Weiss Medea D/A processor
But how good, really, is studio equipment? Although most professional components are not readily available to the average audiophile, Stereophile has reviewed a few of them. I've reviewed the Z-systems rdp-1 digital EQ and the Nagra PL-L preamp, which are equally, if not more, useful in the recording studio than in a home system; and a respected studio monitor speaker, the B&W Signature 800. The Weiss Medea digital/analog converter has a professional audio heritage, and made an immediate impression on me at the 2002 Consumer Electronics Show. Its provenance and construction shouted "Studio Professional!," but its sound quality and sleek design whispered "high-end audio." As soon as Weiss opened US distribution, I jumped on the Medea.
Daniel Weiss, an alumnus of Studer/ReVox, founded Weiss Engineering Ltd. in 1985 to make digital audio equipment for mastering studios. His clients include Telarc, Sony, BMG, Abbey Road, Gateway Mastering, Sterling Sound, Bernie Grundman Mastering, and Masterdisk, among others. Weiss decided to enter the high-end hi-fi market in 2001, and the Medea is his first such product. The Medea's ultra-clean lines are the result of good design and a minimum of controls. In fact, the Medea is the least interactive component I'd used in a long time.
There are five buttons on the front. One, on the left panel facet, is the power switch; the other four select inputs. Above each input button, a blue LED blinks as the Medea attempts to lock on to the selected input, and remains illuminated when lock is achieved. The outputs are muted until lock is achieved. On the far right facet are two trimmer potentiometers for adjusting output level; a precision ceramic tool is provided for the purpose.
The rear panel is nearly as simple. Each digital input channel has two jacks, although only one jack at a time can be used. Inputs 1-3 have both RCA and XLR connectors, while input 4 has both RCA and TosLink. Two pairs of analog outputs, RCA and XLR, are provided, along with an output-level switch. There is a 12dB difference between the level switch's High and Low positions; Weiss's distributor, Damoka LLC, recommends using High as long as the following input device can handle it. The manual, however, suggests sticking with Low unless one cannot achieve enough drive with the trimmers turned all the way up. The High and Low outputs are adjusted with the front-panel trim potentiometers. The states of all buttons are stored in non-volatile memory when the unit is powered down. Finally, there's a dual-voltage IEC power panel.
Its external simplicity aside, the Medea is no plain-vanilla DAC. In addition to accepting up to 24 bits and sample rates of up to 96kHz at each input, inputs 1 and 2 can be used simultaneously to process rates of 176.4kHz and 192kHz, though I had no sources that could take advantage of those higher rates. All inputs are automatically upsampled to 352.8kHz or 384kHz (depending on the original sample rate), and all inputs are subject to two jitter-reduction schemes.
The first such scheme is Weiss's scrupulous attention to internal design. This includes electrical and magnetic shielding, good grounding, power-supply decoupling, and correct signal transmission from the clock generator to the D/A chip itself. I confirmed the care given to this when I removed the heavy top and bottom plates of anodized aluminum, only to find the circuitry still completely shrouded by heavy copper-clad steel plates, with many "finger-grounds" to the main circuit board. The board itself was scrupulously clean and well-labeled, its different sections shielded by dividing walls and interconnected by ribbon or shielded wires. Very impressive.
The other part of the anti-jitter battle is fought by careful feedback control of the PLL, which locks the DAC clock to the input signal. Weiss says the PLL's action is analogous to that of a "very slow-reacting flywheel" in that it can follow long-term variation in input frequency but resists any short-term fluctuations; ie, jitter. This is not a new idea. I recall a similarly named off-the-shelf device, the Digital Flywheel (footnote 1), offered more than 10 years ago as a tweak for studio digital equipment. The trick, of course, is in the adjustment of the parameters of the adaptive circuitry to achieve the goal of jitter reduction and to be able to lock to a somewhat unpredictable input signal.
Signals are upsampled partly in the digital filter and partly in the DSP, to ensure a minimum of phase-shift and ringing. This also permits the addition of shaped dithering of the signals presented to the D/A stage. The analog reconstruction filter is a third-order filter at 176.4kHz or 192kHz. The output stages are class-A, with an extremely low output impedance to achieve very low distortion (even when operated open-loop) and to isolate the DAC from the load.
Footnote 1: The term "Digital Flywheel" is currently also used by EAD and Simaudio for circuits with a similar goal.—Kal Rubinson