Meridian 208 CD player/preamplifier
Philips having already committed the 14-bit design to silicon, they would not have a 16-bit DAC ready in time for the medium's launch in the Fall of 1982. They were thus faced with the problem of squeezing four times the resolution from their existing 14-bit DAC. The result was an ingenious digital filter that combined 4x-oversampling and noise-shaping—the latter is effectively a digital feedback loop, the error produced when the filtered digital data are truncated to 14 bits being fed back to the beginning—to give a digital system with full 16-bit resolution.
Philips's true 16-bit DAC chip, the TDA1541, followed in 1985, but the seeds of ingenuity had obviously been sown: if the combination of oversampling and noise-shaping can increase the resolution of a DAC using too few bits, then why not go all the way and implement a system that used a simple 1-bit DAC and make up for the shortfall in resolution by taking the oversampling, noise-shaping process to the limit?
The result was a D/A system, internally referred to by Philips as "DAC3" (the two earlier systems were DAC1 and DAC2, of course), which was introduced in the summer of 1989. I discussed the design of the system in detail in June 1989 (Vol.12 No.6, p.57), but briefly, the SAA7321 DAC3 chip massively oversamples the input data at a 256x rate, interpolating the new sample values to produce a 17-bit datastream sampled at 11.02MHz (footnote 1). The data words are then fed to a 1-bit DAC, with the 16-bit error fed back in a noise-shaping loop. Mathematically, this should—and does—result in the pulse stream output by the DAC having the full 96dB+ dynamic range of a conventional 16-bit system. But unlike multibit systems, this "Bitstream" DAC is inherently linear and monotonic over its entire range—the reasons why were given in Peter Mitchell's "Industry Update" in January 1990 (Vol.13 No.1, p.36)—requiring no laser-trimming of on-chip resistor values or in-production adjustment of linearity, both of which add to a CD player's manufacturing cost, hence price.
I was told by Philips last year that the Bitstream DAC was therefore intended to be used in low-cost and portable players, the company saying that they would remain with their TD1541-based chip set for high-performance players. Very rapidly, however, once designers had tried the Bitstream approach, it became apparent that, correctly implemented, it could surpass traditional D/A conversion in the preservation of low-level detail. (Even Philips has now introduced a Bitstream player, the LHH500.)
Hard on the heels of the introduction of Meridian's 206 CD player, which I favorably reviewed last July and which uses Philips's conventional multibit DAC, the English company redesigned their 207 CD player/preamplifier to incorporate the Bitstream DAC. The result was the 208, the subject of this review.
Superficially identical in appearance to the 206, the 208 consists of two extruded aluminum chassis faced with glass and bolted together. The 208's transport is on the left, this sliding out in its entirety to allow discs to be loaded and unloaded. When closed, the transport is sealed against airborne vibration. The right-hand side carries function buttons, some of these lit by LEDs when operated, and a green 7-segment display panel. Three LEDS also display signal polarity, disc pre-emphasis, and errors. Two yellow buttons on the far right control volume up and down, there being 64 discrete steps as well as a mute function.
There are also source-select buttons for analog line, tape, and the optional phono inputs, these latching on the first push, reverting back to the CD output when pushed a second time. Although there is a back-panel power switch, Meridian recommends the 208 be left on continuously, a front-panel Standby switch shutting the display down but leaving the circuitry powered. All front-panel functions are duplicated with the supplied model 209 remote handset, this also having an absolute-polarity button, fast search buttons—unusually, the output mutes during search—and a numeric keypad for direct track access.
The rear panel carries three pairs of analog outputs—fixed from the CD player, variable from the preamp section, and tape out—and three pairs of analog input phonos. There is also a headphone output. A pair of TOSLINK optical data outputs are provided, as is one coaxial phono data output. There is also a DIN "Communications" socket to allow the 208's functions to be controlled by the remote supplied with the Meridian D600 "digital" active loudspeakers.
The SAA7321 incorporates two channels of digital filtering and DACs as well as output op-amps; a single chip can thus be used for the complete analog signal reconstruction for a stereo player. Meridian uses two chips in a more sophisticated manner, however. Each is fed both the normal digital datastream and an inverted version of that data. One chip outputs pulse streams representing normal and inverted analog left-channel signals, L and -L, and the other pulse streams representing R and -R. These four pulse streams are passively low-pass filtered, reconstructing the analog waveforms in the process; then, rather than being fed to the on-chip op-amps, they're taken to two NE5534 integrated-circuit op-amps, these low-noise types with a good output drive capability. The two inputs of each op-amp are fed the non-inverted and inverted analog signals for one channel; its output therefore represents the difference between the two signals, and any even-order harmonic distortion or correlated (common-mode) noise will be canceled as a result. Departures from linearity common to both DACs on the one chip will also be canceled, with increased low-level resolution being the result.
A second 5534 appears to be used in this stage's overall feedback loop, while an LF353 op-amp provides a DC-servo action to keep the direct-coupled output's average value at ground potential. FETs pull the output to ground when the mute button is pushed.
As with other Meridian products, the construction is first-rate. All the audio circuitry is carried on computer-grade, four-layer printed circuit boards, these incorporating a full ground plane. Much design attention has also been paid to the Philips transport mechanism, and a high-precision central quartz-locked clock is used to minimize the effects of data jitter (see later).
Footnote 1: Meridian launched their 602 transport and 603 D/A processor at the Summer CES in June, the latter using Philips's second-generation Bitstream DAC chip, the SAA7350BS. This runs at a 384x-oversampling rate of 16.9MHz and allows its user to match it with a proprietary digital filter. These products will be more expensive than the 200-series components, however.—John Atkinson