December 15, 2009

Ayre Acoustics QB-9 USB Digital-to-Analog Converter

2009 is proving to be a watershed year for computer-based audio. High-performance audio companies have begun in earnest to unleash their creativity, endeavoring to maximize the performance opportunities computer audio can provide. Likewise, software such as Sonic Studio’s Amarra is becoming available to ensure "bit-perfect" integrity and native-rate decoding. Riding this wave of momentum, the first-ever Computer Audiophile Symposium was held in Berkeley, California, in June; in October, the Rocky Mountain Audio Fest featured multiple computer audio seminars; and Ayre Acoustics has introduced the subject of this review, the QB-9 USB D/A converter ($2500 USD).

Ayre Acoustics, of Boulder, Colorado, has lately been on a roll. Quietly inventing elegant solutions to problems inherent in high-performance sound reproduction, Ayre unhurriedly releases one or two new products a year to appreciative connoisseurs around the world. Formed in 1993 by Charles Hansen, formerly a designer of loudspeakers at Avalon Acoustics, Ayre builds its solid-state audio equipment on three principles that were encoded in its corporate DNA on Day One: all circuits must be fully balanced, zero-feedback designs; power supplies play a paramount role; and, with customers and dealers -- even reviewers -- always follow the Golden Rule.

Ayre spent most of 2007 and 2008 introducing its state-of-the-art, megabuck R series -- the MX-R monoblock and KX-R line stage, which won awards and rave reviews. For 2009, Ayre has attacked the less expensive end of its lineup, upgrading a trio of older products to MP versions. Its disc spinners (the C-5xe^MP and C-7xe^MP) now feature Minimum Phase digital filters, while its K-5xe^MP preamplifier uses a new Maximum Performance analog output stage. All earlier editions of these three products are fully upgradeable -- a long-term practice maintained by Ayre. Interestingly, the advancements incorporated in those upgrades originated with Ayre’s newest product and the first member of the company’s new 9 series, the QB-9.

The QB-9 is a member of a component category cryptically referred to as a USB DAC -- i.e., a universal serial bus, digital-to-analog converter -- the sole purpose of which is to turn a computer into a source component equivalent to a digital transport by tethering a PC or Apple Mac to an audio system via a USB cable. Surprisingly, the QB-9 is Ayre’s first-ever DAC. Notwithstanding Ayre’s position for the past decade as a leader in digital audio technology, all its prior digital source components -- CD players, DVD players, universal two-channel audio players -- have been self-contained, single-chassis devices. According to Hansen, Ayre’s focus on the one-box solution allowed it to bypass the unavoidable jitter-inducing flaws inherent in the Sony/Philips Digital Interface Format (S/PDIF). But how can something as lowbrow and pedestrian as the ubiquitous USB connection change Ayre’s approach?

Big things in small packages

Arriving in a box of brown cardboard (recycled and recyclable, rather than bleached white), the QB-9 is sheathed in a thick polyethylene bag and suspended in air between two opposed "trampoline-style" frames. Not only does this Ayre Suspension System successfully reduce any chance of shipping damage, it minimizes the use of packing materials and shipping weight, all while helping to conserve the environment.

Cute rather than sexy, the QB-9’s appearance is sleek in its silver or black livery. Of a compact size at 8.5"W x 3"H x 11.5"D and tipping the scales at 5 pounds, the QB-9 could well be described as a "quarter"-sized component. By comparison, the full-size Ayre C-5xe^MP, my long-term reference disc spinner, is just over twice as wide, almost twice as tall, and weighs five times as much. In any event, the QB-9’s size makes it suitable for placement almost anywhere -- in a rack, on a desktop, paired with a headphone amp, you name it.

Being a dedicated USB D/A converter, the QB-9 has a minimum of connections, the primary being the single USB input (the squarish Type B connector designed for computer peripheral devices), which currently accepts signals of up to 24 bits at sample rates of 44.1kHz (i.e., the "Red Book" Compact Disc standard), 48kHz, 88.2kHz, and 96kHz. To complete its D-to-A mission, the QB-9 also has two pairs of analog audio outputs, balanced (via XLR connectors) and single-ended (via RCAs). There’s an IEC connector for the power cord, and two AyreLink Ports to enable various Ayre products to be daisy-chained using standard two-line telephone cords to control an entire audio system.

Tech-laden

Charles Hansen emphasizes that his sole goal in designing the QB-9 was to achieve the best possible audio playback from computers for the least amount of money. Despite such a narrowly focused design brief and the product’s small size, the QB-9 is packed to the gills with original thinking and unique technology. In addition to zero feedback and fully balanced circuits, the QB-9 is the first product from anyone to combine Streamlength Asynchronous USB data delivery, Minimum Phase digital filters with single-pass 16x oversampling, galvanically isolated grounding between the USB receiver and audio boards, and an EquiLock analog output circuit. It all sounds impressive, but what does such technology really represent?

For the QB-9, everything begins with the Streamlength Asynchronous Transfer Mode for USB input, the brainchild of Wavelength Audio’s Gordon Rankin. By taking advantage of the asynchronous-delivery alternative embedded in the USB protocol (Rankin invested over 900 man-hours in writing proprietary code to allow the Texas Instruments TAS1020B Stereo USB Audio Interface chip to operate asynchronously), Streamlength facilitates a high-precision, fixed-frequency clock in an outboard DAC to be the master to which the computer’s output is slaved, resulting in vanishingly low jitter. A white paper expounding the technical aspects and performance benefits of Streamlength is available on Ayre’s website.

Recognizing the inherent superiority of this delivery system -- at least for those who believe it better to avoid a problem in the first place than to try to fix it afterward -- Ayre is the first licensee of the Streamlength asynchronous delivery system. Hansen stresses that it was Streamlength’s potential for ultra-low jitter that enabled the QB-9 to sidestep the limitations in the S/PDIF standard and match the jitter performance -- measured in single digits of picoseconds -- achieved by Ayre’s one-box disc players. The QB-9 currently accepts 24-bit/96kHz digital files, but will support higher-resolution files of up to 24/192 in the near future.

Having decided to build a standalone DAC for computer audio, and intrigued by Peter Craven’s 2004 paper on "apodizing," "minimum-phase" digital filters published in the Journal of the Audio Engineering Society, the Ayre engineering team decided the time was ripe to investigate new approaches to digital filters. Minimum-phase filters primarily differ from their linear-phase brethren (the standard design used in over 99% of all digital recording and playback devices) in that they avoid pre-ringing. Craven hypothesizes that it is this unnatural pre-ringing that has crippled digital media, especially the "Red Book" Compact Disc, a standard compromised from the start. Using a C5-xe disc player as a test mule, and taking advantage of the flexibility offered by Xilinx’s Field Programmable Gate Array (FPGA) chips, the Ayre team spent months experimenting, comparing apodizing filters, myriad other minimum-phase designs, and the filterless approach many prefer. A separate white paper on Ayre’s website details the digital-filter R&D project.

The result is the implementation in the QB-9 of two user-selectable Minimum Phase alternatives. The first, labeled Measure on the rear panel, closely follows the apodizing design espoused by Craven and adopted by Meridian in its 808.2 Signature Reference CD player. The second, tagged Listen, features a more gradual slope that significantly reduces the post-ringing of the filter to about one cycle, albeit at the expense of a slight rolloff in the uppermost frequency response and a partial loss of the theoretical benefits of "apodizing" (surmised to eliminate pre-ringing embedded in digital media during the recording process). As Hansen often notes, there is no such thing as a free lunch in design, only a balance of tradeoffs, and he feels that any tradeoffs inherent in the Listen approach are overcome by the enhanced musicality and naturalness of the sound. Both of Ayre’s Minimum Phase designs use a one-pass 16x upsampler rather than the normal cascade of 2x upsamplers, and thereby reduce significantly the opportunity for compounding rounding errors in the data.

While using a computer to store and manage a large library of music has become for many the preferred audio "transport," the computer itself introduces significant barriers to good sound, from switching power supplies that radiate radio-frequency interference (RFI) through connecting wires (e.g., USB cables) and power cables (into the AC wiring itself), and dozens of competing clocks of varying frequency. Hansen is more blunt: "The bottom line is that the last thing you should do is to hook up a computer to your audio system, but since that is what people are doing, we do everything possible to minimize or even eliminate the electrical pollution that a computer inherently creates."

Ayre attacks these problems in two ways. They galvanically isolate the computer’s electrical ground (and that of the USB receiver board within the DAC) from the QB-9’s chassis and audio board by the use of high-speed optocouplers. This keeps RFI from infecting the audio system via the USB cable. Additionally, the QB-9 uses full-size Ayre Conditioners -- proprietary RFI filters that absorb RF energy and dissipate it as heat -- to clean up the AC before it enters the QB-9’s transformer, which then employs separate windings for the segregated digital and analog circuits. Even the "dirty" power drawn by the USB receiver board from the computer via the USB umbilical is separately cleaned by two micro Ayre Conditioners.

Finally, as a result of technology trickled down from Ayre’s R-series components, the QB-9 has a new analog output stage featuring EquiLock circuit design. In essence, the EquiLock circuit adds a transistor that fixes the operating point of the actual amplification transistor and holds it steady, allowing it to operate more linearly and with reduced distortion. While this new output stage relies on bipolar transistors rather than Toshiba’s now-discontinued audio-grade FETs -- a move that necessitated a return to the drawing board -- Hansen claims it has paid off in an incredible level of performance and "an even greater insight as to what creates the sound quality that we look to achieve with our designs." As mentioned above, the positive results attained with this new output circuit have now been incorporated into the K-5xe^MP preamplifier.

System

Given its relatively low price and high aspirations, I examined the QB-9 in the context of both my primary and secondary systems. In both cases I used as the source a tricked-out MacBook computer upgraded with 4GB of RAM and Samsung’s 64GB solid-state drive, ran Apple’s iTunes and/or Sonic Studio’s Amarra player software, and a 1TB external hard drive on which I’d stored my music as uncompressed AIFF files. The computer and external drive receive power via an Audience aR2-T high-resolution power conditioner to isolate computer-generated noise from the audio system on the power-line side, while the umbilical between computer and QB-9 was the excellent and affordable Wireworld Starlight USB.

In my secondary system I connected the QB-9 to an Ayre AX-7e integrated amplifier driving a pair of Avalon Acoustics Mixing Monitor speakers. Wiring consisted of a Stereovox BAL600 balanced interconnect, an 8’ pair of Stereovox Firebird speaker cables, and a suite of Cardas Golden Reference AC cords. A Billy Bags I-Beam audio center housed the lot.

When in my main rig, the QB-9 fed, in turn, my Ayre KX-R line-stage preamplifier, Ayre MX-R monoblock power amplifiers, and Vandersteen 5A speakers. Wiring consisted of Cardas Clear balanced interconnects and internally biwired Cardas Clear Beyond speaker cables, along with Cardas Golden Reference AC cords and Ayre L-5xe passive power conditioners. The MX-R monoblocks, sitting atop custom Harmonic Resolution Systems M3 platforms, resided next to the speakers to keep the speaker cables’ length to no more than 1m, with all other equipment in a custom Billy Bags rack.

Listening

Ayre equipment favors burn-in time, so the first thing I did was hook up the QB-9 to a backup computer and log over 250 hours of continuous use. It must have been sufficient; as soon as I inserted it in my system, the Ayre DAC sounded fantastic, and I heard no change in its sound thereafter.

Once the QB-9 was ensconced in my system, I hit it with all nature of music, from the jazz piano in Ahmad Jamal’s Chamber Music of the New Jazz (CD, Verve/GRP 268202), which features Jamal’s groundbreaking use of silence, space, and surprise, to hip-hop in De La Soul’s 3 Feet High and Rising (CD, Tommy Boy/Warner Bros. 01019) -- a veritable cauldron of inventive and deftly layered rhymes and samples. The QB-9 sorted complex harmonic structures and pacing with aplomb. With its Minimum Phase filters set to Listen, the time-domain stability was palpable. Free of pre-ringing, the upper registers sparkled -- definition and musicality happily coexisted, even with the severe compromises foisted on the music by the "Red Book" standard.

After favorably sampling a bevy of tracks previously ripped from my collection of CDs, I moved on to some brand-new releases that exemplify the very best the CD format has ever offered. A new edition of Stan Getz and Joćo Gilberto’s Getz/Gilberto (Verve/First Impression Music LIMK2HD036) features the 2KHD Mastering process championed by First Impression Music (FIM). This title is not only one of my standards, it’s a favorite of my wife’s, so it’s played frequently here. The ends to which Winston Ma’s company goes to reduce the destructive effects inflicted by typical production processes were justified by what I heard through the QB-9. This album had never sounded so good. Its sound surpassed that of my SACD copy, and though not as tonally saturated as my vinyl pressing, this new 2KHD edition removes a certain nasal quality from Astrud Gilberto’s vocals and provides levels of life and three-dimensionality that are simply startling for a 16/44.1 file.

As good as "Red Book" can sound when reconstructed in the analog domain by the QB-9, it is only with high-definition music files that computer audio in general, and the QB-9 in particular, fully shine. I’ve downloaded from iTrax.com 24/96 files of several masterpieces published by the 2L label, including the Grammy-winning Divertimenti, by the TrondheimSolistene (2L50). This spectacular recording embodies a refreshingly modern variety of classical orchestral music. With this truly high-definition file, the characteristics already revealed by the QB-9 were intensified, and the resulting experience more detailed yet relaxed, engrossing, and "analog-like."

Another Grammy winner, Alison Krauss and Robert Plant’s Raising Sand (Rounder 11661 9075), is available as a 24/96 download from HDtracks.com. While I found the CD edition enjoyable, I began to understand what all the fuss was about only when I heard the hi-rez version. The interplay between Krauss and Plant evoked a newfound electricity, and the bass clearly emanated from particular instruments rather than an unidentifiable bass machine. HDtracks promises over 1000 titles available in 24/96 by the end of 2009, ensuring dozens of quality performances to suit any taste. The future sounds brilliant.

So, too, I reveled in the 24/192 master-file quality of Kent Poon’s Audiophile Jazz Prologue III (Design w Sound DWS-8001), as well as multiple HRx titles from Reference Recordings, each a bit-perfect copy of one of Prof. Keith O. Johnson’s 24/176.4 master files. Given the current limitation in bandwidth, playback of these titles required downsampling to 24/96 and 24/88.2, respectively, but the vast majority of these hi-def files’ splendor remained. Such downconversions will soon be things of the past, as the previously mentioned 24/192 (and 24/176.4) receiver board will become standard on the QB-9 in a few months’ time, and will be available as an upgrade for all original-spec QB-9s for less than 10% of the retail price of a new QB-9. Who said you can’t have your cake and eat it, too?

A high-performance shakeout

To validate the QB-9 as the current state of the art, I compared its performance with that of my C-5xe^MP player ($5950) and Wavelength Crimson USB DAC (from $7500). I also reflected on what I heard in summer 2008, when I listened to a Wavelength Cosecant v3 and a Weiss Minerva DAC ($4500). At that time, I concluded that "Red Book" data played through the Minerva all but equaled the quality from corresponding CDs played through my C-5xe (pre-MP upgrade). However, in my secondary system, I noted a preference for the Wavelength over both solid-state units. While not quite as neutral or extended, the Cosecant had a beguiling midrange purity, and with many recordings was more forgiving in the upper registers (the Avalon Mixing Monitors can be brutally revealing).

I subsequently purchased Wavelength’s top-of-the-line Crimson, which can be customized balanced or single-ended, with copper or silver transformers, and with one of three alternative DAC modules. Thereafter the Crimson did nearly universal service in my Avalon system, relegating my Ayre C-5xe to primarily SACD duty. (Sadly, SACD and computer audio don’t mix.) The C-5xe found new life, however, with the upgrade to MP status and the Minimum Phase filters, which provided startling sonic gains -- the Listen setting is my preference. Consistent with what I then heard through the QB-9, a veil was lifted in the midrange with the MP filters in place, but the real advances were audible on top -- massed strings lost their congestion, cymbals their sting (the pre-ringing imposed by linear digital filters could be the culprit behind the artificial "edge enhancement" that both distracts and fatigues with 16/44.1). With "Red Book" sources, the C-5xe^MP proves the predominant winner, although the Crimson continues to offer greater convenience in accessing files, provides exceptional playback of downloaded 24/96 content, and nevertheless remains my preference for certain recordings.

Now for the shakeout. When compared head-to-head with the C-5xe^MP, the QB-9 was, however narrowly, the consistent winner with "Red Book" material. That preference became even more marked when I used Amarra, which provides positive results with nearly every album I sample (though its benefits are most pronounced with CDs released in the 1980s and early ’90s). There will always be those who prefer the single-chassis plug’n’play CD player to computer audio, but for those who decide to go the latter route, a C-5xe^MP or other cutting-edge disc spinner in the same system is simply redundant unless you have a significant collection of titles on SACD. Extrapolating to the Minerva, whose sound didn’t quite equal the old C-5xe’s, the QB-9 reigned sonically superior. Keep in mind, however, that the QB-9 is a dedicated USB DAC, while the Minerva possesses "digital hub" flexibility due to its dual S/PDIF inputs (AES/EBU and RCA) in addition to its FireWire link to a computer.

Comparisons with the Crimson offered interesting contrasts. When optimized in my systems, the QB-9 bested the results of the tubed DAC, but the Wavelength nevertheless expressed a beguiling midrange purity and a more forgiving nature that many suitors will prefer. To obtain optimal sound quality from the QB-9, it needs to be supplied a native-rate datastream, as it proved far more sensitive to upstream variables and preprocessing in the computer.

Using iTunes, native-rate playback can be a real pain because it requires manual switching of output datastreams (e.g., from the "Red Book" frequency of 44.1kHz to a hi-rez frequency of 96kHz), through the process of closing iTunes, opening the Audio MIDI settings, changing the output frequency to any new "native rate," closing Audio MIDI, and reopening iTunes. This becomes especially annoying as music-file libraries grow more varied with the ever-expanding availability of hi-rez content. (Amarra addresses this iTunes limitation by featuring native-rate auto-switching.)

With the Wavelength Crimson, my practice was to leave the output setting at 88.2kHz and let the computer simply double the "Red Book" frequency, halve any 24/176.4kHz files, and otherwise perform an asynchronous sample-rate conversion for 24/96 and 24/192 data. With the QB-9, however, a clearly perceptible deterioration in performance occurred with such a set-it-and-forget-it approach, since only two of my 1200 albums are stored as native 24/88.2 files. When I pointed this out to Hansen, he acknowledged that upstream sample-rate conversion in the computer (which is tantamount to inserting another digital filter) can significantly degrade the sound. As a result, and until the arrival of Amarra’s auto-switching, I was forced to set the Audio MIDI output at 44.1kHz to accommodate the vast majority of my audio files, sourced mostly from CDs. To manually change the output setting to 24/96 to critically listen to files downloaded from iTrax and HDtracks is an extra and nontrivial step that places the QB-9 behind the Crimson in terms of convenience -- a primary part of the attraction of computer audio in the first place.

The Crimson was also less sensitive to changes in USB cables. One of the biggest differences between USB cables seems to be their effectiveness at limiting the transmission of RFI along the ground, and in the shielding of power-delivery conductors. (A USB cable has multiple signal conductors, plus one that transmits power from the computer to auxiliary devices.) With the QB-9, Wireworld’s Starlight USB improves on their Ultraviolet USB, which in turn is an improvement over my older Kimber USB and Belkin USB2 Gold. To effectuate its ground-isolation scheme, the USB receiver board in the QB-9 relies on power via the USB cable, while the Crimson draws no power at all from the computer. Note, too, that tube-based systems, with their transformer-coupled inputs and outputs, seem more immune to the performance-robbing affects of RFI, which may account for their resurgence in the high end. Given the RFI-radiating engine that is a computer, the transformer-coupled Crimson employs a topology with some inherent advantages. Such distinctions may explain the differentiation. Accordingly, depending on the nature of the system and the user’s preferences, the Crimson’s attributes may offer a superior balance, albeit at a significantly higher cost.

Watershed moment

My time with the Ayre QB-9 amplified my certainty that computer-based audio is not only the future, it is the here and now. Providing significantly greater performance than any similarly or lower-priced alternative, and undoubtedly one of the best computer-audio DACs now available regardless of price, the QB-9 is a watershed component in this watershed year. It is what it is, nothing more, nothing less: a USB DAC unabashedly focused on making the best possible computer-audio sound for the least amount of money.

A certain irony must be at play in the Ayre Acoustics universe. Hot on the heels of the KX-R line stage -- Ayre’s most expensive product and, in my opinion, its most sophisticated, musical, and aesthetically refined -- the QB-9 is not only Ayre’s least expensive product, but it’s equally ground-breaking, and probably more significant to the world of high-performance audio at large. Whereas the KX-R will grace mere hundreds of systems, I’m sure that many thousands of QB-9s will soon be enhancing the lives of many thousands of computer audiophiles.

. . . Peter Roth
peter@ultraaudio.com

Ayre Acoustics QB-9 USB Digital-to-Analog Converter
Price: $2500 USD.
Warranty: Five years parts and labor.

Ayre Acoustics, Inc.
2300-B Central Avenue
Boulder, CO 80301
Phone: (303) 442-7300

Website: www.ayre.com