Digital Processor Reviews

Back in the summer of 2009, USB-connected D/A processors that could operate at sample rates greater than 48kHz were rare. Ayre Acoustics had just released its groundbreaking QB-9, one of the first DACs to use Gordon Rankin's Streamlength code for Texas Instruments' TAS1020 USB 1.1 receiver chip. Streamlength allowed the chip to operate in the sonically beneficial asynchronous mode, where the PC sourcing the audio data is slaved to the DAC. But high-performance, USB-connected DACs like the Ayre were also relatively expensive back then, so in the January 2010 issue of Stereophile I reviewed a pair of soundcards from major computer manufacturer ASUS , the Xonar Essence ST and STX, which, at $200, offered a much more cost-effective means of playing hi-rez files on a PC.

I was impressed by what I heard from these cards, and concluded in a Follow-Up that "the Xonar Essence STX and its PCI-bus equivalent, the Xonar Essence ST, can be recommended to those on restricted budgets who wish to incorporate a PC into their high-end rigs." So when ASUS announced that it was introducing a version of its standalone Xonar Essence One D/A headphone amplifier fitted with JRC's high-performance Muses op-amps, which I had first experienced when I reviewed the Esoteric D-07 D/A processor in January 2011, I asked for a review sample.

The Muses Edition
The Xonar Essence One is a hefty processor housed in an elegant, black-painted enclosure. The basic Essence One costs $599; the Muses Edition, which can be distinguished by the black color of the stylized lion graphic on the top of its extruded-aluminum sleeve, costs $899. On its front panel are, from left to right: a power button; buttons to select Upsampling, Input, and Mute (the selected input LED turns from blue to red when Mute is selected); a large volume control for the line outputs, to its right an arc of blue LEDs; a smaller volume control for the headphone output; and a single ¼" (6.3mm) stereo headphone jack. Because there are independent volume controls for the line and headphone outputs, the line output doesn't mute when headphones are plugged in. On the rear panel are pairs of RCA and XLR jacks for the single-ended and balanced line outputs, respectively, and input jacks for USB data and S/PDIF data on TosLink and coaxial links.

When the upsampling function is off, the sample rate of the incoming data—44.1, 48, 88.2, 96, 176.4, or 192kHz—is displayed by the arc of LEDs. When the upsampling button is pressed, data at 44.1kHz are upsampled to 352.8kHz; data at 48kHz and its multiples are upsampled to 384kHz. ASUS calls this "Symmetrical 8x upsampling," because the upsampled frequency is an integer multiple of the incoming rate. None of the LEDs illuminates when upsampling is engaged. However, there is an LED at the top of the arc labeled Bit Perfect; though this never lit up when I used the Xonar processor with my Mac mini or MacBook Pro, it is supposed to do so when the Essence One is connected to a Windows PC and the necessary ASIO driver (supplied on a CD-ROM) is installed. (I couldn't verify this, as all my auditioning and measuring was with Macs.)

Inside the box, the circuitry is neatly laid out on a large printed circuit board, with a cutout in the board for the toroidal power transformer. S/PDIF data are routed to an AKM AK4113 receiver chip; USB data are handled by a C-Media CM6631 USB receiver (the same chip used in the Schiit Bifrost DAC reviewed by Jon Iverson in August 2013). The audio data are passed first to an Analog Devices SHARC ADSP-21261 40-bit floating-point DSP chip, then to a pair of TI's PCM1795 DAC chips.

The PCM1795 is a two-channel, 32-bit–resolution device that is pin-for-pin compatible with the earlier and widely used PCM1792 chip; according to its datasheet, the PCM1795 is DSD-capable, but the Essence One will not decode DSD data. Also according to its datasheet, the PCM1795 is intended to operate up to a sample rate of 200kHz, so I'm not sure how ASUS is able to use it at 352.8 or 384kHz when upsampling is engaged. I wondered, as the chip is a two-channel part, each channel could be fed alternate samples, to give an effective doubling of the sample rate, as was once done by Stan Curtis in a mid-1980s Cambridge Audio CD player. However, my measurements (see Sidebar) suggest this isn't the case.

Unusually, all of the eight-pin op-amp chips, a mix of JRC and Muses devices, are socketed. The six Muses 01 dual op-amps, which follow the DAC chips, are made with advanced fabrication techniques said to reduce crosstalk and produce better-balanced left/right channel symmetry, and use oxygen-free copper leads. The headphone amplifier appears to be based on a pair of TI's LME49720 high-performance op-amp chips and an LME49600 high-current output driver; the line outputs appear to be based on TI's LM4562 ultra-low-distortion, low-noise, high-slew-rate op-amps. Other than the headphone output, all the analog audio circuitry is heavily bypassed with local electrolytic and plastic-film capacitors. Overall, the parts count and the quality of those parts are very high for a relatively inexpensive product.

Sound Quality
I used the Xonar Essence One Muses Edition for all my regular headphone listening during the fall of 2013, as well as during the preparation of my review of the Audeze LCD-X headphones elsewhere in this issue. I also used it in my big rig (see the "Associated Equipment" sidebar). Although the Xonar DAC had already been reviewed by Michael Lavorgna and Dinny FitzPatrick on, respectively, our sister websites AudioStream.com and InnerFidelity.com, I didn't read my colleagues' reviews until I had finished my own auditioning. But my impressions of the Xonar's sound quality to some extent echo theirs.

Used as a DAC without upsampling engaged, the Xonar Essence One didn't resolve recorded details as readily as the NAD M51 and Auralic Vega, though it's fair to note that those DACs cost very much more. There was a smooth, rounded-off quality to the Xonar's line outputs that was a benefit with typically overcooked rock recordings, such as the Pretenders' "Talk of the Town" and "Back on the Chain Gang," from The Singles (ALAC files from CD, Sire/WEA; and yes, I am rediscovering and relishing that delicious sob in Chrissie Hynde's voice).

But this character obscured the fact that my reissue of Richie Havens's 1969 album Richard P. Havens, 1983 (CD, Polydor 835 212-2) had apparently been mastered from an LP rather than from the original master tapes. And the dry acoustic of Yamaha's YASI recital hall in Attention Screen's "13 Trojans of Vundo," from their live recording Takes Flight at Yamaha (16/44.1 master file for CD, Stereophile STPH021-2), also seemed a little suppressed compared with the Auralic Vega's presentation. Chris Jones's fretless Fender Jazz bass guitar also sounded a touch softer, but the Essence One still revealed the hit in sound quality resulting from the lossy audio encoding in the video from the concert that I posted on YouTube, to which I had added the audio mix from the CD.