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!