D/A converter 

Originally printed in the August 1994 issue of Gramophone.

The conventional CD player is packaged as a single box but it really consists of two complementary sections between which undesirable interference is possible. The first comprises the mechanism and control electronics which spins the disc, tracks the laser source over the disc surface and converts the bursts of reflected light into a digital signal. This we can call the "transport". The second is purely electronics and changes this digitally encoded signal into good old-fashioned analogue, at about 2 volts peak, suitable for feeding to any of the line-level inputs on an amplifier or receiver. This is the DAC (Digital-to-Analogue Converter).

Surprisingly soon after the 1982-3 launch of CD, some of the more up-market players began to appear with a choice of output sockets—a standard pair of L/R phono sockets labelled "analogue out" plus new "digital out" terminals which allowed the user to bypass the built-in DAC and feed the digital bitstream direct to a separate and hopefully superior interference-free DAC. The latter could be a stand-alone unit or, occasionally, arrive as an integrated component in a 'digital ready' amplifier. One idea was that digital sound sources would proliferate and the separate DAC would cut costs as the source units could be built with only digital outputs. Well, we do now have lots of digital media — CD, CD-I, DAT, DCC, MiniDisc and cable/satellite radio for example — but these steadfastly continue to incorporate their own DAC so the cost-saving motive has not really worked out in practice.

However, the second motivation has caught on: this is to squeeze as much hi-fl performance as possible out of the CD medium by optimising the designs of the transport and DAC as separate units. These tend to be price-no-object designs and too expensive for most consumers. Audio Alchemy in California have produced a series of such products, including some at relatively modest prices but the unit reviewed here breaks new ground.

The DAC-in-the-Box, which they abbreviate to DITB, is almost incredibly small and affordable at £200. It therefore suits the budget conscious buyer who is less ready or willing to shoot for audiophile pinnacles of perfection but simply wants to upgrade an existing CD player or LaserDisc machine whose built-in DAC may not be of recent or high-class design—but nevertheless has a digital audio outlet. The DuB main case measures only 135 z 28 x 98mm, with the front panel and plastic feet extending this by a few millimetres all round.

There are no switches or controls, the fascia simply carrying identical red LEDs to indicate "Power on" and "Locked" to a valid digital input signal. On the rear panel are both coaxial and Toslink optical digital input sockets (equally responsive to digital signals at sampling frequencies of 32, 441 or 48kHz), a pair of phono sockets for L/R analogue output and a mini socket for the DC power input. The latter comes from the DITB's in-line power supply unit which consists of an encased transformer/rectifier with captive mains in and DC out cables each about five feet long. Actually, the Power Station One included in the price of the DITB (in the UK only) is a superior model listed on its own at £60, making this package quite a bargain. The mains cable is terminated in a two-pin moulded plug which should be replaced with a standard 13-amp three-pin for use in the UK unless you have a suitable adaptor. The long cables and tiny dimensions of the DITB enable it to be hidden away and (almost) forgotten. Heat dissipation is not a problem as the DITB runs only moderately warm when carrying a signal (perhaps screening was given a greater priority than ventilation when the unperforated case was being designed) and the power unit remains stone cold. If both the coaxial and optical digital inputs are in use, say to connect both a CD and a LaserDisc player, the system automatically switches to the source which is 'sending', and it makes obvious sense to switch off the other unit when not required. However, if both sources are actually playing, the DITB defaults to the coaxial input.

Construction is reassuringly high in quality, though the plain black finish and projecting bolt heads will appeal more to engineers than to home stylists. Internal assembly on a single substantial PC board is immaculate, with all components labelled. It uses 18-bit eight-times oversampling conversion via high quality proprietary input and converter circuits, and a Bi-FET Class A analogue output configuration, to produce a well above average performance specification.

While run-of-the-mill audio or video interconnection cables will work quite well as coaxial digital connectors, there is a good case to be made out for employing special leads designed for the several Megahertz signals met in digital audio. Path Group, the UK distributors for Audio Alchemy, supplied a sample British-made Kontak Link interconnect cable (£49.95) for which they are worldwide distributors, and I am bound to say that it performed supremely well. The gold-plated plugs have an additional locking cover (a really good idea, especially when the cable weighs nearly as much as the unit itself) and low impedance is maintained over a 800MHz bandwidth using identical OFC conductors in both the send and return wires.

Performance

Despite the increasing climate of opinion which denigrates audio measurements and points to their inability to predict or even correlate reliably with the sound quality as heard, I never regard technical testing as a waste of time. At some stage, therefore, before or after the admittedly more meaningful listening sessions, I like to check the makers' specification point by point. Without 'testing to destruction' I can often get a better feel for the unit's ability to stand up to rough handling and the quality of construction and electrical alignment than is possible from mere music listening. There is the added benefit that objective tests can be carried out to international standards on an individual unit in isolation, whereas 'listening' to an amplifier, for example, is only possible when a CD deck or other source and chosen loudspeakers are hooked up, with their own idiosyncrasies. Then there is the effect of the listening room...

The DAC-in-the-Box checked out superbly well. Each specification claim was exceeded in practice, with the insignificant exception that low-level linearity began to drift off from –90dB downwards. Violent shaking of the unit introduced no apparent glitches. Subjective tests involved a number of CD decks and my workhorse Quad 44/606 amplifiers and ESL-63 loudspeakers. In all cases the DITB sound came over as lively, exciting and with an overall warmth and presence that seemed a millenium away from the tight dry sound we were offered by the first generation CD players. Compared with the sound from the analogue outlet on my 1987 vintage Philips CD960, for example (one of the best sounding designs from CD's inventors), running the DITB from the CD960's digital outlet produced a less pinched effect in the treble, better focused bass and sharper attack.

I have no doubt that the best of the two and three box CD systems can leave the DITB well behind in ultimate performance—at a price.

But this £200 upgrade could give most CD players a new vitality and musical realism. If the idea appeals to you, get your dealer to provide a demonstration. What would be interesting would be for Audio Alchemy, or somebody else, to market a £200 CD transport-in-abox to match the DITB and let everyone into the joys of affordable two-box CD t Specification (test results in brackets) Frequency response 20-20,000Hz ±0-1 dB (agreed) D/A conversion multibit, 18-bits resolution Total harmonic distortion 0-007% (0-005%) Signal-to-noise ratio 100dB (105dB A weighted) Channel separation 85dB (91 dB) Linearity at -90dB: ±0-5dB (2.5dB) Output 2-5V (2.6V) Dimensions (I/V x H ,< D): 135 x 28 x 98mm Manufacturer Audio Alchemy Incorporated, 31133 Via Colinas Ill, Westlake Village, CA 91362, USA UK distributor Path Premier, Unit 2, Desborough Industrial Park, Desborough Park Road, High Wycombe, Bucks HP1 2 313G. Telephone 0494 441736 UK retail price 2199-95

John Borwick