The quality of your AC mains influences the sound of your system more than you might think – it's possible to see distortion levels as high as five percent, or sometimes even more. In addition, noisy radio frequency interference makes life more challenging for your delicate hi-fi components, which is why so many audio enthusiasts invest in mains treatments to clean up their power act.

The £1,995 connected-fidelity AC-2K Balanced Mains Supply is one such product. It's a passive device that features a massive cryogenically-treated 2kVA toroidal mains transformer to provide balanced voltage to power your hi-fi components. The transformer is a 1:1 type, so mains voltage remains unchanged. It feeds the components in your system with a symmetrical pair of out-of-phase voltages, says the manufacturer. Positive and negative connections are inversely-phased so that as one pushes, the other pulls – a bit like two legs on a bicycle. This delivers a lower noise floor because equal and opposite currents meet and cancel each other out at the central grounding point, so the theory goes. Any noise picked up is thus effectively eliminated – something known as Common Mode Rejection.

Build quality is very good, and there is much care taken over small but important details. For example, the toroidal transformer is impregnated with varnish to reduce mechanical noise. Likewise, the centre hole is filled with resin to absorb vibration. It works, inasmuch as there's no transformer buzz to speak of. Some toroidal transformers are prone to buzzing, but the one in the AC-2K Balanced Mains Supply proved very quiet mechanically. Even with my ear close by, I could hardly tell the item was switched on.

The transformer is fixed to a non-magnetic 10mm thick anti-vibration platform which is decoupled. Anti-surge electronics prevent fuse-blowing when the AC-K2 is switched on, but these are not in circuit when the powering your equipment. There's a handy digital voltage readout, but presumably, the display is slightly noisy, so there's a switch on the back that lets you turn it off for best results. The standard version is housed in a nice aluminium box measuring 180x230x330mm and weighs 16.8kg; an optional larger rack mounting version (shown below) is also available. It's very simple to connect up – just plug the AC-2K into a wall socket, and power your hi-fi from it. That's it!

THE LISTENING

The mains supply to my hi-fi goes via a Russ Andrews 6-way distribution board wired with Kimber cable. I also have a couple of passive mains purification/distribution boxes from IsoTek and Isol-8, plus various specialised mains cables, including Kimber. I'd say the mains quality in my central London abode is quite good and very consistent. According to the AC-2K's front panel display, the mains voltage is mostly around the 250V mark. It varies slightly between about 248V and 252V, which is interesting.

Since getting the Russ Andrews Kimber block, Kimber mains cables, and IsoTek/Isol-8 devices, I seem to achieve greater day-to-day consistency. Even so, adding the connected-fidelity AC-2K Balanced Mains Supply made an audible improvement to the sound I was getting. Designer Michael Osborn says the use of a balanced mains supply helps the equipment's power transformers to work more efficiently while greatly reducing levels of harmful noise. Subjectively, the main thing I noticed was increased refinement and a greater sense of ease.

The music sounded cleaner, with reduced glare. Everything I played seemed more transparent and easier to listen to. The sound was better focused at low listening levels and less 'loud' and aggressive at higher levels. There was an overall increase in naturalness. The AC-2K Balanced Mains Supply made my system sound more relaxed and true-to-itself – as though I were listening to it at those times when (for reasons unknown) things sound at their very best. For many enthusiasts, this will be when listening after midnight...

The top end definitely sounded cleaner, so much so that I was able to increase the output of my Townshend supertweeters by two or three notches. I've used these for a quarter of a century or more but have always preferred them set to level 1 (minimum output). Over the years, I've tried higher settings (both in my own and in other users' systems) but always felt level 1 gave the best result. However, with the AC-2K, I found I could play the tweeters at 4 or 5 with beneficial effects, which is interesting. Setting the supertweeters so they're louder increases the effect they produce, but I felt I could hear a slight smearing effect if you set them too high. The quietest setting tended to give the best overall result. But, now the sound is cleaner, thanks to the AC-2K, I can have them louder.

Michael Osborn reckons that, while the benefits produced by balanced mains work across the board, subjectively, the AC-2K has the biggest effect on low-level components like phono stages, preamps, and source components like CD players. He claims digital noise is measurably reduced by the AC-2K. Playing the old Rafael Kubelik DG recording of Mahler's 9th Symphony on vinyl from the late nineteen sixties, I was gratified to hear it sounding so clean and detailed, without excessive brightness or thinness. A good recording in its day, it can sound its age unless your hi-fi system plays to its strengths.

While the recording still sounded bright, the sound had much greater tonal body than before. You could now hear a nice sense of hall ambience, while strings and brass had more richness and weight. I was pleased to hear this challenging recording being reproduced so cleanly. A few days later I removed the AC-2K and tried the Mahler 9 again. The sound definitely wasn't nearly as clean and precise as it had been. The violins sounded a bit glassy/grainy, while cellos and basses seemed a bit blowsy and less well-defined. Adding the AC-2K restored things...

Next came a Hansler CD of Mendelssohn's string symphonies with Thomas Fey and the Heidelberger Sinfoniker. I'd previously found this recording slightly acidic, harsh, and lacking in warmth. So it was nice to hear the sound lose much of its thinness and tonal hardness with the AC-2K in place. Again, the recording still sounded bright, but no longer unpleasantly so. Before, the sound had been slightly congested and edgy, making the listening experience a tad uncomfortable. The AC-2K took away most of the nasty bits, cleansing the sound of glare and harshness. I noticed something similar playing Prince's Purple Rain. The sound was crisp and clear, and at times surprisingly holographic. There was plenty of bite and impact, but although immediate and crisp, the sound wasn't thin or prone to glare.

This extra refinement was achieved with no loss of detail or immediacy. Things just sounded cleaner and more crisply defined. I couldn't detect any downside. Regardless of the music chosen, every recording I tried sounded clearer, less congested, and (in a word) better. Bass also improved, becoming firmer and more tuneful. There seemed to be an increase in bass weight and dynamics, with the primary impression being one of greater clarity and less tendency for the lower frequencies to wallow and spread.

After a couple of days, I disconnected the unit, and that lovely cleanness and ease I'd started to take for granted was suddenly no longer quite so apparent. The sound was still good, but it definitely wasn't as special as before. I used a Prima Luna EVO 200 tube pre/power amp for my early tests. This produces a warm, almost lush sort of sound. Going over to my regular Musical Fidelity Nu-Vista 800, the sound grew sharper and leaner, but AC-2K helped ensure there was no edginess to speak of. The improvement produced is quick and very easy to acclimatise to. You soon get used to the improved clarity and refinement. It's only when you take it out of circuit that the penny drops, and you fully appreciate what it was doing.

The connected-fidelity AC-2K Balanced Mains Supply is not supplied with a mains cable, on the basis that you might already have something suitable. If not, connected-fidelity offer a Unity Two power cable option. The basic AC-2K has a single 13A output socket, but can be customised to offer a double 13A socket, a single Schuko socket, four Schuko sockets, or a double US socket. A multi-tap voltage version (220/230/240V) is also an option. See the website for prices.

THE VERDICT

Having heard what it does in my system – which I'd say is relatively well-balanced and in some ways quite-forgiving – I'd highly recommend auditioning connected-fidelity's AC-2K Balanced Mains Supply. It will really help you get the very best from your hi-fi. But be warned; once you hear the improvement it delivers, you may have difficulty listening without it. That's the situation I find myself in at the moment. Oh dear!

Visit connected-fidelity for more information