NAIM NAP 500 POWER AMPLIFIER

Naim’s long awaited all-new flagship power amplifier sets the pace for the 21^st century.

By Martin Colloms

The development of this completely new power amplifier has been a major undertaking for Naim. Up to now, its designs have been essentially variants on the late Julian Vereker’s founding concept, which led to the first NAP250 stereo chassis some 25 years ago. Even now, the company’s biggest current model, the NAP135, is a monoblock variant of the ‘250, with more generous power supplies allocated to the separated channels, and fan cooling. The output ceiling has remained at 70-80 watts per channel for this series of amps. As Julian pointed out to me some 20 years ago, you could get a more powerful amplifier by operating a ‘250 in mono bridge. The inverting nature of the circuit allowed you to connect the output of one channel, suitably and resistively attenuated, back into the input of the other to get a self-contained, single-ended input, mono 250W amplifier.

At that time, Naim did not readily support this mode of operation, since prolonged drive with less than truly comfortable 8 ohm speaker loads quickly warm-up the ‘250 and result in premature safety shutdown as the case-sensing thermal trip took control. And as if that were discouragement enough, it was considered that bridge working detracted from the amp’s innate speed and bass grip.

Louder, more powerful Naim hi-fi systems have generally meant active crossovers, operating with multiple amplifiers working in packs. So it’s perhaps ironic that Naim’s newest and most powerful amplifier is designed from the outset to operate in bridge mode. This was a design goal from the outset, with the bridge topology in part dictated by a special development of output transistor.

The carefully-honed Naim power transistor technology of the 1980s and ‘90s was coming to the end of its life due to the changes in the available silicon processes. The project began in earnest with a search for a new type of output transistor, which would see the company through into the first decade of the 21^st century at least. There’s no equivalent to the original Naim transistor, a custom-built very fast TO3 device with heavy-duty internal terminations. And it was essential that the new device should have the desired sound quality potential.

Exhaustive selection and subsequent fine-tuning led to a good choice from Semelab, but one where the favoured operating area was a combination of higher current and lower operating voltage than is customary these days. This aspect primarily drove the design to bridge mode, where each channel comprises two high current ’40 watt’ amplifiers operating in bridge differential. There’s upwards of 150W of power per channel under normal conditions: a worthwhile gain of some 4dB in power headroom over a NAP135.

The new NAP500 is Naim’s most expensive power amplifier by a substantial margin. Priced at 10,000 pounds, it is intended to be seen as a reference model, and was guided through its long gestation by Julian Vereker himself; he signed it off for production late in 1999.

Under development, it became clear that the amplifier was reaching beyond anticipated areas as it rapidly outgrew Naim’s traditional alloy box casework. Short signal paths meant a compact construction; high power implied the requirement for significant heat dissipation.

The need for compactness has led to an advanced design with deeply finned integral heatsink casework, and the opportunity was taken to add some industrial design to the mix, finally breaking the Naim mould (or more precisely, the extrusion die for its slab cases). The NAP500 amplifier enclosure is made of beautifully machined, anodised alloy panels finished to a high standard. For access, the side panels hinge away elegantly. Since a proximate integral power supply was found to damage sound quality, there is a separate PSU enclosure, also in machined alloy. Two hawser-like cables with military-grade locking connectors link the units. The smaller power supply box is of a simpler form, not required heat-dissipating fins. Mains power via a single IEC cord.

Looking at the amplifier casework, you’d be right in thinking that the fins were insufficient for a unit which should deliver a total of 0.6kW or more into a pair of 4ohm loads. The explanation lies in the installation of a low-noise fan for forced ventilation. It doesn’t stir at idle and normal powers, but if the amplifier is taken towards its limit the temperature-sensing system switches in the fan.

TECHNOLOGY

Naim explains that this amplifier was required to deliver twice the power, or 3dB more than its existing models; the new level was to be nominally 140W into 8 ohms and also be comfortable with reduced 14ohm loads, where the output rises to 280W/ channel respectively even with continuous running. Early work had shown that available combinations of parallel or series output transistors were unsatisfactory, and that a new single device of great power was required; and this was finally obtained after six years of co-development. The British-made Semelab device proved so powerful that sonically invasive protection devices could be left out. Bridge design not only ‘balances’ the output circuit; it ‘balances’ the power supply so that return currents from the speaker do not flow through the ground system.

Weakness in this area has consequences for the dynamic range and noise level of the input signal; but circuit ground practice is a critical issue. Indeed, reduction of induced electrical noise in any form is one of the key design platforms for the NAP500.

The concept of fast, high-quality supply regulation, long used in Naim’s top amplifiers, has been taken further here. The toroidal transformers and the first stages of the power supply are (as noted) remote from the power amplifier and linked by heavy duty multi conductor cable. There are ten stages of local regulation per channel, the supply box providing a total of 12 separate DC lines to the amplifier proper. Compared with the NAP135, the ‘500 is claimed to have five times greater open-loop gain, a quarter of the distortion and a dynamic noise floor 100 times reduced. Specification claims for this fully regulated power amplifier are: greater than 140W per channel 8 ohms; voltage gain 29dB (as with the other Naim amplifiers); a modest input impedance of 18k ohms; a 1.5Hz  to 100kHz frequency response (limits unspecified); and an economical 60VA idle power. Judging by the case temperature I judge the idle power to be still less.

A 29dB gain is close on 30 times., so a CD player’s 2V output can give ‘60V’ from the amplifier. This means that, if you wish, the ‘500 will operate successfully with a passive line controller.

Internal build quality is excellent throughout and, as with its predecessors, as much design thought and care has been lavished on the high-speed, low-coloration regulated supplies as on the power amplifiers themselves. The power unit  has a single unit 1.2kVA toroid specially built to Naim’s specifications for both sound and quality and minimal mechanical noise- often a problem with good-sounding toroids. Separated secondary windings feed the two channels.

Armed with the powerful , discrete circuitry buffer amplifier developed for the CDS II, Naim chose an unusual route to create the balanced signal internally for bridge mode. Instead of using a bi-phase stage, one section of the bridge amplifier is operated as a shun feedback current input, the other in voltage mode, so generating the required balanced phase across to output terminals. In overall concept, the amplifier is actually constructed as two mono amps in one main enclosure.

The CAD modelling was so accurate that Naim has been able to remove pre-set components, e.g., for bias, which have a finite reliability. In production, the amplifiers are set up, then instead of the previous preset components, aligned and selected resistors are soldered in place, resulting in improved sound quality and reliability. The special power transistors are 350W devices with a 70amp rating which Naim has chosen to operate singly, well into the safe operating area to allow omission of the often-used protection relays. In fact, a total of 16 of these very costly transistors are fitted, eight per channel: four for the regulators; and two for each mono amplifier section, forming one of the bridged channels. The classic circuit H C Lin-based circuit is operated as quasi-complementary with the usual linearising diode present in the relevant signal path.

The amplifier was modelled over some months in B2 Spice, an electronic CAD programme , and using device models specified by Naim. Supply-line noise was considered to be a vital issue, and by painstaking analysis the sources of supply and other noise where tracked down, and solutions found. At high power, eg, 200 watts into 4 ohms, the supply noise is better than 125dB down, a major achievement.

SOUND QUALITY

To begin with, I tried out the NAP 500 in an all-Naim system with the NBL loudspeakers, and was thrilled by the degree of edge-of-the-seat excitement and drama. Maximum level for the system was felt to be on a par with the bigger and more efficient DBL speakers when fed by six actively coupled NAP135s. Dynamic range is a key quality of the NBL  speaker: an ability to read fine low level detail in the presence of heavy, complex modulation. With the ‘500, that quality was even more clearly registered. Some synergy was occurring!

The NBLs didn’t suit the wide open spaces in my studio listening room, so I moved them down to a ground floor room which has a long brick wall, ideal for speakers, which find a solid boundary more to their taste. The next part of the test saw the ‘500 working in my reference listening system, before I reunited it with the NBL/Naim system downstairs.

The NAP500 needed careful matching to do it full justice, and even then I couldn’t be certain the end result wasn’t influenced by the foreign interfaces (foreign to Naim that is).

When slotted into my audiophile system, my reactions to its sound were mixed. A special cable set was made up for me by Van den Hul UK to allow me to link the Conrad Johnson and ART pre-amplifier to the Naim via uniquely wired single-channel XLR plugs. I chose Van den Hul Intergration, first series, which has always given me good results in the rhythm and timing department; additionally, it meant that comparisons with other amplifiers where much more straightforward.

I used Naim NAC5, 4mm to spades for speaker connections, as well as Van del Hul 352 and Transparent XL with Monster XL locking adapters, 4mm to spade. Hard-wired phono-to-XLR adapters allowed me to try a range of other interconnect cables including Transparent XL, though I felt that the dedicated Van den Hul option was preferable in this case.

There’s little doubt that if Naim wants wider acceptance of the NAP500, then its connection practice should be made more universal – even on a customer option basis. (I said as much in my review of the admirable CDSII ). When all was optimised, the ‘500 at last showed what it could do. The drive to the Wilson 6 was to a high standard, confident, with a wide dynamic range and consistent in quality over a wide span of loudness. It didn’t cave in or compress at peek levels, and its ability to hold tight to complex detail, irrespective of power, was a hallmark of its high quality design.

There were major strengths, which remained evident, in or out of a Naim system. Dynamics were lively and sparkling. Rimshots, small drums and delicate percussive nuances were all very well expressed, with explosive power when required and great subtlety on finer-grained transient sounds. Timing was undoubtedly very good by international standards, and rhythm line was firmly and strongly carried in the best Naim style.

The sound was up with the best for listener involvement but fell short of the higher standards for certain other, more cerebral aspects of fidelity. So far, the Wilson System 6 loudspeakers have proved to my most transparent and revealing speakers, showing that greater stage width, depth and focus than the ‘500 could provide was possible with them. The Naim did not score so well in this area and was surpassed, albeit at higher cost, by a Krell FB600 – also possessing  a deeper reaching ‘chunkier’ bass than the ‘500.

Picking at detail, the ‘500 bass was nevertheless very good: notably articulate and resolved, well extended , load capable and fleet of foot. There was a trace of ‘roundness’ in the mid bass (an added warmth if you like, for example when compared with a big Krell, or for that matter the Naim NAP 250).

The mid was open and crisp, highly neutral, not creamy as was the case with the Krell FPB series. It was mildly projected, faintly nasal, reminiscent of the ‘250 but generally sounding more neutral than the ‘250.

Through the treble the ‘500 was very pure, beautifully balanced, light and sparkling, lacking the marginal ‘dryness’ of the ‘250.

In my reference system, and applying my usual listening-test scoring, I put the ‘500 a little ahead of the ‘250 on points, perhaps 27, a lead which it must be said was partly the result of the increased maximum level (always worth something) as well as a wider perceived dynamic range.

In the full Naim system the demands were noticeably different, and I felt that the ‘500 was definitely more at home. Frankly, I couldn’t lay my hands on, even think of, a better amplifier to drive the NBLs – a DBL, for that matter. However it would be tantamount to overkill for the smaller speakers.

Some synergy was afoot with the NBL; the combination was sweeter and better balanced than with a ‘250, while other amplifiers could be heard to clamp down on the known dynamic range of the system and perceptibly slow the beat.

The ultimate margin over the ‘250 was easy  to appreciate at decently high sound levels, where the capable ‘250 didn’t have the grander scale, tonal grace and reach of the NAP500; with the ‘500, the NBL sounded as if the bass units had grown and there was more detail and purity to be had.

Conversely, if the system wasn’t driven too hard, the older ‘250 held up superbly well, seeming a touch quicker on its feet, sharper focused and more ‘direct coupled.’ I for one would find it hard to choose between activation with a NXO and three-pack ‘250, or alternatively, passive operation via a single ‘500. Substantially more sound level with the ‘500 is one answer; but if you are a successful dotcom entrepreneur or have a lottery win you could go active with multiple 500s. I imagine such a set-up would break a few hearts among the Naim fraternity!

This amplifier definitely warms up. From new it verges on the clinical, with a highlighted treble. After a week or so, the full bloom and natural balance developed, and I felt I could leave it on indefinitely. Even after long running it was still really cool. It is clearly very efficient and idles at very modest standby power: a welcome feature.

With the NAP500 and NBL in combination, the percussive quality of the bass was outstanding, like a fist to the chest, equally outstanding were its speed and articulation. More comment on the NBL must be left to a forthcoming review, but this speaker, when referenced against its sound with other amplifiers , did help illuminate further aspects of the ‘500. It was the fastest, best-timed I have heard at the power, while the full bore dynamic range was simply outstanding.

It defines the art in this respect, bringing this often neglected aspect of amplifier design and performance into clear relief. Hearing the ‘500 reveals exactly what effect supply line modulation has on sound quality. The great stability of its stereo soundstage, solid  and immovable no matte how hard the going, was perhaps also to be associated with this aspect of the amplifier. You felt that immense grip and control were exerted on the loudspeaker. Particularly when partnering complementary Naim equipment, the NAP 500 had an upbeat lively and involving sound, and in a Naim system I gave it an overall, highly creditable, score of 33 points.

CONCLUSION

The lab results describe a top quality amplifier with low noise and distortion, fine stability, wide bandwidth and excellent dynamic range. It proved relatively easy  to drive, and was load tolerant down to 4 ohms. However, its electronically defined peak current limit ruled out really difficult loads. But it does have about 50% more peak current  than previous Naim designs and is also about 3dB louder.

In the listening tests this amplifier proved a class act as part of a Naim system, delivering all that was promised- admittedly at a price. No Naim aficionado could fail to be enthralled by the superior dynamic range and power, the improved neutrality, more open treble and deeper bass.

However, in my view this great strength is also this amp’s Achilles’ heel. The NAP 500 rated proportionately lower when used outside a Naim system.

Out in the audiophile world, its fine dynamic expression and strong rhythm place it in the highest category; but at the price and in its context, for me this amp misses the bull’s-eye in respect of stage width, transparency and image depth. Here it rates as ‘good plus to very good’ rather than excellent. To conclude: this costly amplifier establishes the state of the art for Naim systems and makes sense for Naim enthusiasts.

Taken out of this Naim-system context it is still a worthy example – of excellent build and finish, strongest on listener involvement – but is complicated by no-standard connection practice and a degree of non-alignment with the received balance of sound quality parameters considered to represent the high end.

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