Installing a dedicated mains spur

Introduction

I have read several times the opinion that adding a dedicated mains spur is the most cost-effective upgrade for an audio system, particularly if one does the work oneself. There is no real consensus within the DIY community as to why providing a separate mains feed appears to improve the sound of audio components plugged into it. Nevertheless, no-one has suggested that it might actually degrade the performance of the system, and I can think of several reasons why it ought to improve it.

It seems that the majority of noise on the mains line in a typical household is generated within the building, rather than entering from elsewhere via the consumer unit (though this doesn't mean that we don't need to worry about the latter source). In particular, there are many appliances that use half-wave rectification and so produce large quantities of second-harmonic distortion, which among other things can make large toroidal mains transformers hum audibly. There are also sources of high-frequency switching noise, including computer equipment and dimmer switches.

These things wouldn't be so much of a problem if everything were connected directly to a very low-impedance voltage source, such as (ideally) the line entering the consumer unit. The trouble is that in a domestic ring circuit each outlet adds an extra, possibly unreliable, junction to the circuit and may be connected to an additional potential source of noise that is separated from the circuit breaker by this extra resistance and inductance. On top of all this, a large power amplifier with a linear power supply (unless it is very carefully designed) itself injects plenty of wideband switching transients back into the mains, and this may well affect the performance of other components of the audio system.

The advantages of a separate line to the consumer unit should now be pretty obvious. It's clear as well that having two lines, one for the high-current power amps and one for the rest of the system, should be even better. I moved into a new house a few months ago, and a complete redecoration of the living room seemed an ideal opportunity to upgrade the mains supply to my assorted audio bits. Given that even the thickest twin-and-earth cable is pretty cheap, and that a pair of decent mains outlets and a miniature circuit breaker cost something like £ 15 between them, it was hard to resist!

Execution

I decided from the start to have two double outlets, each wired separately to the consumer unit; this way I can ensure a good mains supply for both of my power amps, while going some way towards isolating them from the rest of my equipment. I live in an almost-new house with chipboard floors and insubstantial internal stud walls. Although this isn't ideal from an acoustic viewpoint (and makes hanging shelves off the walls perilous), it did make the job relatively painless.

First of all I removed the skirting board so I could do all the dirty work without having to replaster the wall (and in this case also allowed me to instal an FM aerial lead). I drilled through two intervening walls and one floor to get the cable from the garage, where the consumer unit sits, to the bottom of the upstairs wall where the outlets were to be installed. I used two five-metre lengths of 6mm2 twin-and-earth cable, each of which required a 13mm diameter hole. This is a step up from the default 2.5mm2 cable usually found in a 32A ring circuit, and is generally reckoned to be the highest capacity cable that is straightforward to work with. I toyed with the idea of using 10mm2 cooker cable, and this was indeed far too unwieldy.

Next I installed the two MK Logic Plus switched outlets into the wall, which was again made easier by the stud wall construction - I simply fixed battens behind the plasterboard, and screwed the metal wallboxes to these. Finally I connected up each cable to its outlet - at this stage I was glad to have only a single end of this thick cable to attach to each terminal.

Finally I wired up the other ends of the cables to the MEM 2000 consumer unit via a new 32A miniature circuit breaker.

First impressions

I'm still using my old ProAc Studio Ones, which, although very listenable and competent speakers don't provide the last word in bass extension or detail. I'm also still not able to use my record deck, since I'm waiting to complete my system rack before I can reinstall this, so I have only listened to FM radio and CD so far (my second and third favourite sources...). I haven't had the chance yet to do any proper A-B tests between the new outlet and the old one, and as a final complication the furnishings in the room have changed since I last plugged my system into the ring main.

All the same, my initial impressions are positive. The aspect of the sound that seems to have definitely improved is the "inner detail" - that is, the feeling of physical space and of the acoustics of the recording venue. There is also more of a feeling of ease to the sound, and high-pitched instruments such as triangles and cymbals sound more natural and "present".

As an example, I bought the recently-remastered version of George Harrison's "All Things Must Pass" on CD a year ago. To start with this album really came across as a typical Phil Spector "wall of sound", and as I raised the volume it started to become tiring. With the new mains supply, at low volumes I could hear plenty of detail I missed completely before. At higher volume settings the instruments and voices just gained in power and presence, without any strain (almost like an LP, even...), and I could bask in the extraordinary emotional glow and excitement of this music.

Links to other mains installation projects

Acoustica

Audio Asylum thread

An interesting discussion on an Australian/New Zealand audio forum

Alex Megann, October 2003

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