Naim's new range-topping CD555 CD player was built to survive a nuclear strike, gorgeously styled with a hefty central loading bay, and priced at an intimidating £14,295.

That's the short and superficial version of the story. The CD555 might have been the sexy looking visual statement into which one places and plays CDs, but lurking a few shelves down on the same Fraim rack is the less glamorous but equally useful NA 555ps.

This similarly massive unit is an external power supply. It's an essential partner to the CD555, and included in its £14,295 price, but it can also be purchased separately for £3,995. It has an equally important role as an upgrade to the XPS and XPS2 power supplies, for those using earlier Naim CD players such as the CDX, CDX2, CDS2 or CDS3 models.

Furthermore, at the time of writing some hundreds of 555ps units have already been built and sold - reputedly, a number sold to Naim dealers for their own personal use - while first examples of the more complex CD555 have only just started to trickle out of the factory.

Indeed, one could argue that the 555ps supply is the more interesting component from a practical rather than a glamour perspective, because of its applicability to provide upgrades for a substantial and well-established customer base. Naim has used similar strategies before.

The (original) XPS first appeared as an upgrade option to the original one-box CDX, and was then supplied as the standard outboard power supply with CDX2. An improved and restyled XPS2 was introduced as an upgrade option with CDX2, again appearing some time before it came as part of the CDS3 package.

Furthermore, in our reviews of those various components and combinations, although both player and power supply upgrades were significant, the supply upgrades often gave the greater improvements.

At arms length

Naim has long practised the 'upgrade via the power supply' technique, in the first instance with its preamps back in the 1970s. It was a relative latecomer to CD, only introducing the original CDS in 1991.

This two-box player follows that same Naim tradition of packaging its power supplies separately. In so doing it flew in the face of the then 'accepted' architecture for a two-boxer, which housed the disc drive mechanism in one unit and the digital-to-analogue conversion (DAC) electronics in the other.

Naim's justifications were partly that it keeps mains voltages and transformers well away from the player proper, feeding a number of low-voltage supplies to the player via a hefty multi-way connector.

No less important, it allows a parallel connection from the drive to the DAC to be used, avoiding the S/PDIF connection. Keeping drive and DACs close together allows a much faster data transfer as the digital signals only travel a few millimetres.

The new 555ps looks very like an XPS from the front, apart from the silkier, shinier surface finish reserved for 500-series components, though there are a number of important differences from the earlier unit. For starters, XPS remains available at £2,550, while 555ps costs £3,995.

From the rear it can be seen that 555ps now has two chunky multi-way Burndy sockets, so that separate hawser-like leads are used to connect the analogue and digital supplies to CD555; only one combined lead is used to connect to CDS3, CDX2 et al.

The transformer of the 555ps is 40 per cent larger than that used in XPS, and this is very evident when you try to pick it up. Internally it uses seven separately regulated supplies, with five secondary windings on the transformer. Particular care has been taken to isolate the ultra-quiet master clock supply from the rest.

Nearly all the auditioning was done using Naim NAC 552 preamp, NAP 500 power amp and B&W 800D speakers, linked using a mixture of Naim and Vertex AQ cables.

Improved sound

Substituting a 555ps for an XPS2 in powering a CDS3 CD player took no time at all. It didn't take very much longer to hear the substantial improvement in sound quality that changing the power supply had wrought. The change was not subtle, and anyone who takes time out to audition the comparison will find it very difficult to resist this new power supply.

Information deep in the mix is clearer and easier to make out, which makes familiar recordings more explicit, interesting and involving. The real delight comes in slipping on a CD unplayed for a year or so, and discovering new and previously unsuspected subtleties well down in the mix, simply because of the superior low-level resolution.

Lambchop's Is a Woman was one such example, the title track revealing the subtle interplay of the guitars much more clearly than had been audible before. Indeed, from a musical analysis (though not a sweetness) point of view, the CD supplied greater detail and clarity than the vinyl version, which came as something of a surprise.

We also subsequently managed to borrow a CDX2 for a brief listening test, and this brought a broader perspective to the whole upgrade scenario. The CDX2 (£2,925) comes complete with a modest built-in power supply, and on its own puts in an entertaining performance, albeit with a rather matter-of-fact 'plinkety plonk' presentation, lacking the sense of scale and space generated when the accessory power supplies are used.

Adding an XPS2 (total cost £5,475) immediately added substantial extra poise, space, air, and gave the sound a much more out-of-the-box character. However, neither authority nor dynamic range seemed fully developed here, and the sonic character might have been a little sweeter.

Replacing the XPS2 with a 555ps (total cost £6,920) took the performance comfortably up to the level of an XPS2/CDS3 combo (total cost £7,875) - indeed, on balance the superior power supply with the inferior player was most preferable, demonstrating very impressive dynamic range, quiet backgrounds, extra authority, better distinction between voices in a choir, and extra sweetness, too.

That said, it never sounded as relaxed as the CDS3/555ps combo (total cost £9,320), which definitely brought a better sense of flow and coherence, with greater security in low-level resolution.

The one negative I could find with the 555ps is that some mechanical hum was occasionally audible (presumably responding to minor vagaries in the quality of the mains waveform or voltage), usually in the wee small hours when background noise was at its lowest.

This was never the case with the XPS, and is presumably because the bigger a toroid, the more it is inclined to hum if the mains voltage drops or its waveform becomes distorted. However, this should not detract from an otherwise brilliant upgrade from Naim. Paul Messenger

THE CDX2 AND CDS3 CD PLAYERS

Both the £2,925 CDX2 (HFC 238) and £5,325 CDS3 (HFC 258) use basic Philips-sourced 'building blocks' - the VAM1250 drive mechanism and the DAC chip sets. The differences lie in the implementation.

The front-loading CDX2 features a non-resonant tray with mechanical decoupling for the disc drive mechanism, hinged at one point. The player also incorporates its own modest on-board power supply. The top-loading CDS3 sits on three hard feet, and both the internal main circuit board and the disc drive are spring-decoupled. The drive is suspended on a wire and leaf spring suspension, carefully tuned to take account of the motor torques, and all relevant surfaces have a light-absorbing coating. All the analogue electronics use discrete circuitry, and the player employs extra jitter reduction techniques and more separately regulated supplies than CDX2.