Digital Processor Reviews

Robert Harley wrote about the SFD-2 Mk.II in March 1995 (Vol.18 No.3):

When I first reviewed the Sonic Frontiers SFD-2 in the December 1993 Stereophile (Vol.16 No.12), I was taken aback by this tubed processor's sound quality. What was remarkable was not so much the fact that the SFD-2 was a contender for the state of the art in digital playback (footnote 1), but that it cost $4695. Yes, this is still expensive, but the SFD-2 was a bargain compared to processors costing well into five figures.

Sonic Frontiers has now revised and updated the SFD-2, giving it the "Mk.II" designation. The newer model has many small refinements that have been continually introduced during production of the original SFD-2, along with a few major changes, the most significant of which is the replacement of the original's NPC filter chip with the High Definition Compatible Digital (HDCD) decoder/filter chip, called the PMD100. (I'm guessing "PMD" stands for "Pacific Microsonics Decoder.") The HDCD chip in the review sample was from the second prototype run.

The SFD-2 Mk.II sells for $5295—a $600 increase. SFD-2 owners can upgrade to the digital board containing the HDCD decoder chip—it's the same size and configuration (a 28-pin DIP) as the NPC filter chip it replaces—for $1200. Depending on their vintage, existing SFD-2s will include all, some, or none of the analog-stage refinements.

The SFD-2 got a functional and cosmetic makeover in the Mk.II version. First, the small front-panel toggle switches—to which I objected in my first review—have been replaced by pushbuttons. Rather than selecting the digital input with a toggle switch, the pushbutton scrolls through the unit's five inputs. The power switch (marked "Standby") and absolute polarity reversal also benefit from pushbuttons, giving the front panel a cleaner, more streamlined look. Other refinements include a thicker front-panel dress plate (now without screws showing), and a heavier top cover (14- instead of 16-gauge steel) with mass damping and a more durable powder-coat finish. Two digital inputs were added, giving the Mk.II a total of five: AES/EBU, RCA, BNC, TosLink, and ST-type optical. Finally, an additional LED, marked "HDCD," illuminates when the unit decodes HDCD-encoded sources—a requirement of the HDCD license.

The analog output stage benefits from a new layout that reportedly reduces THD, IMD, and crosstalk. Larger output coupling capacitors lowered the output impedance at low frequencies by more than three times [the highish output impedance gave rise to a bass-shy sound with some preamplifiers—Ed.], and Teflon capacitors now bypass the output coupling caps. Four critical resistors have been upgraded from Holco brand to state-of-the-art Vishay types.

On the digital side, the digital circuit board was redesigned for lower jitter. The UltraAnalog AES21 input receiver in the Mk.II has been specially selected for low jitter, and the Mk.II uses UltraAnalog's new wide-bandwidth pulse transformers.

Listening: I started out with non-HDCD-encoded discs to get a feel for how the Mk.II sounded in relation to the original unit. Just seconds into the first piece of music, I knew that this was a very different-sounding unit from the original—it didn't sound like a more refined version of the original SFD-2, but like a completely different processor.

The most salient difference was the Mk.II's overall perspective. Where the SFD-2 was forward in the mids, the Mk.II was laid-back. In fact, the entire presentation was relaxed, more distant, less immediate. The SFD-2's forwardness was a liability, and was my primary criticism of the unit in the first review. Although clean and free from grain, the forward mids made the original SFD-2 sound less refined and sophisticated—almost as if it was too eager to make an impression (footnote 2).

The Mk.II seemed to have gone in the opposite direction, with a very distant presentation—images existed more behind the loudspeakers than before them. If the SFD-2 put me in Row B, the Mk.II put me in Row T. I preferred the Mk.II's interpretation, but SFD-2 owners who have balanced their systems around the processor's forwardness may find the Mk.II too laid-back.

Along with this more easygoing perspective was an astonishing resolution of soundstage depth. In fact, the Mk.II had the deepest, best-defined, most spacious soundstage I've heard from a digital processor. The unit resolved layers upon layers of space that seemed to recede into infinity. The air between images was more palpable, with a real sense of distance between performers. As good as the SFD-2 was at soundstaging, the Mk.II produced a tremendous improvement in the resolution of spatial information. Image focus was tighter, with sharper outlines. Moreover, the soundstage had greater transparency than the SFD-2, with a "blacker" background. The difference in soundstaging was very similar to what I hear when I put the Sonic Frontiers UltraJitterbug between a transport and processor.

Textures were noticeably smoother and more natural, lacking the edge and synthetic character associated with digital replay. Massed violins were sweeter and smoother, yet at the same time much better resolved. The string section sounded more like several instruments and less like a homogenized unit.

Bass was somewhat better-defined and tighter in the Mk.II. Pitch was better resolved, and I was able to hear more inner detail. The Mk.II's overall bass balance, authority, and weight were comparable to those of the SFD-2.

The most significant improvement, however, was in the Mk.II's resolution of fine musical detail, and its ability to keep individual instrumental lines separate from the whole. The sound was composed more of separate entities than of variations on a single sonic tapestry. Low-level detail only hinted at by the SFD-2 was fully resolved by the Mk.II. This quality really hit home on one of my favorite recordings, Frank Zappa's orchestral The Yellow Shark (Barking Pumpkin R2 71600). The Mk.II revealed so much more of what each instrument was doing that it was almost like hearing the music for the first time. Such complex orchestral music really benefited from the Mk.II's high resolving power.

As stunning as the SFD-2 Mk.II was on conventional CDs (those not HDCD-encoded), I wasn't prepared for what I was about to hear with full HDCD encoding and decoding. Playing the handful of Reference Recordings discs made with the HDCD process was nothing short of a sonic and musical revelation. Forget about the old analog vs digital debate; with HDCD encoding and decoding, digital wins in every department—hands down. HDCD didn't produce a minor improvement to what was already a terrific sound, but took a state-of-the-art processor and elevated it an order of magnitude.

One thing I heard from HDCD that I've never gotten from conventional digital was the total individualization of separate musical lines. I was able to hear each instrument as a separate entity, not just as a sound submerged within a continuous fabric. For example, you may hear the woodwinds play a repeated figure very softly during a quiet passage, only to have them disappear when the rest of the orchestra comes back in. With the Mk.II, I was still able to clearly hear, without strain, the woodwinds beneath the much louder brass and strings—they were perfectly and naturally resolved. Solo piano proved another good example: the left- and right-hand notes didn't merge into one sound, but maintained their separate identities—just as in live music. On every HDCD disc I played, I was amazed at how the music had a natural and unforced resolution.

I found myself listening at lower playback levels with HDCD—I didn't feel the need to increase the level to hear more musical detail. It was all beautifully resolved at any playback level. This difference may sound minor, but the musical consequences were profound. When all the music was right there before me without my trying to resolve it, I felt a much greater sense of ease and involvement. I was overwhelmed by the sheer amount of music before me—it was a moving and gripping experience. I can't overstate how important this quality was to the musical experience.

With HDCD-encoded discs played through the Mk.II, the music seemed to have been recorded in a bigger hall. This impression was created by HDCD's greater resolution of spatial cues. This resolution was so great that, with some passages, I was actually able to hear the first reflections from the walls as discrete entities, with the reflections becoming more diffuse over time. All other digital tends to fuse the reflections into the musical fabric rather than allow me to hear them as separate events. Moreover, the sense of air surrounding the images was much bigger, more transparent, more "tangible" with HDCD. Gone was the flat, synthetic, cardboard-cutout sterility that has characterized even the best digital until now. The result was a much more lifelike presentation.

The soundstage had an openness in the treble that gave the impression of unlimited extension. I've found all previous digital to have a kind of closed-in quality in relation to LP or master tape. It wasn't that the treble sounded rolled-off on conventional CDs, but that the sense of air riding "above" the music had been missing. HDCD seemed to restore that missing extension.

Next, HDCD provided a much more realistic portrayal of instrumental timbres. When I first heard the cymbals on Mike Garson's terrific The Oxnard Sessions, Volume Two (Reference RR-53CD), it struck me that I'd never heard treble reproduction like that from digital. The metallic edge that so often overlays the treble simply wasn't there with HDCD. Instead of the treble energy being aggressive and making my ears want to close, it was smooth, rich, warm, and full of character. HDCD's superb treble rendering threw into sharp relief the synthetic treble sound we've become inured to over the past decade of the compact disc.

Just as the treble was free of edge and grain, the mids were wonderfully liquid and pure. There was a palpable realism to instruments and voices that was astonishing. On more than one occasion, listening with my eyes closed, I was physically startled by the entrance of an instrument, so real was the rendering of timbre. I attribute this quality to the lack of glare and hardness, the stunning harmonic purity, and the fine resolution of inner timbral detail. This lack of glare was particularly apparent on Mike Garson's piano: for the first time in digital, the piano's texture wasn't overlaid by a glassy glare on the higher notes' transient attacks.

Finally, HDCD-encoded discs played through the SFD-2 Mk.II had unbelievable dynamic contrast. Keith Johnson's HDCD discs on Reference are dynamic to begin with, but with the HDCD decoder, the dynamic window was opened much wider. You must be careful when setting the level if the music begins softly—the louder passages will have you running for the volume control. For example, the difference between the quiet beginning and the peaks of "Festival Day in Seville," from Trittico (Reference RR-52CD), and also on the HDCD sampler disc, seemed wider with HDCD decoding.

This wide dynamic contrast was enhanced by the music's ability to retain its coherence and palpability at very low levels, and by the lack of congealing and hardening of the sound during loud and complex musical passages. The sound had a sense of dynamic effortlessness, a lack of strain, and a wonderful ease that I've never heard from digital—or analog, for that matter.

Perhaps the best testimonial to HDCD sound quality is the feeling I got when it was taken away. After my wife and I had spent about 45 minutes listening to The Oxnard Sessions in full HDCD sound through the SFD-2 Mk.II, I switched to another good processor that doesn't have HDCD—without telling her what I was doing. Even before I got back to the listening seat, she was aghast at how the magic, the total mesmerizing involvement in the music, had disappeared. The non-decoded HDCD presentation was flat, synthetic, and sterile by comparison.

Without a doubt, HDCD more than delivers on its promise of elevating digital audio to a new level of sound quality and musicality. It has forever changed my standards of reproduced music. HDCD simply doesn't sound like digital—in fact, it doesn't even sound like analog master tape; it sounds more like a live microphone feed. This technology is a stunning achievement in high-quality music reproduction.

I felt frustrated, however, that once I'd played the few HDCD-encoded Reference discs, there was no other music to enjoy in HDCD sound. Pacific Microsonics needs to get the professional HDCD encoders (not available until the middle of this year) into the mastering studios so that their technology isn't relegated to a just a few audiophile labels. The pleasure of HDCD should be available to all.

Having said that, however, I must report that many of the remarkable sonic qualities I heard with HDCD were apparent through the SFD-2 Mk.II on non-HDCD-encoded discs as well—but to a lesser degree. I heard newfound detail, space, and liquidity on very well-known discs. I can't believe that its vast improvement in sound quality over its predecessor was primarily the result of the analog-circuit refinements and lower jitter. Instead, I'm led to believe that the PMD100 HDCD decoder chip's integral digital filter is simply better (footnote 3) than the NPC filter nearly every manufacturer has used until now.—Robert Harley

Conclusion: The Sonic Frontiers SFD-2 Mk.II with its HDCD decoder/filter chip establishes a new benchmark in digital-playback sound quality when playing back HDCD-encoded discs. In HDCD mode, the Mk.II is the best-sounding processor I've heard to date, eclipsing even the non-HDCD-equipped Mark Levinson No.30.5. Even with CDs that were not HDCD-encoded, the SFD-2 Mk.II's sound featured many of the qualities heard with full HDCD encoding and decoding. The beautiful portrayal of timbre, the stunning spatial presentation, and the astonishing resolution of low-level musical information were without precedent. I must attribute these qualities largely to the better filter within the HDCD chip.

You simply must hear HDCD—in a converter of high enough quality to exploit the technology's potential—to believe how good digital audio can sound. I've never been tempted to say that any converter beat the sonic performance of my Well Tempered Turntable, WTA, and AudioQuest AQ 7000nsx cartridge—a very good, but by no means state-of-the-art, LP playback system. With HDCD recordings through the SFD-2 Mk.II, I can say, for the first time and without hesitation, that the sound of the CD was better in every way than that of LP. HDCD is a landmark event in high-quality music reproduction.

The SFD-2 Mk.II is worthy of my highest recommendation. Its sonic performance is second to none, the new styling and functionality are first-rate, and its $5295 price puts it within reach of audiophiles who could never spend five figures on a digital processor. What more can one ask?—Robert Harley

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Footnote 1: The SFD-2 received Stereophile's "Digital Source of 1994" award.—Robert Harley

Footnote 2: A designer whose ears I have the greatest respect for disagrees with me, believing the SFD-2's up-front perspective is more like that heard from live music.—Robert Harley

Footnote 3: It's possible that many of the qualities I heard in the SFD-2 Mk.II will be available from lower-priced products once the HDCD chip becomes more widely used.—Robert Harley