Not keeping tabs on other reviewers, I can't say if it's normal practice to use as a reference something which members of the public cannot purchase. Hot-rodded hardware, prototypes which never made it into production - there are plenty of wild one-offs to tempt reviewers but which actually serve no purpose if the readers cannot hear them in the shops. It's why, for example, I'm always afflicted with a massive guilt complex every time I write about the Marantz CD-12/DA-12 K.I. Signature.

It was OK when my CD-12/DA-12 was standard form. Hundreds were sold in Europe, and the Philips 1000 sold in greater numbers is said to be similar. But then I had the Marantz tweaked within an inch of its life...along with only a half-dozen other lucky souls who got in there before the chip supply was exhausted. So I cast around for something I could use alongside it, something which was its sonic equal, but with unrestricted, 'real world' availability. And I didn't have to look far, because Theta's Pro Gen V had been in daily use in my main system ever since it first arrived, a couple of years earlier.

No, I hadn't deliberately neglected it, forgotten it, dissed it or ignored it. It's just that, well, I suffer from the same thresholds as everyone else and I kinda took it for granted. To my shame, it had been reduced to acting as the DAC in my home theatre system, processing laserdisc sound more often than CD signals. I stand rebuked, because it deserves much better. And, as if to remind me in spades what a callous putz I'd been, Theta went ahead and uprated the model to Gen V A status, demanding a reassessment.

As Theta maintains a policy of anti-obsolescence, the Gen V in my possession went back to the importer for a retrofitting session. When it left it was a 37lb, 19x3.5x15 5/8in (WHD) gunmetal gray box with a minimum of frills. Unlike the very first models, it was HDCD-equipped, so the centre recess contained three rather than two blue LEDs. These indicated power on, the detection of an HDCD signal, and general signal lock-on. The three LEDs are flanked by two pairs of slim toggle switches. Those on the right choose power on/off and polarity inversion (hooray!!!), while those on the right choose between tape and source, and between three digital inputs. That's it: classic frill-free DAC front panel sparsity.

Not that the back offers much more for those who assess a product's worth by the number of fitments. Two versions of the Pro Gen V have always been available - with or without balanced outputs in addition to the standard sockets - while the version in my possession has always been XLR-clad. Where the fun comes in is with the selection of input types. In bog standard form, a Pro Gen V comes with coaxial RCA (phono) sockets for (digital) tape in and tape out, and with three coaxial phono sockets for digital signals. The all-coaxial unit I have wears a phono socket, a BNC and an XLR for inputs 1, 2 and 3; the optical options include TOSlink and AT&T. So, while I cannot use the Pro Gen V (or the Gen V A, since I stuck with the original sockets) for AT&T-equipped optical transports, I can always stuff a TOSlink into it via the Theta TLC, which can convert TOSlink to coaxial.

In its non-'A' form, the Pro Gen V uses Burr-Brown 20-bit DACs, over-specified double-regulated power supplies, proprietary ROM chip-based software and Theta's own fully discrete Class-A analogue stages devoid of ICs and with very low overall feedback. The resultant device has always been a disarmingly smooth, 'undigital' converter which - like the original California Audio Labs Tempests, the Marantz CD-12/DA-12 and many other top-flight CD playback systems - did its best to divest the sound of a synthetic feel. What I always loved about the Pro Gen V, especially in the context of watching and listening to a two-hour-plus laserdisc with its invariably heightened sonics, was the Theta's relaxing, gracious, non-aggressive stance, its 'hospitality' if you will. Which, I hasten to add, reinforces my belief that you can sense the personality of a product's guiding light in its sound. And Theta's Pro Gen V is very much an audio version of Neil Sinclair.

Upgrading complexity from an earlier Pro Gen V depends, naturally, on the state of the donor machine, perhaps requiring even a complete motherboard, but the bottom line is that the upgrade is comprehensive - not just changing a few resistors. [See box.] What's comforting is that the turnaround is quick and it's performed here in the UK; existing owners will not be without their Thetas for long. Buying from new, in a way, is a bit of an anti-climax, because you won't get to experience that delicious thrill of hearing an old, faithful component's rejuvenation. And in this case in particular, the gains came where they were least expected. By my reckoning, single most important area of improvement denoted by an 'A' is an added level of refinement, not that the original was ill-mannered.

At the risk of making the original Pro Gen V sound like a wussy's dream, its silky smoothness and lack of grain (even compared to the Marantz DA-12 K.I.) were such that I'd imagined it couldn't be taken any further without possibly affecting detail, transparency and, eventually, dynamics. It would have meant 'reeling in' the sense of limitless performance, in some attempt to make the sound even sweeter, more digestible, more 'analogue'. What the 'A'-status Pro Gen V does is improve the coherence, the seamlessness, the 'wholeness', without compromising any of the machine's other virtues.

Using the Theta Data III, Marantz CD-12 and Marantz CD-63SE transports, with the GRAAFiti WFB TWO pre-amp, GRAAFiti 50/50 power amp and Quad ESL63s, I settled in for sessions which I would have wagered to be stripped of any unpleasantness. I played Big Daddy's Mathis-like 'With A Little Help From My Friends', plenty of Dino, a dollop of Emmylou Harris and much Joan Baez, chosen specifically as examples of source material which are sweet almost to the point of a case of Canderel. Would the resultant performance be so pale, so pastel, so sugary as to lack any bite at all?

No way. With the skill of a chef juggling chillis and saffron in the same dish, the Pro Gen V A ensured that all the flavours were there to be sampled by the non-smoker's tongue. So far, so good. The pseudo-Anita Kerr background vocals on the Big Daddy track took on a new sense of scale, a more enveloping wash of sound behind the main vocalist, with each voice more clearly separated from its fellows - and yet there was no sense of heightened hygiene. It's an odd sensation, one sampled best in a recording containing numerous overlapping, complementary sounds rather than in a sparse recording which doesn't challenge a system in that particular area.

Always a champion when it comes to conveying three dimensions, the Pro Gen V A seems to create an even larger soundstage. And it's a uniform increase, depth and height and stage width all up by a few noticeable and similar percentage points. What also changed in this area of presentation is a relocation of the 'line' of the sound. Played through the WATT/Puppy System V.1, the pre-A Theta placed the key sounds in line with the speakers. In the A-state, the sound has been moved back a row, a boon for those who don't want any trace of in-your-face forwardness. This, too, contributes to the notion that the Theta is especially polite and well-behaved, despite the company's wares often showing a dB or so more output than is the norm.

Bass extension remains unchanged, but the absolute control and the nature of the decay on acoustic bass notes exceed the capabilities of the non-A Pro Gen V. What this gives the techno lover is a new bass 'dynamic', a slam with attack and crispness but devoid of aggression. Hard to explain, I know, an oxymoron just begging to be attacked, but it's one of those inexplicables like hearing a system at far louder levels than you prefer, yet which somehow doesn't make your ears ache.

The move from Gen V to Gen V A is not a subtle, minor refining of a great product. It actually warrants calling the machine Pro Gen VI. The price increase makes the single-ended version a £4300 purchase, the balanced edition a cool £6500, but this is reflected in performance which leads me, for one, to regard the Theta as among the very best standalone converters on the market. And, unlike the peerless but unobtainable/unrepeatable Marantz DA-12, you can actually buy one.

Absolute Sounds, 58 Durham Road, London SW20 0DE. Tel 0181 947 5047.

BOX: FROM GEN V TO GEN V A

Here, in Theta's own words, is what defines A-status:

"The analog section of the Generation V A contains 52 transistors. Through a laborious process, six matched octal sets (totaling 48 transistors) plus the remaining four uncritical transistors are now installed in every analog board. This matching process includes testing tens of thousands of devices and then hand selecting the ones that meet stringent criteria. The result of this work is a sweeter, more detailed presentation and less listener fatigue.

"In-depth research into analog filters has also wrought surprising improvements. In a digital-to-analog converter, it is imperative to filter out the sampling and higher frequencies from the analog output. Our previous designs have implemented inductors to achieve this effect. Recently, we found that the elimination of the inductors improved sonics as long as the filtering was properly performed. New filter topologies that are just as effective at eliminating the sampling frequency yet do not utilise inductors were created. Wima capacitors were found to be complementary to the new filter topology. The sonic improvements are not subtle. Less sibilance, tighter image focus and better tonal quality are immediately apparent.

"Theta Digital has always been at the forefront of jitter-reduction technology, the first implementation of which occurred in the original DS Pre. From that time on, we have continued our research into the effects of jitter and the meaning of lower jitter specifications to reproduced sound. A PLL works by continuously comparing an input signal (in this case the word clock which has been extracted from the SPDIF data stream) to a controllable oscillator's output and then producing an error signal which corrects the oscillator's frequency to precisely match the incoming signal. Some of the timing errors (jitter) present in the incoming data stream are always 'passed through' the correction part of the circuit and, as a result, adversely effect the oscillators output. The trick in PLL design is to minimize these jitter effects while allowing the correction part of the circuit to perform properly. A result of our more recent research into this area is the third part of the 'A' improvement. A more seamless sound, along with more detail are the most obvious improvements that come with the new PLL.

"The final part of the 'A' version of the Generation V is a new algorithm. Approximately two years ago our algorithm designer proposed the idea of adding dither to the program run in the Gen V. This, he proposed, would make the D/A converters operate in a more linear fashion at lower levels. Since that time, continuous listening tests based on dither variations have been conducted. The product of those tests is the new algorithm. Better low level resolution and better decay are clearly evident with the new program."