AudioMedia - October 2008 - (Page 49)

POWER FACT & FICTION > conditioning and heating plant. No connections were made with other pieces of equipment, so as to prevent any RFI entering from any other source than the mains cable or the earth wire. A significant transmitter of unwanted RF that is sometimes used in a studio is the compact fluorescent lamp (known to its friends as the energy saving light bulb). By wrapping an open ended cable around the lamp, I could get up to half a volt of RFI, but only mV in a power cable that connected to the mains but not earthed. I got no RFI in the mains cable at all when the cable was properly earthed. Lighting dimmers can also cause problems, and if your studio is near unsuppressed dimmers, such as in a block of flats, the RFI they cause can bleed into the mains. To test the levels of RFI carried in the mains, I set up stage dimmers, each cutting the phase to its own W PAR lamp at exactly . The result was less than expected at about mV when not earthed. With a poor earth, this increased to between mV and mV, depending on equipment used, and the effects of the dimmers could be clearly heard. I also tested the effects of multiple flat screens and CRT monitors, but these did not produce any mains-born RFI that I could measure, except when using the poor earth and then, the mains cable had to be lying right next to the source of interference, just to get mV of interference. The studio analogue mixing desk with its large PSUs showed no sign of mains-born RFI, and I could not test it with the poor earth or with earth lifted, as this would mean unplugging several hundred cables, just to isolate the effects of the mains interference. The Lexicon L did not seem to mind whether it had a good earth, a poor earth, or no earth at all – it remained serenely quiet at all times. Other than that, nearly every other piece of equipment increased its noise floor significantly, when the poor earth was used. This varied from piece to piece, and there seemed to be a tendency for the digital stuff to suffer most. Adding a power conditioner to the above tests had no effect on any units that were properly earthed. When the poor earth was used, some improvement could be measured in most units that were of an earthed design. This was far from massive, but the effect was there. The earth-free designs varied, with one unit, a small desk-top mixer that used the dreaded wall-wart, having slightly more thyristor buzz from the dimmers when the power conditioner was used. balanced lines for microphones or line feeds, that it rejects interference from outside and causes less induction in neighbouring cables, as the opposing voltages cancel one another out. All power supplies in Europe are either single phase AC and about volts, or they are threephase, each phase being out of step with the other phases to provide a voltage of about volts AC. All studio equipment is single phase. European law on mains supplies is complex and has to take into account such things as current and application, so as to make such uses as electric cattle fencing and high voltage industrial DC motors possible. But as far as studios, commercial or domestic, are concerned, any voltage above volts is deemed to be a mains supply and must adhere to the strict laws governing their installation. Because each leg of a balanced supply is independent of the other, should the chassis become live as the result of a fault, the fuse to that leg may be triggered, but the chassis will now have a potential of volts to both ground and any neutral wires not within the system. For this reason, both sides of the balance have to be fused at the wall socket or within the equipment, and the fuses have to be linked, so that, if one side is tripped, the other side is switched off as well. An alternative is to use a power relay or contactor that switches off the entire system. In the EU and in the US, balanced power may only be installed by a qualified electrician. A DIY installation of balanced power is definitely dangerous to both people and equipment. Again and again, the topic of balanced power is brought up and often hailed as the one thing you can do to your studio that will improve signal to noise ratios more than any other. Subjective statements are sometimes added, like, “After installing balanced power, we got clearer highs and better defined lows.” Sometimes, almost magical properties are ascribed to it. Let’s see what balanced power is – and what it is not. Balanced power is created by installing a simple : separation transformer, the difference being that, instead of ground or earth being linked to the neutral wire, it is linked to a central tap that has volts difference (i.e., half the mains voltage) from either of the two live ‘legs.’ The idea of this arrangement is the same as for O ff B alance It sometimes happens that one sees the uninformed disconnecting the ear th or ground wire in an attempt to interrupt a mains hum loop. In a word – don’t! The law requires all equipment to be either earthed, or to be of an earth-free design and, in a commercial environment, lifting the earth is a breech of health and safety laws in every European country. Also, nearly all audio equipment has circuitry that suppresses interference by bleeding HF to earth via the chassis. If the earth is lifted by disconnecting the green and yellow earth wire (or even using a dedicated earth-lift switch) under the right circumstances, this circuitry acts as a voltage divider and part of the mains supply is sent to the now earth-free, disconnected chassis. People have died, sometimes live on stage, because some idiot decided it would be a good idea to lift an earth to get rid of a mains hum loop. Disconnect the screen on the offending audio lead instead! Laying O ut The G round Rules I am often asked, “How do I know if the earth for electrical supply is OK?” The simplest test is to take a multimeter and measure the resistance between the earth lead on any electrical socket and a metal structural part of your building. This should read zero Ω. You may of course get something like . Ω, but if you get the same value by holding the leads to the multimeter together, then recalibrate your meter (or just make allowance for that value) and test again. If you get any value other than zero, call an electrician and get it fixed. Rather like modern cars, accessing the metal > Contac ting Planet Ear th > AUDIO MEDIA OCTOBER 2008     http://www.sommercable.com http://www.sommercable.com

Table of Contents Feed for the Digital Edition of AudioMedia - October 2008

Audio Media - October 2008 Content Recording Post Broadcast Geofocus:�Portugal &�Spain What's Up UK IBC Report Microtech Geffel UM 930 Steinberg Cubase 4.5 ADK�Hamburg &�Vienna Final Cut: House of Fallen Lexicon PCM96 Review The Conch Awards Feature Sound Devices 788T Review Lexicon PCM96 Allen &�Heath Zed R16 Power: Fact & Fiction, PT.2 Product Sampler Plug-ins Video Guide AMSR Plasa Report Audio Technica IEM�M3 Rushes Live Mixing Techniques:�Bruce Johnston Drawmer 4x4 Kickbox

AudioMedia - October 2008

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