Digital Processor Reviews

As I was writing this review, it had been 8 years since Stereophile reviewer and gifted pianist Bob Reina passed away from an aggressive esophageal cancer. I had used the DirectStream MK1 to play several of the recordings I had made of Bob's free-jazz ensemble Attention Screen back in theday.

The DirectStream MK2's maximum output level is 2.3dB higher than that of the original DirectStream, so after switching to the new DAC, I matched its level to within 0.3dB with the 1kHz warble tone at –20dBFS on Stereophile's Editor's Choice CD (STPH016-2). Playing Attention Screen's Live at Merkin Hall (16/44.1 ALAC, Stereophile STPH018-2) with the MK2, Chris Jones's fretless bass guitar solo at the start of "Fruit Forward" had an increased feeling of low-frequency definition. The timbre of his bass was clean, and when drummer Mark Flynn accompanies Jones with softly brushed cymbals, these had better definition than they had with the original DirectStream but without any exaggeration of the sizzle.

Flynn's drums in "13 Trojans of Vundo," from Attention Screen's Takes Flight at Yamaha (16/44.1 ALAC, Stereophile STPH021-2), which I recorded live in Yamaha's YASI recital hall in Manhattan, had more low-frequency impact than they had with the MK1. The soundstaging on this track was precisely defined, accurately presenting what I aimed for in the mix: the piano's stereo image extending from stage center to far right, the drum image covering the full width of the stage (with the kickdrum a little to the left and the snare drum a little to the right of center), the guitar panned hard left, and the bass guitar center stage. When I played with the interchannel phasing and reverb on Don Fiorino's lap steel guitar at the climax of "13 Trojans of Vundo," to give the impression that the instrument was both expanding in size and rising up and away from the listener as its frequency increased (footnote 4), the combination of the DirectStream MK2, Parasound amplifiers, and KEF speakers accurately reproduced the intended effect.

Similarly, the off-stage viola in Eriks Esenvalds's "In Paradisum," from our June 2020 Recording of the Month, Translations, with the Portland State Chamber Choir conducted by Ethan Sperry (24/96 WAV, Naxos 8.574124), was presented as a stable image to the far left of the stage, behind the ethereal-sounding singers and the front-and-center solo cello. The viola was set a little farther back in the soundstage with the MBL N31 than with the DirectStream MK2.

I always start my critical listening to a new component by using recordings that I made myself, like the Portland Choir and Attention Screen albums. That way, I avoid Floyd Toole's "Circle of Confusion," where you are judging a product with unknown sound quality with a recording of equally unknown sound quality. But once I had a handle on the DirectStream MK2's sonic signature, I turned to other recordings. One such was a favorite Bruce Hornsby live album, Noisemakers Summer 2007. This was only available as a 256kbps MP3 download, but even with the lossy compression, the kickdrum and bass guitar in the medley of "Fortunate Son" and "Comfortably Numb" were reproduced with good articulation and low-frequency authority, if not quite in the same league as the Benchmark DAC3 B's presentation.

Since the start of the pandemic, I have established a daily routine where, after I have fed our cats at 5:30pm, I spend the next two hours enjoying music in the listening room, mostly picking a track at random then allowing Roon's Radio function to follow it with tracks its algorithm thinks I will appreciate. With the PS Audio DirectStream MK2 in the system, I often continued listening longer than the allocated two hours.

One discovery was Vaughan Williams's The Lark Ascending in an arrangement that replaces the solo violin's orchestral accompaniment with a piano (24/192 FLAC, Resonus Classics/Qobuz). As reproduced by the PS Audio DAC, the image of Duncan Riddell's violin floated stably in the center of the stage—but, peculiarly, above the plane of the speakers. It must have been captured that way on the recording; this is a further example of the PS Audios soundstaging prowess. Mark Bebbington's piano was recorded a little closer than I would have liked, but this was still a spellbinding presentation of this familiar music.

I finished my critical listening with the recording of Schubert's Piano Trios and other works with Christian Tetzlaff on violin, Tanja Tetzlaff on cello, and the late Lars Vogt on piano (24/96 FLAC, Ondine/Qobuz). This album was very positively reviewed by Jason Victor Serinus in Stereophile's March 2023 issue. With the new PS Audio DAC, I didn't feel I was missing any of what JVS described as the "superb, committed playing of Vogt and the Tetzlaff siblings."

Noise and resolution
For logistical reasons, I performed the measurements of the DirectStream MK2 earlier in the review period than I usually do. (I usually leave the measurements until toward the end of the auditioning to ensure that what I hear is not unduly influenced by my knowledge of a device's measured behavior.) When I measured the MK2, I found that its output had a relatively high level of noise, 20× higher than I had found with the original DirectStream running the Windom firmware. This noise is primarily ultrasonic, but the audioband noisefloor was sufficiently high in level to corrupt some of my test results. However, this audioband noise did not have a negative affect on the DAC's resolution, at least down to the 16-bit level.

With the original DirectStream's volume control set to its maximum of "100" and the Parasound monoblocks set to high sensitivity driving the KEF LS50s, I could hear no hiss with my ear pressed against a tweeter. (For my auditioning, I used the Parasounds set to their lower sensitivity and the PS Audio's volume set to "76," ie, 12dB below the maximum.) Repeating the exercise with the MK2, some noise was audible, but I couldn't hear it if I moved more than a few inches away from the speaker. The level of this noise didn't change if I lifted the USB ground via a Menu setting, and it wasn't affected by the setting of the MK2's volume control. It might be best, then, to avoid using the built-in volume control in a system with high-sensitivity speakers, like the Klipsch La Scala Alex Halberstadt reviewed in April 2023, where the music volume may need to be turned down to levels at which this noise becomes troublesome.

For years, I have been correlating a digital processor's sound quality with its calculated resolution, which in turn can be correlated with its measured analog noisefloor, ie, a reduction in the noise by 6dB is equivalent to an increase in resolution of one bit. This correlation was observed with the Weiss DAC502, Mola Mola Tambaqui, Okto dac8 Stereo, dCS Vivaldi Apex and Rossini Apex, MBL N31, and Benchmark DAC3, all of which offered measured resolution of 20 bits or greater and all of which sounded superb. However, while my auditioning suggested that while the PS Audio DirectStream MK2's retrieval of recorded detail was not in the same class as those high-resolution overachievers, my enjoyment of the music didn't seem unduly impaired.

This is an enigma in that the musical whole seemed greater than the sum of the measured parts. For want of a better word, the PS Audio DAC preserved each recording's musicality.

Conclusions
Overall, I enjoyed the time I spent with the DirectStream MK2 processor and how it played music for me. While its low frequencies don't have quite the drive I appreciate with the Benchmark and MBL processors, it betters its predecessor in this respect and sounds more open in the highs.

However, I remain concerned about the relatively high level of noise in its output. This is a product would-be owners need to audition in their own systems before purchase.

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Footnote 4: At 5:25 in the video.