Studio Mains Supplies (Part 3)
Ben Duncan continues his epic voyage into the world of less than perfect mains supplies.
The latest facts on ring mains are here exposed in yet another article from Ben Duncan.
Last month, we looked at mains filters. Ideally, these should be factory fitted to all audio equipment, but in reality, manufacturers like to save money by omitting refinements you won't immediately notice you're missing (until, a couple of years and many ruined recordings later, you come to the conclusion that the omission of basic filtration represents the undesirable face of 'cost-effectiveness'). Meanwhile, it's easier, cheaper and more elegant to begin with a single, overall filter in line with the studio's incoming mains supply. In the next section, we'll see how a reconfiguration of the studio supply wiring can enhance the big clean-up.
Figure 1 depicts the familiar ring-main: a 20 Amp cable runs around the house, with 13 Amp sockets hooked across it at convenient points. The overall current rating of the ring is only 30 Amps, and this is limited to 13 Amps per socket, but the system works on the assumption of diversification. Few appliances draw as much as 13 Amps, and many draw less than 1 Amp. Moreover, it's unlikely that everything will be switched on full-bore, all at once. On this basis, the number of sockets permitted for a given floor area is unlimited. At first sight it's a potentially elegant arrangement for audio, where there are never enough outlets, but the current drawn is minute.
The ring main was first introduced in 1947 as an improvement over a maze of radial cables. Prior to this, every power socket had its own dedicated cable (Figure 5), heading back to the fusebox. This was tedious in big houses, but in 1947, there weren't many electrical appliances in the average house. Fortunately, someone was farsighted enough to see that there'd surely be a lot more by 1985.
The idea was to simplify wiring, save on scarce copper and provide a large number of outlets without tangles of spaghetti and massed ranks of fuse-boxes. The trouble is, the ring main can all too easily become a 'ring sewer'. As shown in Figure 1, noise currents produced by an uncivilised appliance are superimposed on the power drawn by all the remaining sockets, even if these are in other rooms.
There's usually a single ring allocated to each floor, but if your abode isn't a building in its own right (ie. it's a flat), you'll certainly have your own ring, - assuming there's a meter reserved for you! The significance of this is that whereas a ring supply is very sensitive to noise and garbage circulating within the ring, it's relatively immune to nasties on any adjacent ring circuits.
Figure 2 shows how well equipped studios make use of this fact to retain a clean mains supply, without throwing away the convenient features of the ring circuit. Here are two rings, one kept exclusively for audio equipment, - the Technical Supply - and one for all the dirty domestic stuff, namely heaters, lights, dimmers, fridges, air conditioners, soldering-irons, calculators and so on; the House Supply. Even though the two rings occupy the same room, and are joined at the fusebox, the clean one will remain free from it's neighbour's garbage, provided the wiring runs are kept apart by several feet. Preferably, they shouldn't run parallel, and should cross at right angles, but these conditions can be waived if the House Supply is run inside an earthed conduit (steel pipes), so that it's shielded.
So why doesn't interference on the dirty mains feed leak back via the fusebox to the Technical Supply? The truth is that it does, but at least it's attenuated, courtesy of the much lower impedance seen at the junction with the incoming supply feeder - usually a 60 or 100 amp cable. In Figure 3, interference travelling out of ring main A encounters this impedance (of about 0.02 ohms = 20 miiliohms) in series with the source impedance of the ring main itself, which is always higher.
At 50Hz, the result is attenuation (typically around 20dB) before the noise can pass into ring main B. At much higher RF frequencies, there's a progressive attenuation all the way along the line, so the least interference is conducted between the most distant points (say C and D), whereas RF interference coupling between adjacent points (like A and B) is more erratic.
Returning to our basic, domestic ring main, how can we filter the incoming mains? The only kosher method is to insert a filter at the fusebox, where the two ends of each ring are joined. The standard ring main fuse assumes loads up to 30 Amps, but it will actually pass up to 50 or 60 Amps before it gets around to blowing in any sort of hurry. Therefore the filter's rating will need to be at least 30 Amps. If you recall last month's article, however, big filters are not too hot on performance, and are more expensive to boot. Figure 4A also reminds us that our expensive investment does nothing for noises within the ring, and may even worsen the situation.
Now let's take a look at a professional studio's dual-ring arrangement. How does it perform? Looking at Figure 4B, you can perhaps visualise the technical ring's double protection by both isolation and filtration. To translate this into figures; there's around 20dB lost through isolation and typically 40dB through filtration. In other words, garbage on the House Supply will ideally be -60dB (= -20dB + -40dB) down by the time it arrives at one of the audio power outlets. The advantage in this case is that once you have shifted all heavy loads, such as heaters, over to the House Supply, the Technical Supply becomes only lightly loaded (assuming there isn't a rack of Fairlight power supplies lurking in a cupboard, or 20kW on the monitoring). Indeed, a fair sized home studio setup is unlikely to draw more than 1 Amp on peak load, and even a large studio's demand shouldn't pull more than 6 Amps, unless there's a lot of heavy digital equipment. This means we can fuse the Technical Supply accordingly, and then safely install a 3, 6 or 10 Amp filter in the line. These sort of current ratings are inexpensive and widely available, whereas 30 Amps is somewhat antisocial. For this reason, the filter shown on the House Supply is marked 'optional luxury' - though it would definitely help to clean up garbage on the House Supply still further.
To return to the typical house of the 1930s, Figure 5 depicts the alternative radial scheme of wiring. Though messy, it had the benefit of each socket being inherently isolated by about -20dB from its neighbours, and being fed directly from a low impedance point; the fusebox. If it helps, you can think of it like the famous star earthing technique, except that we're dealing with the live and neutral conductors, instead. Alas, providing a mere five power outlets in one room alone involves lots of wires, and it's inevitable that you'd want to run these neatly side-by-side, as they approach the fusebox. In 1930, mains wiring often had lead sheathing, which shielded adjacent cables from one another. Running modern PVC-sheathed cables along side each other, however, will wreck the isolation at high frequencies and the shielded alternative, conduit wiring, would be too expensive.
Nevertheless, the radial concept has a part to play, in the form of 'spur' circuits, taken off an existing ring main (Figure 6). Given that the spur is exclusive to the studio gear, it can be fused down to 5 amps, and then filtered separately. It's perhaps not as good as a separate ring main, but it's cheaper, and easier to install. The only special part required is a fused spur outlet, (eg. a type 5445WHI from MK Electric). What's more, you needn't feel left out of this if your entire studio set-up is running off a makeshift plugboard, via a single extension lead. This, in effect, is the same as the spur shown in Figure 6, so you too can clean up your incoming mains by wiring a filter in line, at the plug top end of your extension lead.
In theory, we should use a 15 Amp filter, but provided you clearly mark the plug 'five Amp max', and fit a five Amp fuse, all should be well with a cheaper and more effective 5 (or 6) amp filter. These particular current ratings are readily available in the box filter format, that's with a 13 Amp plug and socket combined, so all you need to do is plug it in. But let's assume you can't source one of these, or you'd prefer to mount up a 15 Amp filter, only available in beancan format. On the basis that someone, someday, will surely replace the 5 Amp fuse, when blown, with a 13 Amp one, Figure 7 gives the practical details for assembling a custom filter box. If you're not confident about your mains wiring skills, please get someone skilled to check the assembly before plugging in, and also be sure to make a continuity check of the ground/earth connections.
Feature by Ben Duncan
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