The combination of the Cobalt's hefty presentation and good dynamics teamed up to present a real sense of slam. Kick drum had a nice punch and drive, adding to the Cobalt's excellent pace. Moreover, the bass was taut and fast rather than slow and bloated. Some processors with weighty presentations tend to put a drag on the rhythm—the Cobalt achieved LF heft with speed and definition. As good as the Cobalt's bass was,…
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Oddly, the Theorem seemed to have as much extension in the lowest octave; the Cobalt's weightier presentation came more from the midbass rather than the lowest frequencies.
Sidebar 1: Associated Equipment
I auditioned the Cobalt 307 DAC through my usual reference system: Audio Research LS2B line-stage preamp, Mark Levinson No.23.5 or VTL 225W monoblock power amplifiers, and Hales System Two Signature loudspeakers with a Muse Model 18 subwoofer. The Cobalt also saw time with the superb Thiel CS3.6 loudspeakers.
Interconnects were primarily AudioQuest Lapis and Diamond or Monster Cable Sigma; loudspeaker cables were 8' bi-wired runs of AudioQuest Sterling/Midnight. Digital interconnects included the excellent Aural Symphonics Digital Standard or TARA…
Sidebar 2: Measurements
Unlike the very high output levels of other Theta products, the Cobalt 307 DAC had a maximum output of 2.027V (left channel) and 2.059V (right). This is very close to the 2V CD standard. Output impedance was 50 ohms across the band. This low value suggests the 307 DAC should have no trouble driving a power amplifier through a passive level control.
Frequency response and de-emphasis error are shown in fig.1. The right channel's 0.3dB higher output can be seen in the top trace. There is less HF rolloff than is usually measured in digital processors; the 307…
Sidebar 3: Specifications
Description: Digital/analog converter with 8x-oversampling digital filter and 18-bit hybrid DAC. Frequency range: 0.05Hz–20kHz (no tolerance given). Filter math resolution: 45 bits. S/N ratio: 108dB. Resolution: 18 bits. Linearity: 18 bits. THD at full output: 0.002%. Inputs: one coaxial on RCA jack, one optical on Toslink jack. Digital output: One coaxial on RCA jack. Analog output: unbalanced on RCA jacks. Output level: 2V RMS.
Dimensions: 9.5" (W) by 2" (H) by 6.625" (D). Weight: 5 lb.
Price: $599 (1993); no longer available (2019). Approximate number…
The $799 Theorem was originally shown at the 1992 WCES in a very small chassis that prohibited adding features or upgrades. Sumo has since become more ambitious, putting the Theorem in a full-sized chassis and offering several upgrade options that would have been impossible in the truncated version.
There's not much to say about the Theorem's features; the front panel has a power "On" LED and nothing else. Selection between the unit's single coaxial input and Toslink optical input is automatic. If a digital signal is present at both inputs, the system defaults to the optical input. The…
First, the Theorem had easily the best soundstage of the four processors. The feelings of space, depth, and air was excellent for any digital processor, never mind one costing a hair under $800. There was a distinct impression of individual instruments, each surrounded by air, hanging in three-dimensional space. The presentation was the antithesis of flat sterility, in which images fuse into a congested mess. Instead, I heard a remarkable openness, front-to-back layering of images, and a wonderful sense of bloom surrounding image outlines. I had this impression within seconds of hearing the…
Sidebar 1: Review context
I auditioned the four processors reviewed in this issue under identical conditions and at matched levels. Additionally, the reference playback system has remained nearly constant for the past year, providing a stable platform on which to audition and compare digital processors. The converters under review fed an Audio Research LS2 line-stage preamplifier, which in turn drove a Mark Levinson No.23.5 power amplifier via its unbalanced inputs. Loudspeakers were Hales System Two Signatures, occasionally augmented with a Muse Model 18 subwoofer.
A Theta Data…
Sidebar 2: Measurements
The Theorem had the lowest output voltage of the processors I review this month, measuring 1.97V when reproducing a full-scale, 1kHz sinewave. Output impedance was a fairly low 152 ohms across the band, which suggests the Theorem will have no trouble driving a passive level control.
The unit's frequency response (fig.1) showed a moderate (0.3dB) rolloff at 20kHz. De-emphasis tracking (also shown in fig.1) was virtually perfect. Interchannel crosstalk, shown in fig.2, was difficult to measure; the true crosstalk didn't emerge from the noise floor until 5kHz…
Follow-Up, from April 1993, Vol.16 No.4
Since my review of Sumo's $799 Theorem D/A converter in October 1992, the company has incorporated a jitter-reduction circuit in this popular processor. My assessment at the time was very favorable: I called the Theorem the "best-sounding converter under $1000." When I heard that Sumo had improved the Theorem's sonic performance, I was eager to audition it again. In addition to finding out how good the newer version was, it would be possible to hear the effects of lower word-clock jitter—if indeed the revision lowered jitter as claimed.
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Sidebar 3: The Jitter Game Revisited
This "Follow-Up" provides an opportunity to clarify a perplexing issue raised in the January jitter article. From those measurements, it would appear that processors with single-bit converters have much lower jitter than the more common 8x-oversampling units that use multi-bit DACs. Indeed, 1-bit converters typically have only a few picoseconds of jitter, rather than several hundreds of picoseconds or even several nanoseconds for multi-bit converters. Is the jitter performance of 1-bit machines intrinsically lower, thus "better"?
Yes and no.…