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.

Read Theta Digital reviews including the Compli, Theta DaVid, Dreadnaught amp, Dreadnaught II amp and Casablanca AV preamp.

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.

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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.

Read more about he Gen V-A on Page 2.