Meridian 203 D/A processor Robert Harley page 1
In 1984, just after the first CD player made its way to America, I remember hearing about a small British company which modified Philips CD-100 machines to make them sound better. This idea created quite a stir and was looked at somewhat askance by the majority of a populace who were convinced the CD medium and players were perfect. "But how can you make something that's already perfect better?" I can still hear them saying. This sentiment was echoed by the mainstream audio press, a key factor in the Compact Disc's successful launch.
Six years later, we can now look back and either laugh or cringe in horror. The same writers and magazines who promoted the CD as "perfect sound forever" and embraced early CD players now refer derisively to the inferiority of those first machines. In just a few short years, we've witnessed a radical transformation of digital playback technology, fueled by a rethinking of conventional wisdom. The search is far from over, however, as each new discovery leads to vast unexplored areas which defy the sensibilities of digital audio theory.
That small British company, in case you don't remember, was Meridian. Engineer Bob Stuart applied his talents to fix what the CD's designers considered insignificant. He proved that there was room for improvement in this "perfect" medium, and was at the forefront of working with this new technology. His ideas were vindicated by the enormous success of both of Meridian's modified versions of the Philips CD-100, the Meridian MCD and MCD Pro. The large number of MCDs sold is even more surprising when one considers the total number of CD players sold in those first few years.
As designer of the Meridian 203 reviewed here, Bob Stuart is again at the leading edge of digital playback design. The $990 Meridian 203 is one of the first outboard decoders to use the Philips SAA7321 Bitstream chip, and it incorporates some interesting and innovative thinking on digital decoder design. Moreover, the 203 is a no-frills design that tries to extract the most musical performance from a reasonably priced product.
The Meridian 203 is very simple and straightforward, both operationally and visually. No controls are provided (digital input switching, polarity inversion, etc.), making the 203 a "fit and forget" product. It is also distinguished by its long, narrow shape and beautiful styling, which match other Meridian components.
The front panel, an elegant arrangement of glass over a black background, contains three status indicator LEDs. These include "Power," "EQ" (de-emphasis circuit switched in), and "Lock," which indicates that the 203 is locked to the incoming data stream. The power on/off switch is mounted on the rear panel, suggesting that the 203 should be left on continuously. Digital converters never sound their best until fully warmed up, a process which sometimes takes several days.
The rear panel also contains RCA analog output jacks, an RCA coaxial digital input jack, and an EIAJ optical digital input jack (also called a "TOSLINK" jack). An automatic digital input-selection system defaults to the optical input when both coaxial and optical inputs are activated. Chassis material is extruded aluminum, with a metal back panel and a glass-over-plastic front panel.
This straightforward design extends to the interior. Removing the 203's cover, I was struck by the compact layout and relatively small number of components. Unlike many digital processors that bristle with power-supply caps, voltage regulators, and dozens of chips, the 203's entire circuitry fits on a single 6" by 8" printed circuit board. This pcb, however, is unusual in that is a four-layer design. The top layer is a ground plane, the bottom layer is reserved for signals, and the middle two sections carry the positive and negative power-supply rails.
The power supply takes up very little room, consisting of a bridge rectifier, a few diodes, and four electrolytic caps. Additional filtering and capacitive decoupling are provided by electrolytic caps distributed near the components they supply. Two 3-pin voltage regulators are located (physically) between the analog output stage and digital input processing section. Separate supply stages are dedicated to the digital section, phase lock loop, and analog section.
The digital section is based on the Philips SAA7274P S/PDIF decoder and two Philips SAA7321GP Bitstream converter chips. Although the 7321 Bitstream chip incorporates two channels and is designed to handle the entire conversion in a CD player or decoder, the 203 employs one chip per channel for differential operation. In this scheme, half the chip, normally used for either the left or right audio channel, handles one polarity of one channel. The chip's other section handles the opposite signal polarity. This is repeated in the other audio channel with an additional 7321 Bitstream chip. Philips suggests (and some other manufacturers have implemented) differencing the two chips rather than the Meridian method of differencing the two converters within the same chip. According to Bob Stuart, the 203's dual-differential design results in better linearity, a 4.5dB reduction of noise, and distortion reduction of 6dB. One technical obstacle that had to be overcome was developing a circuit that reformatted the data to each device as left/left-inverted or right/right-inverted, rather than left/right as it arrives from the S/PDIF decoder, all without introducing additional jitter into the data stream.
The result is a balanced output for each audio channel that, after a second-order passive filter, drives a Signetics NE5534 op-amp differentially. Because an op-amp amplifies the difference between its two inputs, any unwanted signals such as noise or digital converter artifacts common to both inputs (the anti-phase converter outputs) don't appear at the op-amp output. This phenomenon is called "common-mode rejection." The op-amp's ability to reject signals common to both inputs is measured by its "common-mode rejection ratio" (CMRR).
The 203's analog section is somewhat unusual in that it features a double-shunt feedback-unbalancing circuit with a second 5534 op-amp. In addition, it uses a DC servo, is essentially direct-coupled, and uses high-quality Nichicon capacitors and metal-film resistors. De-emphasis is performed by passive components next to the 7321. The analog output RCA jacks and digital input jacks are mounted on the pcb, eliminating point-to-point wiring. The 203's circuitry is nearly identical to the circuitry found in the Meridian 208 CD player/preamplifier. (JA's review of the 208 is scheduled to appear next month.)
The SAA7321 Bitstream chip is a tiny surface-mount device that replaces quite a bit of circuitry found in a standard (R-2R ladder) digital converter (footnote 1). First, the 7321 incorporates digital filtering, obviating the need for an additional chip to perform this function. Second, because of the nature of Bitstream D/A conversion, no external current-to-voltage (I/V) converter (typically an op-amp) is required. Also due to the Bitstream process, no external MSB trimmer is needed. The SAA7321 incorporates two audio amplifiers per channel—integrator and filter—but Meridian uses only the integrator. All this results in a dramatic reduction in parts count, pcb real estate, and cost.
The motivation to develop Bitstream, I believe, was primarily to save money in mass-produced CD players. Although Bitstream dramatically improves linearity (0.2dB typical error at –120dB), it provides a significant reduction in parts cost and labor over conventional multi-bit decoders. In addition to reducing the number of parts, Bitstream obviates the need for someone to adjust the MSB trim pot at the factory. Even tiny savings in parts or labor at the manufacturing level translate to huge price differences at the retail level.
One disadvantage of the 7321, however, is that designers are beholden to use the digital filtering and other functions of the chip, along with its D/A converter. It's a package deal: all or nothing. If the 7321's digital filtering is felt to be less than optimum, there is no way to replace it. This situation will soon change, however, with the imminent introduction of the Philips SAA7351 differential-mode Bitstream chip. It allows the designer to use external components that may be more suitable for specific applications. Some of the next generation of Bitstream CD players and digital decoders, including the Meridian 603 and 606 converters, will incorporate this new chip. However, the cost will be higher because of the outboard digital filter, and its low-level linearity is said be inferior to the 7321's.
Footnote 1: See my review of the Wadia X-32 in Vol.13 No.8 for a discussion of jitter in the recovered clock in outboard digital decoders.—Robert Harley