As chief engineer, Søren Kristensen is the technical guru at Soekris Audio. While his main business focus for many years has been designing and manufacturing embedded communication computers, he has always had a strong interest in high-end audio. Two things jump out immediately about the Soekris line of DACs. First, they’re all discrete R-2R sign-magnitude designs. In particular the dac1421 features a 27-bit precision ladder built with over two hundred 0.02%-tolerance thin-film resistors. Second, all Soekris products are designed and built in Denmark—no “designed in the West and built in the East” paradigm here—and yet the product line is remarkably affordable.
The R-2R sign-magnitude DAC technology was developed by Burr-Brown in the early 1990s to address the zero-crossing problem of a conventional R-2R DAC when one-half of the bit-switch circuits turns off and the other half turns on. This degrades signal-to-noise ratio of small signals due to the large noise contribution of switching transients. A sign-magnitude architecture takes a rather clever approach by actually starting at zero crossing and either adding or subtracting current from the resistive ladder to obtain the analog signal. On average, the smaller step sizes result in less noise and greater precision at low levels. To do this requires two internal DAC sections and the splitting of the binary bitstream into positive and negative polarities. The first bit of each binary word is designated as a sign bit with “1” denoting positive polarity and “0” a negative polarity; the rest of the bits specify an amplitude. One DAC section converts positive amplitudes while the other converts negative amplitudes. In the end, the output currents are summed to give the final analog signal. [See the sidebar for more technical details.—RH]
Søren tells me that he always liked the Burr-Brown series of sign-magnitude DACs, and that during 2014 he was thinking about designing a DAC for himself as a side project. It turned out that the PCM1704 chip, his first choice and widely considered to be the best, was designated as end-of-life by Texas Instruments, the parent company of Burr-Brown. With future production in mind, he decided against the PCM1704, but the problem was that weren’t any other similar chips available. Necessity being the mother of invention, and inspired by MSB Technology (at the time the only manufacturer of discrete sign-magnitude R-2R DACs), he set his mind on doing a discrete design. That ended up being the dam1021, the first low-cost R-2R DAC module for the DIY market, which is still being manufactured and sold today.
You may wonder, as I did, how Soekris was able to pull off a discrete R-2R DAC at an entry-level price of $899.One reason had to do with the availability of reasonably priced, precision (0.01%), surface-mount resistors from suppliers such as Digikey, which meant that no matching was necessary. Søren believes that one reason the PCM1704 was designated end-of-life was due to the expense of being manufactured on an old process line requiring laser-trimming. It’s also a question of smart engineering and manufacturing, says Søren, and personally taking charge of every operational aspect. The main PCB has been optimized for automatic manufacturing, using as much surface-mount technology as possible, and, hence, no wires. Parts were selected on a cost/performance basis rather than on audiophile reputation. All high-precision resistors in the R-2R network are highest-quality Vishay, but not better than needed. All important connectors are high quality and gold plated. Søren is not afraid to use parts some might object to for anecdotal reasons, like NP0 ceramic capacitors, MOSFET relays, and switch-mode power supplies. Costs are also being held down by using a direct online sales model, with the maximum of a single reseller between the manufacturer and the end user.
There are two SPDIF inputs (RCA and BNC), as well as a USB input, selectable from the front panel. The USB input is based on a standard XMOS interface chip, with the firmware modified to allow switching between Class 1 and Class 2 USB audio, the latter requiring USB 2.0, which is pretty standard these days. All of the digital inputs are routed to a field-programmable gate array (FPGA), which buffers and reclocks the bitstream and performs anti-aliasing filtering. It is followed by a digital volume control, whose output is clocked by an ultra-low-jitter oscillator driving the sign-magnitude R-2R resistor networks. The DAC can also accommodate a DSD input signal, which is internally converted to PCM by the FPGA.
The resistor network output voltage is amplified and buffered by a high-speed, all-discrete, zero-negative- feedback amplifier, operating in high-bias Class AB. The output stage can be switched to either line or headphone output, using MOSFET relays. When driving headphones, the amplifier is set for higher Class A bias current and gain. A crossfeed circuit may also be activated during headphone listening to control soundstage size.
There are four anti-aliasing filters that may be selected via the “Filter” button on the front panel. This gives the user enormous flexibility in tailoring the DAC’s sonic signature to suit particular tastes and system needs. The filter chosen is indicated by the LED color. Red is a sharp linear-phase filter, equivalent to a classic “brickwall” filter. Orange is a mix between linear- and minimum-phase filters. Green is a reasonably sharp, short-delay, linear-phase filter. And finally, the LED off indicates a soft Butterworth minimum-phase filter, which comes close to no oversampling (NOS).
My first order of business was to tweak my planar-speaker reference system toward full neutrality, really an important factor for accurate evaluation of any source component. That meant leaving tubes, i.e., tube colorations, out of the chain and routing all DACs through Ed Schilling’s totally truthful The Truth photocell volume control. Two critical cable changes were also implemented. Canadian Take Five Audio’s cryo-treated Mogami 3103 speaker cable turned out to be an exceptional fit, featuring an even tonal balance coupled with exceptional transient speed and control. Another major upgrade was DH Labs Silver Sonic D-750 digital cable in a one-meter length. I just couldn’t believe my ears; it brought about a substantial increase in soundstage image focus and transparency.
The first few rounds of listening tests were dedicated to Red Book CD via SPDIF input. The Soekris was paired mano a mano against several respectable DACs in my collection. First up was the Altmann Attraction DAC, which I’ve been powering lately off a lithium iron phosphate battery. I’ve lived with this NOS, TDA1543-chip-based design for over a decade and know its sound character quite well. The attraction here is the midrange: vivid, engaging, with excellent tonal color fidelity. With the digital filter set to soft Butterworth, the Soekris equaled the Altmann in these respects while delivering greater treble precision and bass drive. Its tonal presentation was consistently neutral without emphasizing any particular frequency band. Soundstage depth was enhanced as was image palpability, which now bordered on the reach-out-and-touch-someone caliber. These findings, coupled with the dac1421’s enhanced transient clarity and superior resolution of micro-detail, made it clear that the TDA1543 chip was being outclassed, and that some 30 years after its introduction it would perhaps be wise to lay it to rest.
Next up was the Metrum Acoustics Amethyst. Just slightly more expensive than the dac1421, this 24-bit R-2R Dutch design proved to be very competitive with the dac1421 in every way. Easy to listen to and featuring enormous layers of detail, it delivered textures legato style, without the digital glaze many sigma-delta DACs dish out. With as much listening as I do to female voice, it didn’t take me long to realize that the Amethyst sounded a touch laid-back through the upper midrange, not so much an issue with Mary Fahl’s earthy alto vocals (The Other Side of Time [Sony Odyssey SK89892]), but somewhat disconcerting on soprano voice. This same tonal aberration was also evident with USB source material.
Enter the Schiit Audio Yggdrasil. I had developed quite a liking for the Yggy, primarily in the context of tube amplification and USB audio. This time, in the context of solid-state amplification, it didn’t fare as well. It showcased a terrific low end and satisfying tonal weight, but the upper midrange, a serious sonic priority of mine, lacked proper harmonic colors.
So far, the dac1421 had managed to best all comers. There was only more challenger left, and that was the Audio-gd Reference-7—a decade-old DAC using a total of eight PCM1704UK sign-magnitude R-2R chips. Luckily, I managed to snag a used unit recently for a good price. Its default setting is 8x oversampling, which does result in a sweet tone, but image outlines shrink in the process. The digital filter may be bypassed internally, and this NOS setting yields superb 3-D image outlines and remarkably accurate tonal colors. No matter how much I tinkered with the filter settings, the Soekris approached but couldn’t equal the natural textures and 3-D imaging performance of the Reference-7 with Red Book CDs. It became clear during this process that there was no perfect filter setting. While the soft Butterworth filter conjured the most realistic image size it also sacrificed a bit of treble smoothness. Most of the time, the Green LED setting (minimum-phase filter) gave the best overall compromise, especially with the volume control set to -6dB to minimize overdrive of the DAC’s analog output stage.
One undeniable advantage of the dac1421 over the Reference-7 is its ability to handle high-res 24/192 files, which are readily available for streaming off Qobuz. To confess, I’ve become addicted to Qobuz streaming over the past year, mainly due to its wide music content and ease of navigation. This brings us squarely to the USB-audio phase of the evaluation.
Initially, I was a bit disappointed with the Soekris. USB audio lacked the focus and rhythmic drive I’d been enjoying with SPDIF Red Book CDs. I suspected that the culprit was most likely a noisy USB stream from my Mac BookPro laptop. So, I decided to investigate UpTone Audio’s ISO REGEN, a device that is inserted between the computer USB output and the DAC input with the objectives of providing galvanic isolation from a potentially noisy computer and of fully regenerating a clean USB data stream. It looked a bit funky dangling from the USB port, but it undeniably boosted the sound quality by a significant margin—I’m tempted to say by a factor of two. The most noticeable improvements were sweeter textures, tighter image focus, and enhanced soundstage clarity due to cleaner transient attack and decay. UpTone Audio’s Alex Crespi loaned me the optional UltraCap LPS-1.2 linear power supply for use with the ISO REGEN. It displaced a generic Chinese linear DC power supply and further boosted overall sonic benefits. The cost of this UpTone combo almost equals that of the dac1421, but if you’re into computer audio you should definitely up your game with UpTone Audio.
My original intention was to review the dac1421 as a DAC, but I couldn’t resist some headphone listening. It’s not really my thing, and my Sennheiser HD600 cans are clearly not state-of-the-art, but I was determined, nonetheless, to glean some sonic impressions. The solid-state disposition of the headphone amplifier was obvious with squeaky clean and concise harmonic textures. The crossfeed function worked as advertised. The medium crossfeed setting (–8dB sent to the other channel) worked for me, pushing the stereo image forward, thus more closely emulating the soundstage from a pair of loudspeakers.
In the final analysis, this diminutive DAC and headphone amp delivered superb sound quality, underscoring the sonic benefits of a discrete R-2R sign-magnitude design.
My dac1421 was purchased direct from the U.S. reseller, Mod House Audio. I actually started off with the entry-level dac1321 model. It sounded so much like a good moving-magnet phono.
Specs & Pricing
SPDIF/TosLink input: Up to 24-bit/192kHz
USB input PCM: Up to 24-bit/384kbps
USB Input DSD: Up to DoP128 and DSD256
THD: <0.008% (-1dB): <0.03% (-60dB)
Resistor precision: 27 bit, 0.02% resistors
Jitter RMS: 0.3pS typical
S/N (20kHz bandwidth): >120dB unweighted
Frequency response: +0.1, –1.0dB (20Hz–20kHz)
Digital volume control: –80dB to +10dB
Line output: 2.0V RMS (RCA), 50 ohm output impedance
Headphone output: 6.5V RMS (6.3mm jack), 2.0 ohm output impedance
Dimensions: 220 x 40 x 205mm
Weight: 1.4 kg
SOEKRIS AUDIO APS
MOD HOUSE AUDIO (U.S. RESELLER)
Loudspeakers: Analysis Audio Omega
Digital front end: Audiolab 6000CDT transport; Mac BookPro running Audirvana 3.5
Power amplifiers: Wyred 4 Sound 1000R monoblocks, Red Dragon Audio S500
Preamplifiers: Lamm Audio L2.1 Reference, Experience Music AVC, Ed Schilling’s The Truth
Cables: Mogami, Acrotec, FMS Nexus-2, and Kimber Select & KCAG interconnects, Take Five Audio cryo-treated Mogami 3103 speaker cable
A/C power: Sound Application power line conditioners
By Dick Olsher
Although educated as a nuclear engineer at the University of Florida, I spent most of my career, 30 years to be exact, employed as a radiation physicist at the Los Alamos National Laboratory, from which I retired in 2008.More articles from this editor
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