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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…

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…

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…

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…

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…

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…

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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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.…

Description: D/A processor. Frequency response: 20Hz–20kHz, ±0.5dB. THD: ±0.004% at 2V output. IMD: ±0.01 at 2V output. S/N ratio: !w100dB. Channel separation: !w80dB. Induced jitter: ±80 picoseconds. Digital filtering: 8x-oversampling. Conversion: hybrid 1-bit/multi-bit DAC. Analog section: Fully discrete, DC-coupled. Inputs: two coaxial (RCA jacks), one optical (Toslink jack), "clock" input (see text), AT&T ST-type optical optional. Outputs: unbalanced on RCA jacks.
Dimensions: 19.5" W by 1.75" H by 9" D. Weight: 10 lb, shipping.
Price: $799 (1992…