It is ironic that 25 years after the introduction of the first generation CD player by the Sony/Philips consortium a groundswell of designs has emerged that challenge one of the mediumís fundamental engineering assumptions. It had been considered a given that an anti-imaging filter is necessary to remove the ultrasonics generated during the digital to analog conversion process. Recall that the earliest players used multi-bit DAC chips followed by analog "brick wall" filters designed to steeply attenuate the image spectra above 22 kHz. While the specs looked good on paper, the promise of "perfect sound forever" turned sour quickly as a chorus of complains echoed a common theme: bright, fatiguing sound that ultimately resulted in digititis Ė an allergic reaction to digital sound. The establishmentís initial reaction was to blame the messenger; namely, digital masters were said to be the culprit. I recall listening to J. Gordon Holtís Sony CDP-101, the first kid on the block with a CD player. This one being a seminal first-generation player, and a gift from the marketing folks at Sony. Out of a stack of some 30 CDs, only a couple managed to sound decent. Slowly, the real problem was recognized to be the brick wall filter and sure enough second-generation players took advantage of evolving digital signal processing technology and incorporated digital oversampling filters positioned prior to the multi-bit DAC. I have no preconceived bias against digital filters; they neither generate new information nor improve resolution, but they do allow the use of much gentler analog filters, which are audibly benign.
Now, just when it appeared that the digital ship had righted itself, a new "dark age" was spawned with the advent of the single-bit sigma-delta converter. Author Ken Pohlmann (The Compact Disc Handbook) gives an excellent analogy of how such a chip works. He likens it to a light switch operating at a high frequency. The two extreme amplitude states are off and on, but also any intermediate level can be achieved by toggling the switch off and on rapidly at a given frequency. The single-bit DAC was seen as offering the potential of increased linearity relative to the older R2R type at lower cost while obviating the need for factory calibration. In contrast, the R2R DAC uses an onboard voltage-divider resistor network or ladder capable of generating 65,536 voltage values. Each bit in a 16-bit data word enters the ladder through a switch. The most significant bit (MSB) enters the ladder at the top, while the least significant bit (LSB) is assigned the last section in the resistor network. A data zero keep the corresponding switch open, while each 1 closes a switch and allows that bit to contribute to the overall output voltage. Granted, the LSB is difficult to maintain in calibration, but the output is free of the RF switching noise that afflicts the sigma-delta type, which requires sophisticated digital noise shaping techniques to work at all. Since noise cannot be destroyed, noise shaping merely shifts the noise: reducing noise within the audible bandwidth while increasing it at higher frequencies. More recent low-bit sigma-delta designs improve the situation somewhat but fail to change the bottom line: improving linearity at the cost of increasing high-frequency noise is a poor tradeoff.
In my experience, designs based on the sigma-delta chip tend to sound bright and/or lack convincing timbre accuracy. Altmann Micro Machinesí progenitor, Charles Altmann, is much more empathic about this issue. In his opinion, there is no music possible with sigma-delta DACs. In his experience R2R chips are the only way to achieve a listenable sound quality. However, the type of R2R chip used, he feels, is of far less importance than the skill of the designer. While from a technical standpoint he appreciates R2R type DACs such as the Burr-Brown PCM1704 (true 24-bit noise-free resolution), they are not necessarily well suited to zero oversampling applications. He believes that he has been able to push the Philips TDA1543 dual 16-bit DAC to incredible results. Incidentally, by virtue of its single +5V voltage rail, it is compatible with battery power supplies. Some might consider the TDA1543 as a relic from a bygone era, but its low-cost, potential for good sound and compatibility with battery power have made it a favorite with many designers. In fact, three of the four DACs reviewed herein use this particular chip.
So Are Anti-Imaging Filters Really Needed?
It was the golden-eared Peter Qvortrup at Audio Note UK who first dared to listen directly to an R2Rís DAC output Ė minus analog filters and what he refers to as "digital trickery." What Peter discovered is a sound quality that is much more closely akin to the vinyl experience. In my experience, zero oversampling gives the impression of a more believable soundstage. The spatial impression in terms of depth and width perspectives is typically better defined relative to oversampling designs. It is as though the auditory system is presented with a better set of cues with which to synthesize a 3-D impression of the auditory stream. Perhaps the Gestalt psychology principle of closure is at work here. This is a mechanism for dealing with missing information, a means, if you will of connecting the dots (to use a visual analogy). An example given by Al Bregman is that of a soft sound being masked or drowned out by a louder one. If the softer sound can be heard both before and after a burst of the louder sound, it can be perceived to continue behind the louder sound Ė even if it is physically removed while the loud sound is being played.
Welcome to the natural voicing of zero oversampling. What a breath of fresh air! Gone is the endemic brightness of early CD players and sigma-delta converters and its associated sensory overload. Yet, the relative paucity of zero oversampling designs makes you wonder whether most DAC designers spend more time measuring than listening.
What about the ultrasonic energy present at the output of a zero oversampling DAC? Is it likely to overload or generate distortion products in the associated analog chain? While that is always a possibility with high-frequency test signals, there is no evidence that this is at all an issue with music program material. The proof is in the listening.
The Jitter Game
If we assume that 20 nS is about the correct threshold for the audibility of jitter, then any player that, for example, deploys the ubiquitous Crystal Semiconductor CS8413/14 receiver chip should handily eliminate the audible effects of jitter. The primary functions of this chip are to recover and decode the audio data from the S/PDIF digital data stream. A low-jitter clock is provided onboard that uses a phase-locked loop and a voltage-controlled oscillator. Jitter is reduced at the output to typically 200 picoseconds (pS) or 0.2 nS. Thatís a factor of 100 lower than the threshold of audibility claimed by some. Yet, many audiophiles have experienced an increase in clarity and speed with decreasing levels of jitter well below 200 pS. Therefore, it can be safely stated that any design whose entire jitter reduction is dependent on an-of-the-shelf receiver chip cannot possibly offer state-of-the-art performance.
The Scott Nixon Audio Tube Dac+
Right out of the box, the TubeDac+ sounded significantly substandard. Even after an extended break-in, the sound was still so grainy and lifeless that I decided to return the unit to Scott Nixon for a checkup. Scottís finding was that he DAC chip was clipping a bit and THD was well over 1 percent. Apparently he has only had a handful of the chips do this, but usually within the first hour of burn in. The fix consisted of changing the fixed 'vref' resistor to a trim pot to precisely balance out the waveform.
Back from its visit to the doctorís office, the TubeDac+ sounded much improved. Sonically, its calling card is a smooth, easy-to-listen-to presentation with a slightly romantic and prominent midrange. The residual level of midrange grain was a function of the type of 6922 used. Be sure to avoid any of the Sovtek variants. Siemens Gold pin and Bugle Boys worked best. Keep in mind that the buffer stage is operated at exceedingly low B+ voltage. That avoids the need for and the cost of a high-voltage supply, but the downside is that the operating point is constrained to a non-linear range of the transfer curve. It would appear that this design choice is responsible for most of the unitís sonic shortcomings.
There were problems noted almost immediately at the frequency extremes. The upper octaves sounded closed-in lacking in air and finesse. That accounted for the fact that instrumental timbres sounded darker than the real thing. Bass lines were prominent but lacked adequate pitch definition. Soundstage dimensions were not fully fleshed out. In particular, the depth perspective was compressed. Image outlines were broad-brush, lacking the requisite specificity to resolve massed instruments. I preferred having the "anti-sinc" filter turned off, as it marginally improved image focus. Veiling of the soundstage-reduced transparency, which meant that visibility of the inner recesses of a recordingís ambient space was impaired. Neither was low-level detail resolution on par with the performance bar set by more expensive DACs. For example, it was difficult to follow instrumental lines backing a lead vocalist. Background harmonies were masked to the point of reducing the presentationís clarity. And finally, dynamic shadings lacked conviction, as transients simply did not rev up as fast as they should.
Bottom line: Easy, inoffensive sound, trapped inside a low-resolution device.
Overall rating: 2.25 blue notes out of 5
47 Laboratory model 4715 DAC
The 4715 has been around for a few years and has garnered several reviews in the interim. I think that on the basis of musicality, it still holds up well to the competition. Harmonic textures are smooth and free from electronic glaze and high-frequency emphasis. Neither is it a detail freak, artificially highlighting nuances within the tapestry of the music. If I may be permitted a tactless moment, I would characterize its sound as lacking flash and zing. If offers the music lover timbre fidelity but without the nervous energy that characterizes much of digital sound. It is the sort of sound quality that is bound to disappoint hard-core audiophiles who are into the sound of a recording per se rather than the music. The tonal balance is reasonably neutral, with just a slight loss of treble sheen. Bass lines were consistently well defined and well integrated with the lower midrange. The core of the midrange exhibited a textural sweetness and purity that has become a hallmark of Kimuraís designs. This alone was worth the price of admission, as it placed no barriers between the listener and the soulfulness of the music.
Since the unit lacks a buffer or output gain stage, proper interface matching becomes essential. A tube preamplifier or line stage, which typically feature high input impedance appeared to work best. Tube magic also helps the 4715 in recreating a reasonable soundstage spatial impression. When compared with my current reference, the Altmann Micro Machinesí Attraction DAC, it became clear that the 4715 falls short in several respects. A slight veiling of the soundstage results in a reduction in transparency and loss of low-level detail. Transient speed is slightly blunted, as is the ability to follow the decay portion into the background of the recording. It follows that the clarity of the presentation is not entirely crystalline in nature. Neither are image outlines as sharply focused as they should be. It would appear that its shortcomings are due to the limitations imposed by the inherent jitter performance of the CS8412 receiver chip. Unless a designer is willing to look for new means to further reducing jitter at the output of the receiver chip, state-of-the-art performance is unattainable.
Bottom line: The 4715ís combination of harmonic sweetness and purity of expression are noteworthy. A decent performer on all fronts, its most endearing gift is the ability to involve the listener in the musicís emotional content. It is not flashy or showy, so itís unlikely to impress hard-core audiophiles, but otherwise it represents a solid recommendation for the music lover who simply wants to kick off his shoes and enjoy the music.
Overall rating: 3.5 blue notes out of 5
Audio Note UK DAC1.1x/II Signature
Both unbalanced (RCA) and balanced (AES/EBU) connections are provided. Taking care of the digital data stream is the Crystal CS8414 receiver chip, coupled via a digital pulse transformer. The converter chip is the Analog Devices AD1865, a dual 18-bit design offering excellent THD+N and SNR. The AD1865 is a complete, dual 18-bit DAC. The first four MSBs of each DAC are segmented into 15 elements, while the 14 LSBs are produced using standard R2R techniques. Both segment and R2R resistors are laser trimmed to provide extremely low total harmonic distortion. The AD1865ís onboard high-performance output amplifiers are said to be capable of fast settling and high slew rate.
An anode follower gain/buffer stage, configured with a single 6DJ8/ECC88/6922 dual triode per channel, is used to keep the output impedance below 2 kOhm. My review sample was outfitted with a pair of Siemens ECC88 types. When I inquired about the unitís output signal polarity, I was told that it is phase inverting. However, to my ears that did not appear to the case, as I obtained the best sound with the amplifier-speaker interface in-phase.
There was plenty of mojo on display, once I got past the Siemens tubes. My first impression was that the soundstage was "condomized" Ė that is, wrapped up in a giant condom. Safe sound is not my idea of listening pleasure. What was needed was more feeling, less veiling, in a word - more intimacy. It was time for a bit of tube rolling. Out went the Siemens and in went the Richardson select new production Bugle Boys. This particular pair had performed extremely well in other contexts and it did not disappoint in this application. The presentation became smooth, round, and cohesive in the finest tradition of vintage tube sound. But most of all, the feeling of Ďbeing thereí was now much more convincing. The midrange was revved up in terms of transient attack and immediacy.
The zero oversampling sensation of natural balance was very much in the air. The lower midrange and upper bass were portrayed with a big tone character, full-bodied and rhythmically alive, basically giving my 6SN7-based tube line stage free reign to express its palette of harmonic colors. Think Tennessee whiskey, Jack Daniel's Old No. 7, which is distilled, then filtered through 10 feet of charcoal, then aged in white oak barrels. The end result is most definitely mellow. Treble extension was a pleasant surprise, as many zero oversampling designs sound closed-in, casting a shadow over the soundstage, and lacking in transient sparkle. Yet, despite its apparent extension, upper octave brightness and glare were absent in action. Listening fatigue was extremely low and totally dependent on the character of the recording.
Bass lines were tightly defined. Dynamic nuances were given full scope of expression. Portrayal of soundstage dimensions was also very good. In particular, the width perspective was as good as Iíve experienced, extending linearly from left to right without any bunching at the speakers, and at times extending beyond the edge of the speakers. As in the case of the 47 Laboratory model 4715, there was some loss of image focus and low-level detail resolution. It was not possible to delineate ambient decay fully into the background of the recording, which translates into a loss in clarity. To my mind, these shortcomings are most likely attributable to the residual jitter at the output of the CS8412 receiver chip.
Bottom line: The Audio Note DAC1.1x/II Signature makes for a safe recommendation for music lovers who have been traumatized by digital sound and are looking for a safe CD haven. If offers a more kinetic presentation relative to the 47 Laboratory model 4715, while retaining the latterís zero oversampling virtues. But be prepared to experiment with tube substitutions.
Overall rating: 3.75 blue notes out of 5
Altmann Micro Machines Attraction DAC
And now for something completely different; and yes, I did leave the best for last! What you see is far from the typical DAC in a box. In fact, there is no box. All components fit neatly onto a small surface-mount circuit board, which is affixed to a piece of spruce that has been treated for vibration control. The Attraction DAC is designed to be powered by a single 12V automotive battery. It is part of AMMís BYOB line Ė that is, Bring Your Own Battery. AMM recommends the Optima Redtop spill-proof lead-acid battery, which is what I use (model 34). You will also need a battery charger, but with a current draw of only 200 mA, thereís a reserve of well over 200 hours of enjoyable listening before re-charging is required. And battery status is monitored by a blue LED; when it starts dimming it is your cue to start a re-charge cycle. Every Attraction DAC comes complete with a gold-plate battery cable and a five years limited warranty.
While the idea of battery power is not new, it is particularly relevant in the case of a DAC. For example, power supply voltage fluctuations induce jitter at the input of an R2R type converter chip, since the reference voltage controls the firing of the switches at the input to the resistor ladder. This is clearly a situation where the purest power supply is needed, and the idea of decoupling it completely from the AC mains represents the right call. No need for power transformers, rectifiers, filters, and regulators with all of their attendant noise. Battery supplies neatly cut through all of issues raised by AC supplies.
An all-sample rate option is available that allows the DAC to negotiate sample rates from CD (44.1kHz), DAT (48kHz),a and DVD-A (88.2, 96, 176.4 and 192kHz). A couple of toggle switches are used to select the actual sample rate. The Attraction DAC is said to be the world's only digital playback machine with static non-oversampling DA conversion up to 192 kHz. Needless to say, the receiver-chip used (Altmann R16) is not a garden-variety off the shelf design. Altmann explains that he had to use a custom design realized with a Xilinx programmable logic chip, as no "useful" receiver chip for 192kHz is available from either Crystal (Cirrus) or from AKM Semiconductor. The digital input signals are recovered with extremely low jitter as they are generated by custom (Golledge, UK) voltage-controlled crystal-oscillators (VCXOs) and employ AMMís proprietary UPCI (Ultra Precision Clock Injection) technology.
Yet another DAC option is Altmannís Jitter Scrambling Decorrelator (JISCO), which may be switched in or out. The JISCO is also available as a stand-alone device that is said to improve sound quality even with the cheapest CD or DVD-A player transports. The goal of the JISCO is to reduce jitter at the input to the DAC chip by manipulating jitter before the digital data stream is processed by an off-the shelf-receiver chip (e.g., one of the ubiquitous Crystal chips). It accomplishes that paradoxically by actually increasing jitter at its output. After getting through the scrambling algorithms, jitter below 10kHz is shifted to a frequency range of several MHz where the receiver chipís internal clock extraction PLL is most effective in reducing jitter. According to Charles Altmann, jitter attenuation of the CS8414 is quite good at 1MHz and above, but poor below 10kHz where much jitter is produced in CD playback. So when all is said and done, the actual jitter at the DAC chips is lowered and uncorrelated with the signal.
The DAC chip used is our old friend, the Philips TDA1543. Altmann is also a firm believer in the Burr-Brown OPA134 operational amplifier because "I am good at getting the best sound out of them. There are many other op amps available, also much more expensive ones, but the ones I have tried out just don't have that "134-sound". A total of five of these devices are used per channel: one for current to voltage conversion and four as an output buffer. The OPA134 has been around for a while and according to Burr-Brown features a true FET input stage and increased headroom to provide superior sound quality and speed for audio applications. Note that the Attraction DAC does not invert signal polarity.
For the record, my review sample was configured as follows: the Attraction DAC (base price of Ä750) with the all-sample rate option (Ä250), and the JISCO option (Ä250) for a total cost of Ä1250. Prices and options are spelled out at www.mother-of-tone.com.
The Truth Changes Everything
Audio critics, on the whole, tend to magnify small differences between competing components. At the highest levels of high-end audio such differences become elusive, and in some cases may well be a function of system and room interactions. With this DAC, the differences were obvious and dramatic. The impression of clarity was immediately startling in its diamond-like precision. There was simply no veiling of the musical fabric. Low-level detail was resolvable as if peering into the soundstage via a high-powered microscope. This was no cheap trick achieved by brightening or etching harmonic textures. Many speakers have sold in the showroom on the basis of a presence region boost or high-frequency emphasis. Such a tonal balance is most impressive, at least in the short run, but rather fatiguing as most consumers discover to their dismay much later. In this case, detail resolution was a function of pristine, pure harmonic textures free of digital artifacts, so that the auditory system had an easier time unscrambling and resolving the various instrumental lines. Vocals, laced in EQ and artificial reverb (standard protocol on pop multi-track recordings), that would turn "corrosive" on most DACs were reproduced with the full splendor intended by the recording engineer. You should really hear the expansion and decay of reverb through this DAC. Absolutely breathtaking! I am not suggesting that I was inundated with detail. Much like observing a school of fish in an aquarium, most of the time it was the ensemble that captured my attention, but it was always possible to focus on an individual and follow its explicit movements.
The tension generated by microdynamic nuances and transient attack was left fully intact. Transients were unfolded with racing-car speed and dynamic conviction. On the flip side, decay of musical lines was preserved down to a velvety black background. Couple that with a soundstage of immense transparency. It felt as though a giant searchlight had illuminated the inner recesses of the soundstage. Depth and width perspectives were fully fleshed out. Image outlines were laser sharp, so that it took no effort to resolve the ebb and flow of individual instruments within the musical tapestry. The Gestalt of listening to a cohesive musical ensemble, in the same place at the same time, usually a strength of analog sound, was much in evidence.
Above all else, the music possessed both body and soul. The musicís full palette of emotions was propelled forward with verve, from a whisper to a scream, from a sweet refrain to a touch of the blues Ė a veritable "soul train" of emotions. This is not the sort of DAC that will ever put you to sleep. Audiophile obsession with detail and imaging is akin to mental masturbation. Thatís no way to reach a sonic orgasm. Fidelity to the real thing starts with timbre accuracy, and this was an area that the Attraction DAC excelled in. The corpus of each instrument was portrayed with a believable foundation and a purity of harmonic texture. Bass lines were full, tight, and in time with the upper octaves. The midrange with associated tube amplification was slightly rich and warm, which is the way I like it. My guess, as I have yet to pin this down, is that this rich and dark disposition is not intrinsic to the DAC but a reflection of the character of the associated tube gear.
Bottom line: It should be clear that the Attraction DAC pushed all of my buttons. Anyone serious about music needs to audition this DAC. It will redefine your expectations of what is possible with the standard CD format. What a enjoyable way to rediscover your CD collection. Simply put: a tour de force of digital design!
Overall rating: 5 blue notes out of 5
47 Laboratory model 4715 DAC www.sakurasystems.com
Audio Note DAC1.1x/II Signature
Altman Micro Machines Attraction DAC www.jitter.de