Meridian D600 digital active loudspeaker

In audiophile circles, it is the "Stuart"—electronics designer Bob Stuart of the Boothroyd-Stuart collaboration—who has received most recognition. The contribution of industrial designer and stylist Allen Boothroyd has gone relatively unremarked. Yet as I unpacked Meridian's D600 "Digital Active" loudspeaker, I was struck by Boothroyd's ability to make the humdrum—a rectangular box loudspeaker—seem more than just that. The man has one hell of an eye for proportion. From the first Orpheus loudspeaker of 1975, through the Celestion SL6 and 'SL600 (where AB did the industrial and package design), to this latest Meridian loudspeaker design, his brainchildren look "right," to the extent of making competing designs appear at minimum over-square and clumsy, if not downright ugly.

"Digital Active?" The "active" is self-explanatory, given Bob Stuart's long history of designing loudspeakers which accept a line-level input to feed internal and integral amplification. But "digital?" This is rather a misnomer as it suggest that the actual principle of operation of the D600 is digital; ie, it directly converts binary input words into equivalent sound pressures. But no, the D600 may accept a standard multiplexed-stereo serial datastream, sampled anywhere from 30–50kHz, but it converts this to analog via an internal digital-filter/DAC before driving the crossover/equalizer/amplifier electronics which in turn are connected to conventional moving-coil drive-units.

I first heard a pair of D600s at the 1989 Winter CES last January, and to say I was impressed would be an understatement. Even in the unfamiliar circumstances of a room at Las Vegas's Golden Nugget hotel (footnote 1), the sound had a breathtaking clarity to it, apparently achieved without any undue treble emphasis. The L'Oiseau-Lyre recording of Handel's Messiah quite frankly took my breath away, and I let Bob Stuart know right away that I wanted a pair for review.

Technical details
Starting at the conventional end, the D600 uses three drive-units, all sourced from SEAS. The tweeter appears to be a descendant of a ferrofluid-cooled aluminum-dome unit the Norwegian manufacturer developed for the English Monitor Audio company, which first appeared in their R652/MD model. (Thiel also uses a version of this tweeter in their CS1.2 model.) Although the D600 uses two 6" woofers, the bottom one is rolled off above 200Hz to reduce the radiating area in the midrange and above. (Widely spaced drivers will produce severe beaming once the wavelength of sound drops below the distance separating their acoustic centers.) The D600's low-frequency alignment is fundamentally reflex, a 7.25"-deep, 2.5"-diameter port venting at the bottom of the speaker's rear panel. I believe that additional filtering is used here, as in other Meridian active speakers, to give an overall 6th-order high-pass characteristic, though two controls on the supplied 609 handset do allow some control of the bass alignment. "Bass" provides shelving up or down of the response below 200Hz of ±2dB, while "Q" offers a choice between flat response and a roll-off of –3dB at 50Hz, this possibly allowing a near–rear-wall placement. (I suspect that the latter is actually the speaker's intrinsic response, the "flat" position applying a degree of electronic boost to equalize the woofers' output to be flat.)

The cabinet itself I understand is internally braced and constructed from ¾" plywood panels loaded with a lead-loaded bituminous compound. It certainly feels extremely dead when rapped with a knuckle—what audiophile can resist that simple test? The ¾" MDF baffle is recessed to allow the drivers to be flush-fitted, while the grille is constructed from black cloth stretched over what appears to be a polystyrene space frame. A green LED display at the speaker's top indicates volume setting and selected source (Slave), the Master also offering a choice between those and track/time information (Meridian 207 CD player) or station/frequency (204 FM tuner). A red LED glows to indicate the Master speaker.

The rear of the cabinet is recessed by some 60mm (23/8"), which both adds some stiffening and leaves space for the D600 electronics. (The cabinet recess is lined with aluminum foil to provide a degree of electromagnetic screening.) Two black-painted aluminum panels enclose this tray—the bottom one is only 8" tall and carries the mains input, on/off switch, and transformer; the second runs the full height of the cabinet and carries five separate printed circuit boards. The topmost carries the digital electronics and control microprocessor, including a Philips B-grade 4x-oversampling digital filter—"B" stands for "Best"—and is screened by a tin can, this mechanically damped by a Sorbothane pad on the rear panel when the screws are tightened up. This is fed an input signal by a 7-pin DIN socket, duplicated by a phono socket. A second digital input is also provided via another phono socket. The DIN socket "hears" from the other Meridian components in the system what they're up to and supplies them with the control information sent by the 609 handset. A second DIN socket sends a duplicate of the input digital data to the other speaker of the pair, this, of course, extracting only the channel information relevant to its own operation.

Two sets of analog inputs are also provided, both balanced and unbalanced, via phono sockets. I found it easy to misunderstand these: selecting "Radio," "LP," or "Tape 1" all routed the analog input from the A1 jacks to the output circuitry. Finally, I sussed what was going on, helped by a glance at the 207 CD player control panel. Yes, it was all the same analog signal, but the different source settings affected the 207's input switching: selecting "Radio" with the remote switched whatever was plugged into the 207's "Radio" inputs to the 207's outputs, hence to the D600 output. The only exceptions are "Tape 2," which assumes that a second digital source is plugged into the D600's second serial digital input socket, and "Video," which assumes a digital source plugged into the EIAJ-standard optical input of the D600.

The analog inputs go to CD4053BCN CMOS switching chips. These common ICs also appear to be used to activate the various switched EQ options and the approximately 1dB/step 64-level volume control, all controllable from the handset. The oversampled, digitally low-pass filtered digital data are converted to single-channel analog by a selected Philips TDA-1541A S1 "Crown" dual-DAC chip, this offering in my experience a genuine 15.5-bit resolution, and followed by what appears to be a discrete-transistor current-voltage stage. Both low-noise bipolar NE5534 and LM351/LF353 JFET-input op-amp ICs are then used to implement the crossover filters and equalization functions.

A separate 70W power amplifier is used for each drive-unit, each one being constructed from discrete transistors and using a single complementary pair of Motorola MJ15003 & 4 TO-3 devices. These are bolted to the rear panel with vibration-proof washers and use the sheet metal as a heatsink. They run fairly cool, however—they are said to operate in Meridian's "unique class-AA non-switching" mode.

Fundamental to any piece of hi-fi kit's audio performance is its power supply. The base of the rear panel carries two 10,000µF electrolytic capacitors and the boards are literally festooned with three-terminal voltage regulator chips, both premium LM317/337 types and less-well-specified 78/79 types being in plentiful supply. Overall, construction is to a high standard.

If I were being picky—and isn't that I'm paid for?—I am faintly irritated that you still have to run an additional set of long interconnects to the D600s to handle the analog input signals. I wait, therefore, for Bob and Alan to introduce their next-generation CD player/preamplifier which will undoubtedly have integral A/D converters to convert all the analog signals to serial digital datastreams to be fed to the D600s down the single datalink in place of the CD data output. (Larry Archibald thinks that this is a dreadful idea, reducing the fine-quality output of, say, a Linn record player to the homogenized level of any other digital source!)

For those who prefer a more conventional active loudspeaker, Meridian's M60 ($3690/pair) appears to be identical to the D600, apart from lacking the remote-control sensor, preamp switching facilities, and digital inputs and outputs.

Any number of D600s can be set up in a particular system, for Dolby-Surround purposes, for example, the 609 control already having buttons to set appropriate levels. Which of a pair of D600s is configured as a "Master"—ie, it controls that room's system—and which a "Slave" is set at the factory. If you need to change this choice, however, it's just a matter of telling the speakers which is which with the 609 remote.

The 609 remote has a "standby" button which puts the D600s (and a 207 and 204 if connected) to sleep. Pressing any function button, or loading a CD into the player's drawer, wakes up the speakers, but I found that an upper-midrange hardness apparent on turn-on took about 30 minutes to be sufficiently ameliorated that I could enjoy the sound.

Footnote 1: One of the town's classier hotels, that is if any Las Vegas hotel can be said to be "classy."
Meridian America
14120-K Sullyfield Circle
Chantilly, VA 22021
(703) 818-3028