Digital Processor Reviews

On first hearing, Aqua Acoustic Quality's Formula xHD D/A processor was highly engaging. It was immediately obvious that the Formula xHD was a relaxed, unmechanical-sounding ladder DAC—a converter that dispenses with off-the-shelf processor chips in favor of a string of voltage dividers, made from discrete resistors arranged in an R-2R or ladder configuration. In less than a minute, I was admiring the deluxe, all-natural quality of its sound. Wow! I thought. So this is what it's like to have a $17,000 DAC in my system!

I'd been in one of my recurrent Sun Ra periods when the Formula xHD had arrived, so the first track I listened to was "Heliocentric," from The Heliocentric Worlds of Sun Ra, Vol.1 (16-bit/44.1kHz FLAC, ESP/Tidal). The sound was lush and easy-flowing. But before this 4:23-long composition had ended I had become distracted by the unusual way in which the Aqua was presenting Sun Ra's extended silences and cavernous spatial perspectives. The core art of "Heliocentric" involves poignant silences interrupted by "solar" eruptions of alto saxophones, piano, large drums, and antique cymbals appearing on a soundstage engineered to resemble the vast depths of outer space.

The digital silence of some DACs reminds me of microphone gating, where electronic noise gates attenuate the microphone's output below a preselected signal level and above some unwanted ambient or electronic noise level. This effect can be vexingly distracting, and is qualitatively unrelated to real-life quiet, or even anechoic silence. To its credit, the Formula xHD's silences didn't sound gated at all. Instead, those moments of emptiness were completely natural and grain-free.

But I heard something else: With the Formula xHD in the system, the deep spaces of "Heliocentric" seemed thicker and darker than I remembered from previous listenings.

That same night, I invited my objectivist audio friend, Mr. O, to come over and give his opinion. He listened carefully to Heliocentric Worlds and said nothing. But he smiled coyly when I played violinist Hilary Hahn and pianist Hauschka's Silfra (16/44.1 FLAC, Deutsche Grammophon/Tidal). "I like this DAC!" he exclaimed. "It's smooth."

As Mr. O and I listened on, I posed a question: "For $17,000, shouldn't this DAC sound a little more sharp-edged and focused? Shouldn't its sound be at least a little cooler and Martini-dry? Isn't that the sonic character we associate with accuracy?"

Mr. O nodded his agreement.

As I listened more closely, I noticed that the attacks in the lower registers of Hauschka's piano and the leading edges of Hahn's violin seemed to be all there—not hard or sharp, a little unpronounced, perhaps, but apparently complete.

That was my first night with the Aqua Formula xHD.

When I woke the next morning, I re-read Jason Victor Serinus's review of the Formula xHD in the June 2018 issue, in which he concluded: "Thanks to the Formula xHD's ability to smooth over digital's rough edges, I don't hesitate to recommend that it be auditioned by anyone with $17,000 to spare, and whose system suffers from bright or harsh sound, or who values, above all else, the warmth and bloom often ascribed to analog sources."

I was already familiar with the sound character of various Aqua Acoustic Quality DACs, as they'd been used in some of the better-sounding rooms at High End 2018, in Munich. I've also been enjoyably intimate with a variety of non-oversampling R-2R DACs from Audio Note, HoloAudio, MSB, and TotalDAC. Therefore, when I read Jason's review, I wondered if he'd failed to recognize the sonic and aesthetic virtues of the NOS-R-2R-no-filters viewpoint. I wondered this because JVS seemed to demonize the relaxed, nondigital character of the Formula xHD, saying, in the first paragraph of his "Listening" section, that "the highs were subdued and the color palette limited."

I streamed the 16/44.1 version (FLAC, SFS Media/Qobuz) of the high-resolution download (24/192 FLAC, SFS Media/HDtracks) Jason had based that observation on: Michael Tilson Thomas conducting the San Francisco Symphony in Berg's Three Pieces for Orchestra. Instead of Jason's impressive system of Lamm Industries L2.1 preamp, Dan D'Agostino Master Systems Progression monoblocks, and Wilson Audio Alexia 2 speakers, I had the Formula xHD feeding a PrimaLuna DiaLogue tubed preamp driving a First Watt SIT-3 solid-state amp powering DeVore Fidelity Orangutan O/93 speakers. And guess what—even through that low-powered, single-ended, no-feedback system, I heard pretty much what JVS had heard: High frequencies seemed slightly rolled off, and vocal tones leaned more toward gray than Technicolor. What was really noticeable was how the Aqua's soundstage seemed filled to near-opacity with some sort of gray humidity. This brought me up short—because NOS R-2R DACs typically sound more atmospheric and naturally transparent than their sigma-delta counterparts.

I couldn't stop playing Three Pieces for Orchestra, so I double-checked the above observations using Harbeth Monitor M30.2 speakers powered by Rogue Audio's 100W Stereo 100 amplifier. While neither the Harbeths nor the DeVores make much energy below 50Hz, both speakers made one thing clear: the Formula xHD DAC could deliver copious bass. But said bass was consistently round—not soft or loose, but round in a way that upstaged its mass and definition.

Both amp-speaker combos were also adamant that the Aqua Formula xHD did not surround instruments and voices with bright, fresh air. This lack of engaging transparency ran contrary to all my everyday experiences with HoloAudio's Spring "Kitsuné Tuned Edition" Level 3 DAC, which is now my DAC of daily preference. (The Spring is also a digital-filterless, NOS, discrete-resistor R-2R DAC.) The HoloAudio Spring's greatest virtues are its grainless quietude (like the Aqua), its image solidity (also like the Aqua), and its uncanny transparency (not like the Aqua).

To further explore the Aqua's transparency, I installed Harbeth's P3ESR 40th Anniversary speakers. Their reference-quality clarity would help me compare the $2649 HoloAudio Spring with the $17,000 Aqua Formula xHD, as I paid special attention to sopranos, pianos, and the spaces surrounding them.

Jason knows way more about sopranos than I do—and his knowledgeable music writing teaches me more every day. In his review of the Formula xHD he said, "One familiar recording that I love, and play regularly to test systems, is Eileen Farrell's Sings Verdi (CD, Sony Classical Masterworks 62358/ArkivCD)." On Tidal I found Eileen Farrell—Verdi Arias (16/44.1 FLAC, Columbia/Tidal), and used this recording extensively in the following comparisons.

I began with the HoloAudio Spring, which I doubt made Farrell's notes float as freely as Jason is accustomed to. Nevertheless, with Farrell singing "Ma dall' arido stelo divulsa," from Verdi's Un Ballo in Maschera, the Spring did what it specializes in: it put a genuine female body behind that famous voice. No floating Farrell notes—just full-spectrum harmonics emitted through an enormous stereoscopic space. The Spring delivered nothing but clean air between Farrell, her microphone, and me.

The Aqua Formula xHD's reproduction of this track seemed smooth and well sorted, but also thicker and more ponderous than the Spring's, whose light-filled clarity put Farrell and her tenor, Verdi specialist Richard Tucker, into a semi-believable, three-dimensional space I could peer into. The relative opacity of the Formula xHD limited these effects.

I still needed to know about pianos. The recording by Todd Garfinkle, founder and chief producer of M•A Recordings, of Man from Plovdiv, an album's worth of Milcho Leviev's hysterical, sublime, and quirky keyboard improvisations (CD, M•A M018A), is ideal for evaluating DACs. Garfinkle placed his two omnidirectional mikes perfectly—not too close to, not too far from Leviev's piano. The result: high-spirited piano improvisations captured in perfect sound. Besides its display of fantastic pianism, Man from Plovdiv is a reliable tool for assessing a component's ability to generate a believably full-sized concert grand in my room.

Compared to the R-2R Aqua Formula xHD, the sigma-delta Mytek Manhattan II DAC ($5995) decoded Man from Plovdiv with more compelling force and substantially higher levels of transparency. Schiit Audio's multibit Yggdrasil Analog 2 ($2399) was more dynamic, pacey, and transparent. The HoloAudio Spring developed the greatest solidity. Unbelievably, the Spring forced a solid, exquisitely toned Steinway grand through the tiny Harbeths and onto the floor of my little room. During its turn, the Aqua Formula xHD made Leviev's instrument seem quite dense, but also muted and indistinct—as if Leviev had kept his foot on the soft pedal.

Aqua Acoustic Quality's Formula xHD D/A processor sounded as smooth and warm and enjoyable as Jason and Mr. O suggested, but I don't regard it as a pleasure machine, nor do I see it as a cure for systems that sound too bright. In my system, the Formula xHD was simply too opaque for a perfectionist DAC of any price. My head is shaking in disbelief.—Herb Reichert