Digital Processor Reviews

Since the first digital processor on the market using UltraAnalog DACs appeared (the $12,000 Stax DAC-X1t, reviewed in August 1990, Vol.13 No.8), there has been a proliferation of good-sounding processors using this extraordinary—and expensive—part. Among these are the Audio Research DAC1, Audio Research DAC1-20, VTL Reference D/A, and the groundbreaking Mark Levinson No.30 reviewed last month.

We can add another product to this illustrious list: the Kinergetics KCD-55 Ultra. Although the KCD-55 Ultra is the latest processor I've auditioned using UltraAnalog DACs, Kinergetics would have been the first to market with such a product had it not been for a quirk of fate. At the 1989 Audio Engineering Society Convention in New York, UltraAnalog had hundreds of thousands of dollars' worth of custom equipment stolen—including Kinergetics' only prototype of an UltraAnalog-based version of their highly regarded KCD-40 CD player (footnote 1). Rather than start over with the CD player, Kinergetics decided to build an ambitious outboard D/A converter instead.

The result is the KCD-55 Ultra reviewed here, Kinergetics' top-end digital product. As often happens after loss or destruction, the opportunity to rebuild creates something better than what was originally lost. Kinergetics used their months of design experience on the KCD-40 Ultra as a launching platform for this more high-reaching effort.

Let's see what they've come up with.

Technical description
The KCD-55 Ultra (hereafter called simply the Ultra) is an unusually and beautifully built component. The thick front, rear, and side panels are made from CNC machined extruded aluminum. These panels are then attached to a separate sheet-metal chassis that holds the printed circuit boards. This construction technique results in both a very solid build and an expensive, elegant appearance.

Two knobs, two switches, and an LED comprise the front-panel controls and indicators. These functions are common to most digital processors: power on/off indicator LED, polarity reversal switch, and selection between two digital inputs. The knobs, however, are unusual. The first attenuates the level from the "variable" analog outputs, obviating the need for a preamplifier or passive level control in CD-only systems. Rare, but not that unusual. The knob on the panel's right-hand side, however, is another story. Marked "Processor," this control reportedly allows the user to tailor the sound to match system and tastes. I'll have more to say about this function later.

The rear panel has two pairs of analog outputs on RCA jacks, one fixed level and one variable, with the latter controlled by the front-panel volume knob. Although the rear panel is machined for two coaxial inputs, only one RCA jack is fitted. The second input's hole is covered by a plug (the circuit topology can accommodate up to four digital inputs). An AT&T ST-type optical input is provided on newer production, reflecting the general agreement that ST-type optical is superior to either coaxial or Toslink interfaces. Kinergetics commendably added the expensive ST-type input without raising the Ultra's retail price.

Popping the top panel, I was surprised by the Ultra's minimalist design; there were fewer parts than found in most converters. The analog output stage, power supply, and "glue logic" (the chips that make everything work together) were all executed with a minimum of parts. A closer examination, however, revealed some unusual and tweaky design techniques.

Starting with the power supply, three transformers are used, one toroidal for the analog stages and two standard laminated types for the digital supply. These are mounted on a separate pcb in the front left-hand corner, away from the rest of the circuit. The digital supply transformers were selected on the basis of their ability to keep RF noise generated by the digital circuits from getting back into the AC line.

There are a total of eight regulation stages for six supply voltages (two digital and four analog). The analog supply is regulated by standard three-pin IC regulators, then, for the critical output stage, is regulated again by a discrete circuit using a pair of OP42 op-amp chips located right next to the output stage. Wherever a rail supplies a circuit stage, a Roederstein polypropylene decoupling capacitor is employed.

The input receiver is the ubiquitous Yamaha YMJ3623B (footnote 2), but implemented with Kinergetics' proprietary jitter-reduction circuit. A Sony CXD1144B chip provides 8x-oversampling digital filtering. This chip is the most powerful (highest number of taps) and expensive of the digital filters, selected after auditioning a variety of filter chips. The input receiver, filter, AT&T optical input jack, and associated components are mounted on a separate pcb toward the back of the chassis.

The rest of the circuit is contained on a large pcb that consumes about a third of the chassis's real estate. I was particularly impressed by the output stage: it's all discrete, class-A, direct-coupled, and uses high-quality parts. The circuit is essentially a "discrete op-amp," with bipolar transistors and JFETS providing gain and a Precision Monolithics BUF-03 acting as the op-amp's output. This is the same part Corey Greenberg was so enthusiastic about in his DIY buffered passive preamp article last November. It can drive large amounts of current (40mA), and has no problem with low impedances (it was designed as a 75 ohm line driver).

Because the Ultra's analog stage is direct-coupled, an NE5532 op-amp and pair of trim pots form a DC servo circuit to prevent DC from appearing at the analog output. The front-panel level control is a high-quality metal-film type ganged pot (with separate left and right channel adjustment) that attenuates the output level from the variable output jacks. Note that this pot is an attenuator after the gain stage, not part of the gain-determining feedback loop. De-emphasis is passive, switched in by a solid-state device instead of by a relay.

Now, about that front-panel knob marked "Processor." According to designer Tony DiChiro, it's a "hysteresis control." This circuit is found in all Kinergetics electronics, but the Ultra is the first product to provide user adjustment of the circuit (footnote 3). The control's range is quite narrow to prevent overuse and sonic degradation, but reportedly produces enough change in sound for final system matching. I'll comment on the control's sonic effects later.

Where the Ultra really gets elaborate, however, is in the critical digital/analog conversion stage. Not only does the Ultra use the best DACs currently available—the two-channel, 20-bit UltraAnalog DAC D20400—but it employs two of them for differential operation. In this scheme, each dual DAC receives the digital code representing the analog signal and the same code inverted. One channel of the dual DAC converts one polarity to analog, the other channel converts the other polarity. This produces two analog output signals of opposite polarity for each channel. When the two opposite-polarity signals are amplified differentially in the output stage, only the wanted difference between the two signals is amplified. Any noise, distortion, or artifacts common to both channels are thus rejected—a phenomenon called "common-mode rejection."

Kinergetics uses this technique in all their digital products. It becomes very expensive, however, when using 20-bit UltraAnalog DACs. For comparison, the KCD-55p, which uses the same chassis, power supply, pcb, digital filter, and discrete output stage as the Ultra, sells for $1695. The only difference? The KCD-55p uses the Analog Devices AD1860 DACs found in the KCD-40 (and the ST optical input is optional). Incidentally, owners of the KCD-55p can have their units upgraded to the Ultra version for $2500, about $200 more than if the Ultra were purchased initially.

Overall, I was impressed by KCD-55 Ultra's build quality, thoughtful design (particularly the additional discrete regulation stage for the analog output and the output section itself), and use of two very expensive UltraAnalog 20-bit DACs.

Listening
A logical comparison for the Ultra was the Audio Research DAC1-20. The ARC is similarly priced ($3500), uses an UltraAnalog DAC, and has established a reference level of musicality at its price. Any digital processor selling for about the same money must regard the DAC1-20 as formidable competition.

After my first listen, I was pleasantly surprised by the Ultra; it more than held its own against the DAC1-20 in some respects. In addition, the Ultra offered a different perspective on the DAC1 that may appeal to many music lovers.

First, the Ultra had excellent bass—tight, controlled, and with powerful dynamic impact. In fact, the Ultra had the best low-frequency reproduction of any of the UltraAnalog-based processors I've auditioned, save the Mark Levinson No.30. I have previously criticized the bass performance of UltraAnalog-based converters as lightweight, soft, and lacking dynamics. I had mistakenly attributed this characteristic to the DAC, not the implementation. The Ultra (and the No.30) set the record straight. The bottom end seemed to extend deeper than heard through the DAC1-20, with a more taut and less fat rendering. Dynamics were also superior, with a greater sense of slam and power. Listen to the kick drum in the tune "Are You Scary?" from the new Sheffield CD The Usual Suspects. Through the Ultra, it had a punch and depth rivaled only by the No.30 and Wadia 2000.

In addition to being tight and punchy, the low end was round, solid, and had a satisfying fullness. Acoustic bass had a warmth and liquidity I particularly enjoyed. Pitch definition was excellent, with clear articulation of each note, even with fast and complex bass lines. Listen to the acoustic bass on "Round Midnight" from Kenny Rankin's Because of You (Chesky JD63, reviewed in this issue). This is a stunning recording; the track mentioned (vocal and bass only) is very revealing of how well a converter conveys the instrument's roundness, warmth, and fine detail. Through better processors, the instrument will sound more "bass-like," and less flat or wooden. Through the Ultra, the bass was superbly portrayed. Overall, the Ultra's bass reproduction was exemplary, and clearly a step above the DAC1-20's.

When it came to presentation of midrange and treble textures, however, I preferred the DAC1-20. The Ultra had a trace of hardness through the upper mids and lower treble that was contrasted with the DAC1-20's ease and liquidity. On Three-Way Mirror (Reference Recordings RR-24CD), for example, the acoustic guitar, flute, and cymbals sounded slightly edgy. In addition, the presentation was more forward, with less sense of ease. It wasn't a case of not enjoying the Ultra—it was very liquid and had more natural rendering of midrange textures than most processors—but it fell short of the Audio Research unit, which excels in these areas. Long sessions with the Ultra tended to be more fatiguing than with the DAC1-20, and there was less inclination to listen at high levels.

Though the treble was smooth, I wouldn't characterize the Ultra as laid-back. Rather, it struck a good balance between revealing HF detail and being overly soft, with a tilt toward revealing detail. Treble textures had a trace of hardness compared to the DAC1-20, and sounded more "digital." In addition, the treble could at times sound a little on the etched and analytical side of reality, rather than soft and gentle. I wouldn't use the words "refined" and "delicate" to describe the Ultra's treble, characterizations I've used to convey the DAC1-20's presentation.

These drawbacks were more than offset, however, by the Ultra's superb rendering of detail and ability to separate individual instruments from the whole. The presentation was infused with a wealth of fine detail; subtle sounds that were blurred through the other processors became vibrant and alive through the Ultra. The brushed snare drum on Jazz at the Pawnshop (Proprius PRCD 7778), for example, was made up of many finely woven components rather than fused into a single sound. It also had a vibrancy and palpability rarely heard through any processor. Inner detail was rendered with a precision and life that made me feel as though I were hearing more music. Without a doubt, the Ultra presents another level of information to the listener. In this regard, the Ultra approached the No.30's stunning resolution of detail, but with less ease and warmth.

There is one important presentation aspect in which the Ultra is superior to just about every other converter I've auditioned, except the No.30: creating the illusion that the presentation was made up of individual images, not merely variations in a synthetic tapestry. On Robert Lucas's Luke and the Locomotives (AudioQuest AQ-CD1004) there was a convincing impression of the individual band members in the listening room (especially on the track "Feel Like Going Home"). The soundstage was beautifully fleshed out, with superb delineation of each instrument. Images were tight, well-defined, and thrown with pinpoint precision. This superb image specificity was accompanied by a feeling of air and space surrounding the instrumental outlines. The Ultra's portrayal of depth and space was excellent, with a distinct three-dimensional quality. Naturally miked music with subtle spatial cues was well served by the Ultra: the listening room assumed a wide range of apparent sizes throughout the auditioning, accurately reflecting the recording's characteristics. Soundstage width was similarly good; the musical presentation was thrown in a wide arc across the listening room, making it easy to enjoy the music and forget about the loudspeakers.

The Ultra's crystal-clear soundstage transparency further heightened these impressions. The result was a convincing illusion of individual instruments hanging in space around the loudspeakers. I really enjoyed this aspect of the Ultra's presentation; music was more lifelike and less homogenized. If I had to name the Ultra's best attribute, it would be this. Moreover, the ability to separate individual images from the presentation is rare in digital processors, and an aspect I find musically important. The Ultra was the antithesis of synthetic homogeneity.

When it came to dynamics, the Ultra was topnotch. Transient detail was razor-sharp, with fast, clean leading edges. Drums had power and energy not heard from the DAC1-20. The sound of the stick hitting the head was well conveyed, giving a greater feeling of the drummer's rhythmic contribution to the music. I recently recorded a great-sounding drum kit with just two microphones in a live room with a very pure, all-tube signal path. Playing back the DAT master tapes revealed the Ultra's ability to recreate the steep attack that gives drums immediacy and life. The Ultra's dynamic quality and transient quickness conveyed an increased sense of rhythm and energy, especially on jazz and blues.

Most of these impressions were gained with the Ultra going through an Audio Research LS2 line stage. Driving the power amplifiers directly from the variable outputs slightly improved the overall transparency and palpability. The LS2 is, however, extraordinarily transparent; listeners with most other preamps that are not as neutral will realize much greater benefits from using the Ultra's variable outputs and front-panel volume control.

Finally, I heard very little difference when using the front-panel "Processor" control. The presentation seemed a little softer toward the counterclockwise side, and more immediate with it turned up. Its effect, however, was far less then the differences described between the Ultra and DAC1-20. I must also add one complaint about the Ultra: there was a loud click when switching inputs. Users are therefore advised to mute their preamps or turn down the Ultra's level control when switching between digital sources.

Conclusion
If I had to count my favorite digital processors on one hand, the Kinergetics KCD-55 Ultra would be included. I really enjoyed my time with the Ultra: It consistently conveyed the music with life and vitality. The Ultra had many strong points that made it musically involving, especially its ability to differentiate individual instrumental images. I also found that its presentation of detail and transient quickness gave music a heightened sense of energy. Further, the Ultra excelled at throwing a spacious, well-delineated soundstage with pinpoint images. To top it off, the bass rendering combined power and punch with liquidity and pitch definition. Overall, I thought it was a terrific processor.

Having said that, I can easily see how many listeners will prefer the Audio Research DAC1-20's greater ease and gentleness over the Ultra's more incisive presentation. The DAC1-20 was more analog-like in its portrayal of instrumental textures, but ultimately presented less information to the listener. Playback systems that lean toward the etched and analytical may benefit from the DAC1-20's greater ease. I enjoyed listening to both processors; I'm sure their different interpretations will each find an audience.

The Ultra's variable output is a powerful attraction for listeners with CD-based systems. No preamp, passive level control, or extra interconnects are needed: just plug your CD transport and DAT recorder into the Ultra, and plug that straight into the power amplifier. Getting an active preamp and second pair of interconnects out of the signal path—especially if they're colored—will greatly improve a playback system's overall transparency and musicality.

After just having spent many enjoyable hours with the $14,000 Mark Levinson No.30, the KCD-55 Ultra struck me as emulating some of that reference product's best characteristics. The detail, image specificity, and dynamic contrast that made the No.30 so stunning were heard in the Ultra, albeit to a much smaller degree. The No.30 did this, however, with an ease not heard from the Ultra. Nevertheless, this says a lot about the Ultra's special qualities.

If you're considering buying one of the megabuck processors, give the Kinergetics KCD-55 Ultra a listen. It's up there with best of them.

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Footnote 1: I was most impressed with the KCD-40 when I reviewed it in January 1990 (Vol.13 No.1). It uses two Analog Devices AD1860 18-bit DACS per channel in a push-pull configuration.—John Atkinson

Footnote 2: This Yamaha receiver chip outputs 16-bit digital data, meaning that it truncates S/PDIF or AES/EBU datastreams featuring bit depths greater than 16.—John Atkinson

Footnote 3: The Modern Dictionary of Electronics (Howard Sams, publisher) defines hysteresis as (definition #5) "A form of nonlinearity in which the response of a circuit to a particular set of input conditions depends not only on the instantaneous values of those conditions, but also on the immediate past (recent history) of the input and output signal. Hysteretical behavior is characterized by inability to 'retrace' exactly on the reverse swing a particular locus of input/output conditions...This term literally means to lag behind."—Robert Harley