OK, so the Accustic Arts' TUBE-DAC II is not the most expensive DAC in the world, but this little baby is hardly cheap. It only handles sampling rates, for example, up to 48kHz, so to most intents and purposes it is half of an exceptionally upmarket CD player, in partnership with the matching Drive I transport (£2,995).

The company behind this assault on the highest peaks of CD replay is Schunk Audio Engineering, a German outfit offering amplifiers, loudspeakers and cables, alongside a top digital source like the TUBE-DAC, and a handful of very slightly less esoteric digital models.

Features in the TUBE-DAC include that well-known 21st-century amplifying device, the thermionic valve, in this case a pair of ECC83 triodes, which is a common enough sight in audio. Used here in a hybrid configuration (which as our own measurements were able to confirm) it differs in some ways from your average valve circuit.

Explaining the cost

But valves aren't that expensive and don't account for the price. A cost that's explained largely by three things, two of which we could see and one we could only read about. The case, made largely of thick aluminium panels is superb and there's a generous sprinkling of ultra-high-performance op-amps, to be precise ten type OPA627. The latter is one of those near-mythical audio components that outperforms standard parts in almost every way. And the part we read about? Accustic's own digital filter, with a 32-bit microprocessor for which great things are claimed.

The 32 bits sound good and so do many of the claims made for this bit of electronic trickery. Accustic doesn't call it a 'filter', just 'digital signal processing', and claims that it's considerably more elaborate than normal upsampling. That may be so, but as far as we can see it's doing a standard upsampling job, with just the same sort of response as most conventional up- or over-sampling players.

Accustic makes much of the additional noise created by regular upsampling, but we've yet to see evidence of that. The firm also seems to imply that normal DACs share digital processing for both channels and so create a slight time delay between them. But we feel that problem (to the extent that it ever really was one) was laid to rest 20 years ago!

So in fact what we have here is a perfectly standard digital filter, one with a slightly sub-optimal performance that does not attenuate rapidly enough as frequency increases above 20kHz. This results in a small degree of aliasing in the output of the TUBE-DAC. Where we will give Accustic high marks is in the elimination of jitter, with the audio output showing no trace of distortion.

As one would certainly hope, everything about the construction of this unit is deluxe, from the very positive control switches to the uniformly high quality of components inside and their immaculate assembly to the circuit board. All the usual high-end touches are there: twin mains transformers, multiple regulated power supplies, nice connectors, even a 'Generalised Impedance Converter' - a nifty circuit configuration for the final analogue filter.

You get both unbalanced and balanced audio outputs and all three flavours of electrical digital input. It's also AES/EBU balanced, which is the preferred mode of connection to the Drive I. Digital outputs are a nice touch, too, facilitating connection of a recorder or remote DAC slaved to the TUBE-DAC's selected input.

Sound quality

We expected some decent sounds from all this high-tech and weren't disappointed. It has all the hallmarks of classic high-end kit, with excellent neutrality, effortless detail and almost tangible grip, authority and control.

It is, of course, impeccably well-mannered and locks on very quickly to the incoming digital signal - whereas some high-end DACs can be confusingly slow.

One of the consistent features we've found across a range of upmarket CD players (and since we spent most of our time with the TUBE-DAC partnered to a Drive I, a CD player is effectively what we were testing) is the high degree of polish they bring to the sound, making cheaper players sound rough by comparison. This one is no exception, joining such company as Meridian and dCS in offering sounds that appear to believe their origin.

During the time that high-sampling-rate, high-bit audio has been slowly penetrating the market, CD recording and replay standards have risen making one question what all the high-res fuss is about. In the practical sense of the terms, this is high resolution. And, thanks to an otherwise uneventful Xmas, we had plenty of opportunity to listen at leisure to Accustic Arts' statement on digital replay and our respect remained high across a wide range of music and recordings.

Among the many discs that we were able to audition on the TUBE-DAC was a newly made CD of operatic voice and piano, recorded so recently that the original sound was still fresh in our ears. The recording itself was excellent, that much was obvious from the most casual listen, but the real extent to which the tenor's high notes rang true was much more apparent via the TUBE-DAC than via our resident (modest but capable) CD player.

The human voice is such a familiar sound that any interference with its complex harmonic structure has a disproportionately large subjective effect, making it a good test of audio equipment. In this case, it was clear that both the recording and replay equipment had admirably discharged their function and the sound was less 'digital' than we were used to.

The jitterbug

It's worth mentioning that, we tried playing the same recording from the hard disc of a computer, rather than from the CD in the Drive I. Computers are renowned for being a jittery source, but there was really very little difference between the two. Accustic Arts may not thank us for saying this, but the TUBE-DAC has very good rejection of incoming jitter and is therefore quite unfussy about the source.

Moving a world away to highly processed contemporary pop, the TUBE-DAC is similarly assured, unfazed by the multi-layered nature of studio productions. Rapid, funky synthesiser can prove a tough test for digital sources, but again the result is confident, detailed, almost calm. Which can come as a bit of a shock if you're used to more coloured
audio kit.

If there is anything to criticise, it's a very slight degree of hardness in the highest registers when reproducing instruments with an extended harmonic structure - a violin being perhaps the most critical. In such cases, the 'air' around the instrument is not quite so pure, the decay at the end of notes a touch less well-defined.

Overall, though, this is clearly a very capable DAC indeed, with plenty going for it sonically and, in terms of ownership, it practically exudes class. This experience has certainly made us keen to visit other Accustic Arts products. And who knows what further treats lie in store?