dynamo-whine.

The Dreaded Line Whistle

This shouldn't now occur, because the 405 line TV Standard which caused the problem has been phased out, as of January 1985. But just for archeology's sake, the VHF Television standard involves a 15kHz line oscillator. Given the base crudity of early TVs, it's not surprising that this frequently leaked onto the mains supply, and has caused more than a few people to go a looking for phantom 15kHz tones. But even without 405 line TV, there are plenty of mains-powered control and instrumentation electronics including UHF TVs and intercoms, introducing tones or other spurious audio signals onto the juice. A bizarre side effect of this is the singing light-bulb; an ageing light bulb makes quite an effective, if well muted, "monitor speaker" in that it will reproduce any whistles or tones present on the mains supply, not to mention Thyristor hash. Hands up if you've never heard an anglepoise lamp whistle when sitting in an otherwise silent room? The best bulbs for intentional monitoring are ageing high power ones, with gangly filaments, eg 750 watt Giant Edison Screw (GES). By the same token if the effect irritates you, stick to nubile, low wattage bulbs! When these begin to sing, they're nearing the end of their life.

RFI

RFI, Radio Frequency Interference, is no more than an extension of the line whistle problem except that the frequency of the interference is much higher. Strictly speaking, impulsive noise and hash are also RF phenomena, but in this category, we're concerned with narrow band RF nasties, that's Radio transmissions or their spuriae, rather than ad-hoc energy splatter. Radio frequency can't be heard directly of course, but who's betting that some part of the studio set up won't perform the demodulation necessary to make a good radio receiver? We will look at this point in a later section. Sources of broadcast RFI include the local radio stations, pirate or otherwise, taxis, the police, Citizens Band users, mobile telephones and to some extent computers. The audible symptoms range from ethereal swooshing noises or a machine clatter, through to an impromptu eavesdropping session as the local policeman strolls past your studio at 3am!

Prevention or Cure

The answer, in a nutshell, is a bit of each. For a start, anything which creates interference on your own patch may as well be suppressed. First because it's cheaper by far than attempting total suppression. Second, because local interference on the mains will invariably propagate itself as a radio wave, cleaning up the mains with a filter will literally leave the back door and the ceiling open! In fact the complete suppression of locally radiated RFI can involve the conceptual application of hundreds of ceramic capacitors and ferrite beads, or even more outrageous, caging the entire studio in a shield. On this basis, the work and small expense of suppressing your own noisy domestic gear is doubly advantageous.

Thereafter, filtration on the incoming mains can play its part in cleaning up residual interference, and tackling the 'long distance' interference. In addition, all the most sensitive equipment can be dealt some extra filtration, right up to the actual power supply. This will clean up any residual RF garbage, re-radiated from the unfiltered mains wiring all around the room.

Checking for It

Because the checklist of contacts needing suppression could prove lengthy, it's handy to establish which switches need suppression most. This means setting up your console and monitors to be sensitive enough to display potential mains interference. If you intend filtering the incoming mains and/or individual power supplies, it's best to get the mains filtration set-up beforehand, otherwise you may waste time and effort suppressing contacts which would have been cleaned up in any event. Failing this, don't wind the gain too high, and concentrate on tackling the causes of the loudest clicks. Another hint, the popular idea that saving the monitors by setting up the console to display impulsive interference noises on fast LED Bar meters, doesn't work too well. This is because LED metering and your eyes aren't so good at catching really fast peaks as your ears should be. Second, as it takes no account of the perceived irritation viz, motor hash in the mix can be painfully obvious, even when it's close to the noise floor. Despite these reservations, it's helpful to note down the console's peak meter readings, just to give us some idea of the order of magnitude, whenever a range of interference sources are being investigated (Figure 2).

Having set up the monitoring, the next step is to walk round the studio, briskly switching all the ancillary gear on and off, a dozen or more times in succession and taking note of the results. The idea of the rapid on/off sequence is to ensure that at least one of the switch operations happens close to the mains zero-voltage point. This arises at the beginning of each half cycle where the rate of rise of current (dl/dt) is highest and consequently the potential for really noisy bangs and clicks is greatest.

Having noted the worst offenders in the studio, it's time to investigate the surroundings. For home studios, it's best to make a beeline for the kitchen. You can then begin by testing the lightswitch on the 'fridge door for clicks, helping yourself to a beer at the same time. Back on track, a slight practical problem is "How on earth can I know what's coming out of the monitors when I'm in the kitchen?"

Alas, the vocabulary needed to convey the nuances of discordant interference noises doesn't presently exist, so it's not a lot of help asking your colleague to shout back the audible effects of each switch: "That one was Loud! Quieter! Not so quiet! Louder! Two bangs in a row - oh, not so loud..!" So was the last switch click too loud - or not? This is where peak metering can come in handy. Another answer is a pair of cans on a long lead, or better still, a micromonitor. Using a CB or an FM intercom is rather self-defeating because the transmission link itself is susceptible to the very interference we're trying to assess.

Suppression Techniques and Evaluation

Figures 3 and 4 display an 'RC' (resistor-capacitor) network, or snubber. It's also called a contact suppressor. Its action is to cancel out/damp down the fast, inductive perturbations that aggravate and prolong arcing across switches and contacts. The result is that a loud bang subsides to a faint, fast click.

In effect, the capacitor provides a receptacle into which excess voltage can be dumped whenever the switch is opened, whilst the resistor limits the peak discharge current. Otherwise, the capacitor would blow-up the contacts when they next closed! The resistor's other function is to damp down any resonance currents, because this ringing would create a disturbance on the line. That would rather defeat the object of the suppressor!

An interesting faux pas that can be spotted on a lot of oriental equipment is a ceramic capacitor wired directly across the mains on/off switch, and no sign of a resistor. This trick is possible if we use a small enough capacitor (say 1000pF), because the peak discharge current won't be enough to zap the switch contacts. Or at least not straight away. Actually, it's much better to uprate the capacitor (to around 100nF) and include a 100ohm resistor - in other words to replace the small capacitor with a bog standard contact suppressor. The results are improved click suppression and a bonus extension in the lifespan of the switch. Application is simple - just connect it across the delinquent contacts, taking care to wire it as close up as possible. Figure 5 demonstrates the idea, by illustrating contact suppressors wired across a domestic light switch and an equipment toggle switch. The idea can also be usefully extended to 13 amp socket and spur outputs, where these are used as everyday switches. A very important point to make here is that if you don't feel absolutely confident about modifying your mains hardware please ask someone who is to help you.

Heavy Duty Suppression

The hash from motors, thyristor controls (eg dimmers and speed controllers) and related sources can't be dealt with by recourse to basic contact suppressors alone. In any case, if the appliance is quite new, it's likely that these are already fitted, because across the EEC, basic interference suppression is now a legal requirement on all domestic appliances.

The archetypal motor suppressor is a triad of capacitors (figure 6), often cast into a single cylinder and called a delta suppressor. It's hooked up between live neutral and ground. This makes the noise currents on live and neutral symmetrical. Under these conditions, there's a better chance that interference will be ignored by power supply transformers in the same way that a balanced input rejects 'common mode' interference.

Figure 7 displays a more elaborate assembly, using small 'VHF' choke in conjunction with the deltacap. These accelerate the attenuation effect of the capacitors, especially at high frequencies. In effect, we've converted the contact suppressor into a filter.

Next month we'll look at the application of wave filters in quelling interference, both at source and destination.