Digital Processor Reviews

My late father-in-law used to say that life is like a roll of toilet paper. The closer you get to the end, the quicker it passes. I was reminded of that when I was asked to review Bel Canto's new e1X DAC/Control Preamplifier ($6800). It didn't seem that long ago that I favorably reviewed the Minnesota company's e.One DAC3, but Google told me that it was 15 years ago! "Other than the jitter performance via its USB input, the Bel Canto e.One DAC3 is the best-measuring digital component I have encountered," I wrote in November 2007 (footnote 1), adding that it "offers some impressive audio engineering in both the digital and analog domains."

The e1X DAC
The first product from Bel Canto's e1X series to be reviewed in Stereophile was the e1X power amplifier, in June 2020, which got a thumbs-up from Tom Gibbs. The e1X DAC was shown at the 2019 AXPONA, but production was delayed by pandemic supply-chain issues. A sample became available for review in the spring of 2022.

The e1X DAC is a smart-looking, full-width component in a black-anodized aluminum case. The front panel features a large, rectangular, white-on-black alphanumeric display, a control knob, and a ¼" headphone jack. The rear panel features AES3, coaxial and optical S/PDIF, and USB digital inputs as well as a UPnP/DLNA-compatible Ethernet port. The USB and Ethernet ports support MQA-encoded data and DSD data in the DoP format (DSD64 via the network connection; DSD64 and '128 with the USB port). There is also a USB-A port for plugging in a FAT32-formatted drive.

In addition to balanced and single-ended analog outputs, each of which can be used with fixed or variable gain, there are four pairs of RCA jacks. Two pairs are for line-level analog inputs, both with a selectable Home Theater Bypass mode, one pair is for a MC/MM phono input—there's also a grounding lug for use with this input—and the fourth pair can be used to feed a powered subwoofer. The signals fed to the line-level analog inputs and the phono input are converted to digital. According to John Stronczer, Bel Canto's founder, CEO, and chief of design, digitizing the phono input makes it less susceptible to picking up noise than a purely analog phono stage. Once converted to digital, analog signals can then be adjusted with the DSP-domain controls like Tilt and Bass EQ, as well as the volume control.

The front-panel knob controls volume; with a gentle push, it also allows selection of different sources. All the other settings, including switching the output between the line-level jacks and the headphone jack, controlling the channel balance, Tilt, Bass EQ, Subwoofer filter and gain settings, Home Theater Bypass, and phono-input gain and load options, plus firmware updates and factory resets, are accessed by pressing the Program button on the remote control then rotating the knob. Changes can then be made with the knob or the remote's up and down buttons. Pressing and holding the knob or the remote's Program button allows any changes to be saved.

The digital-domain Tilt control is virtually identical to the analog control found in the 1980s-vintage Quad 34 preamplifier. As the name suggests, this control tilts the frequency response by up to ±3dB above and below a hinge frequency close to 1kHz. Each of the 10 steps changes the tilt by ±0.6dB. The Bass EQ control applies a boost or cut of up to 3dB below 100Hz, again in 0.6dB steps. The Subwoofer control adjusts the gain at the subwoofer output and sets the low-pass filter's turnover frequency between 40Hz and 120Hz in 10Hz increments. The Main outputs can have a similar high-pass filter applied or left off.

Pressing the Display button on the remote cycles through several screens. The first displays the source and the volume setting, the second the incoming sample rate and the volume, the third the firmware version, the fourth the equipment selected (in this case, "E1X DAC"), and the fifth turns off the display. (As the display is very bright, I tended to leave it dark.)

The e1X DAC is Roon Ready, so it's optimized for playing music streamed from Roon. Bel Canto also offers a free UPnP/DLNA iOS app called Seek, which allows audio to be streamed from Qobuz, Tidal, Spotify, and vTuner internet radio and files to be played from DropBox, OneDrive, and iCloud folders, from a drive plugged into the USB-A port, or from local storage on their iDevice. Seek supports playback with resolution and sample rate up to 24 bits and 192kHz.

Technology
A white paper by John Stronczer on the design of the e1X DAC explains that it shares its architectural approach with Bel Canto's Black system design (footnote 2). "We use a powerful digital architecture, combining asynchronous interface retiming, ultra-low noise master clocks, 32/64-bit DSP and proprietary digital link technologies to achieve superior analog performance," he writes, adding that the analog performance is defined "by our two-stage High Dynamic Resolution (HDRII) DAC core."

The DAC chip used, not just in the e1X's core but in all of Bel Canto's digital products, is a Burr-Brown (now Texas Instruments) PCM1792A. This two-channel, current-output chip accepts 24-bit data with a sample rate up to 192kHz and offers up to a 130dB dynamic range, depending on the output voltage. (The six most significant bits are processed by a conventional R-2R section; the 18 least significant bits are decoded by a five-level delta-sigma section running at 64 times the clock speed.) The first version of this chip dates from 2003—it was used in the Bel Canto e.One DAC3—and while it's still in production, the Texas Instruments website says that it is "not recommended for new designs." Nevertheless, Stronczer says that the PCM1792A "is a high-performance DAC whose musical qualities we have refined over the past 15 years. While newer DAC technologies have come along, none have provided the ultimate performance and unique analog characteristics of the PCM1792A. Continually refining the circuitry and design choices surrounding this DAC core results in a highly dynamic and musically revealing sound."

The e1X signal path starts with what Stronczer calls an Asynchronous Multi-input Processor (AMiP) board, fed from a dedicated, isolated, linear power supply. This board carries the digital and analog input interfaces as well as the critical pair of ultralow-noise master clocks that retime the digital data. As previously mentioned, the analog inputs are digitized, and all the audio signals travel through the same digital path. The AMiP board includes five digital signal processors, one of which contains the MQA Decode and Rendering function. This processor bypasses the 8×-oversampling filter incorporated in the Burr-Brown DAC chip, instead implementing MQA-derived filters for PCM and DSD data. A 32-bit ARM processor is dedicated to the user interface and internal control functions, and a 32-/64-bit DSP core is used for the Tilt, Bass EQ, and Subwoofer crossover controls.

"The digital signal from the AMiP board is sent to the HDRII DAC board through a proprietary interface," Stronczer writes. "The HDRII board includes its own asynchronous ... ultra–low-noise master clock. This and the AMiP board provide two series stages of aggressive jitter rejection from any incoming digital data source. ... Our architectural choices preserve signal integrity by maximizing the performance of both the I/V and voltage amplifier stages. In this way, the 32-bit digital volume and balance controls are used to preserve the >126dB dynamic range of the DAC core."

The e1X's I/V and voltage-gain stages operate in class-A, and the passive components "are biased with constant DC current and voltage for highest dynamic linearity."

Listening with Roon
The e1X DAC is a complex and complicated component. For my first round of listening, I set the processor's gain to variable, with the default maximum setting of "85," and used Roon 1.8 to stream audio data from the internal drive in my Nucleus+ server over my network. I fed the Bel Canto's balanced outputs to the Parasound Halo JC 1+ monoblock amplifiers. The amplifiers were set to their lower input sensitivity, which meant I used the e1X's volume control set with Roon, Seek, or the remote control between "67" and "77."

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Footnote 1: Dick Olsher reviewed Bel Canto's first digital product, the Aida D/A processor, in Stereophile's November 1994 issue.

Footnote 2: Michael Fremer reviewed the Bel Canto Black system for Stereophile in June 2015.