Audio Research DAC1 D/A converter Page 2
Like many D/A converters, the DAC1's circuit topology includes the ubiquitous Yamaha YM3623B S/PDIF decoder chip (footnote 1) and NPC 8x-oversampling digital filter. Unlike most other digital processors, however, the DAC1 employs what is far and away the best-sounding (and -measuring) D/A converter extant: the UltraAnalog D18400 dual-channel DAC. The D18400 is a combination of monolithic and discrete components encapsulated in a 3" by 2" module. The only better DAC I know of is UltraAnalog's D-20400, a 20-bit converter, variations of which are used in the Stax and VTL converters.
It's fair to say that the UltraAnalog DACs perform like no others: the exacting manufacturing process and calibration procedure is unique to UltraAnalog. Each module is driven with 100,000 digital code transitions and the resultant analog output signal measured and recorded. A computer then calculates the exact resistor trim values necessary to achieve near-perfect values for each "rung" in the resistor ladder. A technician then installs discrete metal-film resistors of the correct value. Because the D18400 is factory-calibrated to such a tight tolerance, no Most Significant Bit (MSB) trimmer is needed in the digital processor's adjacent circuitry. Obviating the need for an MSB trimmer has many advantages: no calibration is required after the digital processor has been built, there is no chance of accidental misalignment, and the unit will stay in calibration regardless of time, temperature, or other conditions (footnote 2).
Such performance doesn't come cheaply, however. The cost of these DAC modules is many times that of conventional IC DACsa fact reflected in the retail prices of other processors that use the UltraAnalog units. The DAC1, however, is by far the lowest-priced converter using UltraAnalog DACs. This was accomplished by using a single dual-channel DAC rather than run two of them differentially as is done in the Stax DAC-X1t and Kinergetics KDP-100 Ultra. Audio Research's goal was to build a digital processor that could take advantage of the UltraAnalog DAC's remarkable performance, yet still be affordable.
Further reducing cost, the DAC modules used in the DAC1 are the 18-bit version rather than the 20-bit type. ARC felt the 18-bit part offered sufficient resolution and musically satisfying results. Remember, designing an audio component to a price involves weighing the relative sonic merits of a huge number of variables. If ARC had used the 20-bit DAC, perhaps the power supply, for example, would have to have been less elaboratea tradeoff the designers felt was unacceptable. Alternatively, they could just as easily have left out the very expensive AT&T fiber-optic input and put the savings into a 20-bit DACthe latter makes the unit more marketable, the former makes the product more musical. In addition, a well-made DAC with true 18-bit resolution is far superior to a 20-bit unit that might perhaps have poor low-level performance. One should evaluate a product's overall design and performance rather than any single parameter. You can be sure that the designers weighed many options before deciding on the final circuitry and parts. Incidentally, we should acknowledge the role of critical listening in high-end product design when determining what parts and techniques produce a more musically satisfying result.
The analog stage occupies the rear right quarter of the pcb. Each of the two (left and right channel) high-current, discrete stages use J-FETs at the input and MOSFETs at the output. As with all ARC products, the semiconductors in the DAC1 undergo extensive in-house selecting and sorting. The TO-220 output transistors are numbered and have colored markings on them, indicating the testing process they have undergone. Each channel has an 8-pin DIP op-amp used as a DC balance servo. The third-order output filter is in the feedback loop of one of the gain stages. De-emphasis is performed in the digital domain, as is polarity inversion. A pair of muting relays shorts the signals to ground so no output appears at the RCA jacks until the unit has stabilized, thus keeping noise and glitches out of your system.
The analog stage design and implementation reflect Audio Research's tradition of preamplifier design. As previously mentioned, all analog-stage DC is supplied from discrete regulation stages, fed by a dedicated transformer. Resistors are 1% metal-film types, and the two capacitors in the signal path are polypropylene and polystyrene.
The DAC1's parts and build quality are extraordinaryeven more so considering the $2995 price tag. In addition, the execution is similarly superb: the pcb is double-sided, with wide, heavy traces; careful layout has eliminated virtually all internal wiring; and the unit exudes craftsmanship and precision. The overall fit'n'finish is beyond reproach.
"Do you think Bob has enough digital processors?" John Atkinson asked Tom Norton rhetorically as they scanned my equipment racks during a recent listening session. Yes, I do keep several digital processors on hand for comparisons with newly introduced challengersall in the line of work, of course.
What first struck me about the DAC1's presentation were its liquid textures and resolution of layers and layers of fine detail. This is one area where good analog clearly beats even the best digital. The DAC1, however, didn't exhibit the loss of fine detail endemic to so many digital processors, instead sounding quite analog-like. Similarly, instrumental textures and shadings lacked the hardness and glare that often plague digital playback. There was a gentleness and ease to the presentation that I found particularly musical. Instruments and voices had a natural silky quality rather than a metallic or dry character. More on this later.
Footnote 1: The Yamaha receiver chip produces between 2 and 5 nanoseconds of clock jitter in a standard implementation. The DAC1 uses a jitter-reduction technique with the Yamaha chip to reduce the amount of jitter in the recovered clock.
Footnote 2: See my review of the Stax DAC-X1t in Vol.13 No.8 for a more detailed description of the UltraAnalog DAC, its calibration procedure, and why such performance is important in D/A converters. There is also a discussion of converter linearity and MSB trimming in this month's "Follow-Up" of the Wadia 2000 and X-32 converters.