Theta Digital Cobalt 307 digital processor

One measure of a high-end product designer’s talent is the musical success of his top-of-the-line product. This is his statement to the world of what he can accomplish—a kind of “personal best” that defines the upper limits of his talent. Because he knows of no way to make the product better, the component stands as the ultimate testimonial to his skill.


Another, perhaps more interesting, way of assessing a high-end designer’s talent is to examine what he can do with the lowest budget. The designer trying to make champagne sound on a beer budget must stretch his skills to the limit when squeezing the most sound out of the lowest parts cost. The easy solutions—a more massive power supply or exotic passive components, for example—cannot even be considered when the product’s parts budget is set very low to meet a competitive retail price point.


Which brings us to the $599 Cobalt 307 DAC digital processor designed by Theta’s Mike Moffat (footnote 1). Mike is best known (and highly regarded) for his all-out designs such as the terrific Theta DS Pro Generation III, not for budget products. The “Cobalt by Theta Digital” line was conceived to bring good-sounding digital products to those who can’t afford Theta’s DSP-based processors.


Designing the 307 DAC was an exercise in tradeoffs: give up something in one area to gain something in another. In fact, the Cobalt’s price constraints were so tight that the unit has three feet instead of four, saving 50 cents in parts costs—50 cents that could then be spent elsewhere in the design. This approach resulted in a product with some impressive parts and build quality for its $599 price: ¼”-thick front panel, dual transformers, more than 32,000µF of power-supply filter capacitance, high-grade op-amps, and Vishay resistors.


But this is just the beginning; the Cobalt 307 DAC has other surprises in store.


Technical description
Removing the Cobalt from the shipping box, I was struck by the unit’s solid, brick-like feel. Although only half the width of standard chassis components, the Cobalt’s ¼”-thick front panel greatly added to the unit’s appearance and sturdy feel.


Two pushbuttons and two blue LEDs are slightly recessed in the front panel’s curved inset. The buttons select between coaxial and Toslink optical inputs and invert absolute polarity. The LEDs indicate power on/off and when the unit is locked to an incoming digital signal. The rear panel has four RCA jacks: a pair of stereo analog outputs, a coaxial digital input, and a coaxial digital output. A Toslink optical jack and IEC AC jack finish off the rear panel.


Looking under the hood revealed a design and execution unexpected at this price level. The unit uses the Crystal CS8412 input receiver (the lower-jitter “C” version), a Sony CXD1244 8x-oversampling digital filter, and the Burr-Brown PCM67 “hybrid” DAC. The Sony filter was an unusual choice: it’s harder to design around (it needs additional clock-conversion circuitry) than the ubiquitous NPC 5803 or 5813, but reportedly has more taps than other digital filters. Mike Moffat—who has always placed a high priority on the digital filter—decided that the Sony filter came closest to achieving the imaging performance he wanted without the expense of a DSP-based filter. Note that the Sony filter has an 18-bit output, rather than the NPC’s 20-bit.


The Burr-Brown PCM67 DAC is an 18-bit, dual-channel hybrid unit that combines a multi-bit ladder DAC on the upper ten bits and a 1-bit type converter on the lower eight bits. This is the same DAC used successfully in the Sumo Theorem converter (footnote 2). The 307 DAC uses the highest-grade “K” version of the PCM67 (footnote 3).


Current to voltage (I/V) conversion is performed by a pair of Analog Devices AD841 op-amps. These excellent and fairly expensive parts are also used in the $15,000 Meitner IDAT as I/V converters. The feedback resistors around the I/V converters are an exotic Vishay type (Vishay resistors cost several dollars apiece; Holco metal-film resistors are about 20 cents). This resistor is a critical link in the digital conversion chain.


The power supply was equally impressive. Two transformers—one each for the analog and digital sections—feed over 32,000µF of filter capacitance and four regulation stages. The electrolytic filter capacitors are the expensive Nichicon Muse caps, designed specifically for audio applications. In addition, the incoming AC is filtered before the transformers.


The direct-coupled output buffer is based on the National Semiconductor LM6321 op-amp. A third-order low-pass filter is wrapped around the output buffer to remove the 8x-oversampled images and other ultrasonic junk. Capacitors are polystyrene and other resistors are metal-film types.


De-emphasis is performed in the digital domain by the Sony digital filter. Unusually, no output muting relays are used. The processor reportedly won’t put out glitches when it locks to an incoming digital source. Most processors mute the output until the unit has locked and stabilized. Despite the lack of muting relays, I never heard a glitch from the 307 DAC during the time it spent in my listening room.


The entire circuit, including transformers, is mounted on a single printed circuit board that consumes the chassis bottom. Build and parts quality, execution, and the product’s feel are far beyond expectations for a $600 digital processor. For example, the pcb is made with 2oz copper plating, and the chassis has pem nuts and bolts rather than just holes and sheet-metal screws. The unit’s economy was realized with volume parts buying, a new extrusion method for the front panel, and careful attention to production engineering. The 307 DAC has no internal wiring, reducing its assembly time to a mere 90 seconds.


Music
From the start, it was apparent that the Cobalt had a good overall tonal balance and a big, gutsy, powerful quality not previously heard at this price level. The Cobalt’s bass was particularly impressive. There was a generous feeling of weight and heft, giving the music warmth, power, and drive. Bass guitar lines were well resolved, and the instrument was portrayed with a full “purring” quality. Similarly, acoustic bass had a round, meaty quality that provided a good tonal foundation for music. By contrast, the Sumo Theorem had a lighter, less robust presentation. Bass lines hinted at by the Theorem were fully fleshed out through the Cobalt. The Theorem tended to make the presentation somewhat thin and lean; the Cobalt imbued music with a sense of power and weight.

Footnote 1: We understand that Schiit Audio founder, Jason Stoddard, then with Sumo, was involved in the design of the Cobalt 307.—John Atkinson (writing in 2019).


Footnote 2: See my review in Vol.15 No.10 (October 1992) and “Follow-Up” elsewhere in this issue.


Footnote 3: Burr-Brown will soon introduce the PCM69 hybrid DAC chip. The newer version will accept a 256x clock, allowing it to interface to the Crystal CS8412 input receiver with no “glue logic” or clock-frequency conversions. The PCM69 will simplify the design, lower the parts count, and reduce manufacturing cost in future digital processors using the chip.

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