Digital Processor Reviews

As explained by Ken Kessler elsewhere in this issue, the English A&R Cambridge company made their name by producing one of the UK's most successful integrated amplifiers, the 40Wpc A60. This neatly styled model was in production for a decade or so and was the basis for a large number of good-sounding but inexpensive audio systems. These days, the company, whose products in the US sell under the Arcam banner, is a major British hi-fi manufacturer, with a product line that includes integrated amplifiers, tuners, loudspeakers, cartridges, and even a CD player. A&R was, I believe, the first UK manufacturer to obtain a player-manufacturing license from Philips, and with the product under review here, has broken new territory for a supposedly "audiophile" company in having a custom LSI chip manufactured to their own requirements.

The Delta Black Box, designed by A&R Cambridge's Mike Martindell, is a stand-alone DAC for use with CD players equipped with a coaxial digital output or with the new generation of CD transports soon to reach these shores from Japan. At present, the Black Box has one digital input, in the form of a standard RCA phono jack that accepts a Philips/Sony standard, 44.1kHz-sampled, multiplexed two-channel, digital serial datastream. A&R recommends the use of 75-ohm characteristic-impedance coaxial cable to connect the player's digital output to the Black Box, supplying a 750mm (29.5") lead with the unit. They counsel against the use of conventional interconnects to carry the datastream, and for those who want to place the transport remote from the DAC box, recommend 10m (33') as the absolute maximum cable length.

Next to the coaxial input jack on the rear panel is a hole for a second input connector, at present screened off with a rubber grommet. The input circuitry and RCA jack is carried on a small printed circuit board that plugs into the main circuit board, and replacement boards with an optical input connector (or with the ability to take data sampled at a rate different from the CD's 44.1kHz, from a DAT machine for example), will be available by the time you read this review. The optical board will cost $149. A small pushbutton next to the data input is to be left in the out position unless trouble is encountered with a particular CD player. (Paul Miller, for example, noted in the May 1988 issue of Hi-Fi News & Record Review that a Technics SL-D990 CD player didn't have a suitable output format.) Pushing this switch in can sometimes help in these circumstances.

The right-hand interior of the slim aluminum enclosure is dominated by two transformers, one each for the digital and analog sections, with a pair of small printed circuit boards attached to the front panel carrying the on/off switch, a switch to change signal polarity, and two rectangular LEDs. The left-hand LED shines green to indicate correct polarity, or red if the polarity has been inverted. All the 110V wiring and switching, including an internal fuse, is well-insulated and safe from prying fingers when the cover is off.

Nearly all the circuitry is carried on one large, double-sided pcb to the left of the transformers, the only exception being the small data input board. The incoming datastream's 5.6448MHz clock frequency is extracted and synchronized with the 11.3MHz system clock, the latter produced by a phase-locked voltage-controlled oscillator. The serial data words are then taken to Arcam's "Black Chip," this a 1000-gate ASIC (Application Specific Integrated Circuit) NGA35037 chip that replaces on the order of 25 conventional CMOS chips. This IC, unique to Arcam, separates the subcode data from the serial datastream, as well as setting the flags indicating whether the signal is pre-emphasized and whether an error is present. It then arranges the interleaved left/right audio data in a form suitable for feeding to the next stage, the 4x-oversampling digital filter.

The inversion of the data bits, which as explained in the November 1988 issue (p.110) has the effect of inverting the polarity of the signal, the absolute phase, is also performed in the Black Chip's output register. By having all this digital housekeeping implemented within one chip, Arcam explains that the amounted of radiated RF hash is kept down to a minimum, as well as improving reliability and reducing the parts cost.

The digital filter is the latest version of Philips' SAA7220P/A chip, which also performs the error correction and interpolation, and the resultant stream of 176kHz-sampled L/R data is taken to a selected Philips TDA1541A twin 16-bit DAC (the only chip to be socketed to allow easy replacement). Unusually, this is placed on a piece of pcb real estate almost totally isolated physically from the rest of the board; it is also independently supported by four Sorbothane bushes to give a degree of isolation from vibration.

It appears from an inspection of the board that every building block in the digital circuitry—system clock, Black Chip, digital filter, and DAC—has been given its own regulated voltage supply, a total of three three-pin LM7805 voltage regulator ICs being present to supply +5V rails, with an LM7915 and an LM337 to provide –15V and –6V rails for the DAC.

The two current outputs from the TDA1541 are taken to a current-to-voltage convertor constructed from discrete transistors running in class-A, then to an all-discrete, two-stage output circuit, which also applies the de-emphasis via a FET-switched network, when appropriate, and the final low-pass filtering, this with a three-pole Bessel characteristic. There are no output coupling capacitors, DC-servo circuits constructed around a pair of LF411 op-amps keeping the outputs at ground potential. A relay mutes the outputs upon switch-on, the muting lifting after about three seconds, while both pairs of output jacks are gold-plated. The analog circuitry is powered with ±12V from a complementary pair of discrete-transistor voltage regulators, these attached to a small heatsink at the rear of the enclosure.

The quality of construction is to a very high standard, with a rational layout, a star-grounding topology, and much use made of metal-film resistors and polypropylene- and polycarbonate-dielectric capacitors.

The sound
I did all my auditioning from the Black Box's higher-level pair of output sockets, this enabling the Mod Squad Line Drive pass ive preamp to be used with its volume control around the 3 o'clock position. All the players used as sources were isolated from vibration with Audioquest Sorbothane feet.