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  • Writer's pictureSteve Elford

Thinking About Mains

Part 1 – what is really on the mains?

We often hear clients say "if the mains power has come all the way down tens of miles of HT power lines, through transformer sub-stations, then from one end of the street to the other, how can the last couple of metres of wiring possibly make any difference?"

But let's consider it from a different angle.

As we know, we've got three lines in our mains – live, neutral and ground. The power comes down the live line. But think of it as a 50Hz power signal from a power amplifier that's connected to the other end of that line. That power amplifier has some really big juicy transistors on it's output with a DAC and preamp input, and voltage rails at +/- 340V (to give 240V RMS in the UK). To create the power, we play a CD which has a strong 50Hz tone on it, and our amp pushes that up to +/- 340V, backed up by tens of amps current capability. At the user end, all we need is say a 50Hz optimised transformer to extract some of that power to run the fridge or make our toast. (a bit like a bass driver is optimised to extract bass energy)

But, unfortunately that CD is not a clean 50Hz signal. Sure, there's a big peak at 50Hz but there's also a huge range of sonic and ultrasonic signals on there. If you played it through speakers you would hear a loud 50Hz bass tone plus all sorts of hissing, pops and clicks, and plenty of harmonic tones in there too (coming from the optimised tweeter!). Even worse, if you used an oscilloscope on the wire, you would see increasing 'noise' all the way up into radio frequencies.

So you see, our power network ships a 50Hz or 60Hz tone around – and that's all it's designed to do. It doesn't take into consideration any noise it carries. It's simply a low frequency with current idea.

We need to consider the neutral and ground lines too of course. Sure the neutral is the return for the live 50/60Hz signal but again it's full of noise. Ditto the ground line. And don't assume that a standard domestic ground is much use at grounding high frequency noise. No, that ground is only there to short a 50/60Hz fault, or a DC fault, and trip a safety feature. That domestic ground does not have good electrical properties for shorting high frequency signals.

None of this would really matter to us though if it wasn't for these following issues:

  • Hifi is in essence a very sensitive piece of electronic measuring and reproduction equipment that is degraded by the presence of noise in its circuits. We call this Radio Frequency Interference (RFI).

  • At high frequencies, signals easily jump around between lines and circuits – everything becomes a highly efficient transmitter and receiver when their size is of the same order as the wavelength of the signal.

  • Not only is our power grid feeding uncontrolled noise into our systems, but we have now drastically increased the sources of domestic noise in our homes with all the digital systems we use.

So, one of the things we have to work on with system building considerations is how to manage the noise problem. And as this series grows, we'll develop the discussion at depth and talk about solutions.

More coming soon :-)

Power lines
Bob says – "There's a lot of RFI on these lines!." If you listen hard to your speakers, you might just hear his walkie-talkie breaking through.

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