Kao Audio UD2C-HP

Average User Rating:
  1. project86
    "Quirky little masterpiece"
    Pros - Excellent sound - clean, transparent, neutral, but not too analytical, quality headphone out, beautiful design and construction
    Cons - Limited inputs, no Hi-Rez over USB (for a reason, but still....)

    There’s been lots of talk over the past few years, on this forum and many others, concerning the Asus Xonar line of soundcards. The Xonar ST and STX have basically become the standards in terms of reasonably affordable, high quality soundcards for the PC. For just over $200, you can add good sound and vastly improve your audio capabilities in comparison to an onboard audio solution.
    Some people wish to take things a bit further with a dedicated external DAC. In most cases this is accomplished by connecting to the computer through the USB interface. Though many options exist on the market, Asus recently joined the party with their Xonar Essence One standalone unit. At $600, the Essence One is a dedicated DAC with a built in headphone amp. It promises to raise the sound quality bar compared to its soundcard siblings. Reviews on HeadFi seem mixed, with some people really enjoying it and some being a bit disappointed. But I haven’t seen an in depth review done by someone I trust, so I can’t say for sure.
    But this review is not about the Essence One. It’s about the Kao Audio UD2C-HP. Kao Audio is an audio company comprised of former Asus employees, all of whom are also music lovers. They left Asus in 2009 to launch a company dedicated to high end audio. The team is almost completely made up of engineers who worked in the notebook and motherboard R&D sections. My contact, Ting-Kuo Kao (referred to as just Kao from here on out) says he was in touch with the project manager of the Xonar team -  the PM would sometimes come to him for design advice, though his suggestions always ended up being too expensive to be implemented. Despite having severed ties with Asus (and now being “the competition”), Mr. Kao was never anything short of respectful when discussing his former employer.
    The first product from Kao Audio is a standalone DAC/headphone amp called the UD2C-HP. It’s just coincidence that Asus came out with a similar product around this same timeframe. The similarities are only in function – these two units are completely different in design and execution, and I’d say that Asus and Kao have fundamentally different philosophies when it comes to audio reproduction. The Kao UD2C-HP is available worldwide via eBay, for the price of $988 shipped.  

    First, let’s examine the name. The UD obviously stands for USB DAC, although the unit does also feature a coaxial SPDIF input. I’m assuming the 2C makes reference to the two separate DAC chips on board. The HP is for the integrated headphone amp (apparently there is a UD2C version which omits the headphone functionality). And there you have it.
    Getting more specific: the UD2C-HP is a fairly compact device. It’s almost exactly the same size as the NuForce HDP in width and depth, but twice as thick. Though it does have a USB interface and could be considered “transportable” due to the small size, it requires power from the wall rather than drawing power from the USB connection. But used as an all in one unit, this device will comfortably fit in places that most other high quality solutions just won’t go.
    In terms of features, the UD2C-HP won’t overwhelm, but what does have should be enough for most situations. You have the USB input which is limited to 16-bit/48kHz (more on this later) and the coaxial SPDIF input which can take anything up to 24/192. The RCA output stays at a fixed level, so this device does not act as a pre-amp. But there is a volume control which applies to the headphone output. To maintain the slick industrial look, this is handled by a touch sensitive pad on one corner of the device.
    Internally, the unit takes a somewhat different approach. I say different in contrast to most of the higher quality DACs I’ve enjoyed lately, but it’s nothing too far-fetched. The focus here is on signal purity above all else – extremely short signal paths, overkill-quality parts (from the Schurter brand headphone socket to the high bandwidth relays to the premium RCA connectors), and a complete lack of what Kao feels to be unnecessary circuitry. That means no opamps (anywhere) and no upsampling, though the Delta-Sigma design of the twin DAC chips does use oversampling. The old audiophile adage of “simpler is better” absolutely comes in to play here.
    Speaking of those twin DACs – rather than using a parallel design like some of the competition (see Cambridge DacMagic, Bryston BDA-1 for examples), the Kao unit dedicates one chip to the line out and one chip to the headphone section. This is relatively uncommon, but it is the same method used in the $4k Resonessence Labs Invicta, so it isn’t unheard of. The line out uses a Crystal CS4351 DAC, and the headphone gets a similar CS4350. Both units contain what Kao says are excellent I/V and filtering circuits integrated into the chips themselves. So when tapping the line out you are getting the pure 2Vrms single ended signal directly off the DAC chip itself with no further tampering. The CS4350 feeds a differential voltage output straight to the TI TPA6120A2 chip for headphone amplification. Kao feels that splitting the signal from a single DAC chip is a compromise, and I’ll cover that further in the interview portion of this review. Volume control for the headphone amp is done in the digital domain, using the onboard feature in the CS4350 along with an external MCU.
    The supporting hardware is fairly simple: a TI PCM2704 converts USB signals to SPDIF before routing them to the tried and true CS8416, which also accepts SPDIF signals directly from the coaxial input. I asked Kao about his choice to not only use an adaptive mode USB interface, but one that is limited to 16/48 or less. He advised that it came down to two main reasons – 1) that the vast majority of music released is still in the Redbook format, and 2) that iPad compatibility through the Camera Connection Kit was a high priority. This lines up with my experience that DACs using 16/48 USB implementations have the least amount of issues with this playback method. So in what could be seen as a compromise, Kao made the choice to go with real world usability over an impressive spec sheet.
    I have a few specs to point out. First is the power delivered from the headphone out: 1W at 32ohms and 85mW at 600ohms. They didn’t have measurements available for other impedance loads at the time, but I’ll update if that becomes available. It should be sufficient for driving most headphones short of difficult planars, but obviously is more powerful into the lower impedance range.
    The second figure is with regards to jitter. Kao repeated what I’ve heard from other designers, which is that exact measurements into the low picosecond range are very difficult to obtain with any degree of confidence. He advised me that the device had measured very low, but didn’t want to put much faith in the specifics, so he asked me not to mention the number (it was quite low is all I can say). I don’t know much about the testing conditions or how this translates to real world use. Since the design seems optimized towards signal integrity, I have the impression that feeding a low jitter signal would get you excellent results, and feeding a poor quality signal would give poor results. There isn’t much in the way of jitter reduction except for what is built in to the DIR and DAC chips themselves, so the design instead relies on PCB optimization and power supply precision to keep jitter under control.

    The textured finish is such that it is hard to photograph without capturing
    blemishes and smudges. In reality, dust is more of an issue.




    Touch pad for volume control - works quite well, very precise.
    I did an interview style email exchange with Kao, but I don’t think it is fair to post it verbatim. His English skills are passable but it gets a bit confusing when we get into technical discussions. I also never specifically asked for permission to quote him so I’ll just paraphrase. My questions will be the ones in blue.
    What are the goals of your design?
    1) We want to recreate a detailed performance without adding anything. That means a flat frequency response. We want people to hear music as was intended by the one who created it, and not embellish that in any way.
    2) Functionally, we want to be simple but effective. We don’t want people paying for things that they won’t use. But for the things that our product does do, we want it to do extremely well.
    3) The external appearance of the product is very important to us. We want users to enjoy music as well as the environment around them. Nobody wants their listening area to look like a factory with unsightly equipment everywhere.
    How did you reach those goals?
    We did lots of measuring, but in the end it comes down to listening tests to determine what works and what doesn’t. Our collective experience as engineers helps with the measurement part, and our experience as music lovers helps with the listening part.
    How do you feel USB sounds compared to SPDIF?
    In most cases I prefer USB because it is more consistent. SPDIF is greatly affected by the quality of the input source. When fed by a very good source, the result can be excellent. But USB is more consistent.
    Why did you choose these specific DAC chips?
    The CS4350 and CS4351 are advanced multi-bit delta-sigma designs. While the top of the line CS4398 is technically the better chip, in our listening we preferred the “lower” models. Part of the issue is that the CS4398 is more complex, with more functions and thus higher load parameters. The CS4350/4351 is a perfect fit for our needs without any unnecessary extras, which is likely why it sounds better in our configuration.
    Why does the headphone section need a separate DAC?
    When splitting the signal from a single DAC chip, you lose some low level details due to trace bifurcation. We found this to be the case even when there is no load connected to one of the outputs. It has to do with signal reflections which can cause distortion. It is a small issue but still something we wanted to solve, to give the ultimate sound quality. It can be avoided by making the PCB traces longer, but this is also undesirable for obtaining the purest signal quality. The reason we don’t use a pair of identical DAC chips is that the CS4350 gives us the differential voltage output which we need for the headphone driver, and the CS4351 gives the single ended signal we need for the line out.
    What other aspects of the DAC contribute to the overall experience?
    We believe that to make the DAC convert digital signals into high quality analog signals, you not only need the best quality power in general, but it must also be suitable for every part of the circuit. For example, USB receiver circuit, headphone amplifier circuit, clock PLL circuit, etc. all have different operating frequencies, so the required power characteristics are different. We use high-frequency switching power supplies with medical and aerospace grade specifications. We carefully tune everything to ideal levels. This makes digital data unit, clock PLL unit, pure analog, and digital-analog circuits each get the most appropriate power.
    A high-frequency switching power supply is still a switching power supply. Electronic switches and filters are used to generate power. But both types have different circuit designs. Output power quality is also very different. In general, when the modulation frequency is greater than 20kHz it can be classified as a high-frequency switching power supply. Our high-frequency switching power modulation frequency is approximately 100kHz. In audio equipment, there are only a few other companies using these types of power supplies, such as Chord and NuForce (with their higher end Reference products). This is because very few people understand noise control technology, especially when it is used in audio equipment.
    CS8416 seems to have higher jitter at 48kHz and 96kHz than some of the alternatives. Can you explain why you chose that model instead of other options? It does seem to be one of the best for 192kHz sample rates, but that isn’t really your focus. Did you consider any other options from Texas Instruments or Wolfson?
    We tried to use the Wolfson WM8805 and TI DIR9001. Only the CS8416 give us the required sound. Another reason – we are mainly concerned with 44.1kHz sound performance, because this is the most frequently used. At this sampling frequency, we believe that CS8416 is the best. We don’t want people to judge strictly from the numbers, and we hope nobody will think less of our product because it might not use the highest specification chips. We’ve spent much time comparing and would rather build something that we believe sounds better, instead of just seeming better on paper.
    Do you have a favorite headphone to use with the headphone output?
    Grado RS-1i and Denon AH-D5000 / AH-D7000. These are our favorites. But we feel that the UD2C-HP sounds great with a wide variety of headphones.


    The UD2C-HP is a very unique device. At first glance it appears to be formed from a single block of aluminum, though closer inspection reveals an upper and lower half. This is the first DAC I’ve reviewed where I wasn’t able to open it up for a closer look – I was afraid that I wouldn’t be able to get it back together properly. So I relied on Kao to provide interior pictures.
    I find the finish striking and components to be of very high quality. Great care was taken during the selection process of every part; for example, Kao says they tried over a dozen RCA connectors before settling on the current choice, which had as much to do with sound as it did with aesthetics and durability. The Schurter brand power toggle and headphone jack both feel great, adding those last satisfying details to the user experience.
    The only downside I can find is that the unit shows debris very easily. It is the opposite of a fingerprint magnet – instead, it shows lint and dust. But this is easily remedied with a cloth such as the type normally used for cleaning sunglasses.




    Kao sends a generous helping of accessories along with the DAC. You get the unit itself, a short USB cable, a longer USB cable, a thick aftermarket style power cable (branded as “Monitor Acoustics”), and a leather base for the unit to sit on. The DAC itself does not have any sort of rubber “feet” or padding on the base, so this leather pad is used to keep from scuffing the table (or whatever the device is sitting on).
    I discussed the power cable with Kao. He maintains that differences in impedance, resistance, and conductance can result in audible differences between cables. A well designed power cable can help you get the most from your device. That explains the massive cable included in the package. I’m not really big on aftermarket cables but I still appreciate the fact that Kao included something nice. I’ve seen many companies include a basic cable and then use it as an excuse when someone isn’t satisfied with the sound of their product – insinuating that the customer need only to supply a quality aftermarket power cable to start hearing what their product is really capable of.




    Monitor Acoustics power cable

    Leather pad for the base
    This is a list of associated equipment I used for evaluating the Kao UD2C-HP:
    Source: Acer laptop running Windows 7 and Foobar2000, JF Digital HDM-03S music server, Pioneer Elite N-50 streaming audio player, iPad2 with USB Camera Connection Kit
    Amplification: Analog Design Labs Svetlana 2, Violectric V200, Yulong A100, Matrix M-stage
    Headphones: Heir Audio 8.A, Sennheiser HD700 prototype, Unique Melody Merlin, Earproof Atom, Audio Technica W1000x, Lawton Audio LA7000
    Speaker setup: Parasound 2125 amp, Octet Matrix DE7 speakers
    Cables used were by Signal Cable including the Analog Two interconnects and Digital Link coaxial cable, and I obviously used the Kao power and USB cables. Music was a mix of various genres, mostly stored in the FLAC format, from 16-bit/44.1kHz to 24-bit/192kHz. I burned in the DAC for well over 100 hours prior to doing any serious listening, and by now I’ve got probably 500+ hours on it.

    Pictured stacked with the Squeezebox Touch, which is very close in size.
    I’ll start by addressing the UD2C-HP as a DAC only. I wasn’t sure what to expect since the design is so different from the rest of my DACs. Would it sound ultra-analytical? Or smooth and rolled off like a NOS design? Thankfully it is neither of those – for such a tiny little unit, this DAC really delivers a big performance, while maintaining a nice balance between those two extremes.
    The sound as a whole is very transparent and neutral. I’m not hearing any significant frequency deviations in the form of boosting or roll-off. More importantly, I don’t hear any glare in the highs; just crisp clean reproduction. We often associate “neutral” with “bright” or “boring”, but that isn’t the case here.
    Vocals were a very strong point. Whether listening to a lovely “audiophile approved” voice like Diana Krall, or a lesser known songstress like JJ Heller, vocals came through with startling accuracy. I felt like I was in the room with them and could hear every vocal intonation, however slight. Yet it never felt forced like some DACs do when they strive for this type of accuracy.
    Bass hit strong and deep but maintained full composure. Listening to “Super Double-Bass: The Artistry of Gary Carr”, a great sounding release in the JVC XRCD format, I heard excellent control and rock solid definition. This unit is definitely capable of accurately resolving low frequencies when it comes to real instruments. Switching to electronic music with heavier bass: from Skrillex to Mistabishi, The Crystal Method to Crystal Castles, the UD2C impressed with deep impact - but it fell a little short of some of my other DACs in this regard. I probably wouldn’t notice without direct A/B comparisons though.
    Over time a pattern started to emerge – I think this DAC has some of the best tonal accuracy I have ever heard. It excels with reproducing the timbre of real instruments and voices. It just sounds utterly natural and convincing. Its soundstage is large enough, and more importantly, accurate enough, to sound realistic without calling attention to itself. At the same time it falls short of my reference units in some other areas - overall resolution, coherency, and the ability to handle complex passages without any congestion. Not that it is actually poor in these areas… by most standards it still does a great job. Just not compared to the absolute best out there (which of course cost quite a bit more money).
    I hate saying a DAC is neutral and doesn’t have much sound of its own. That seems like such a generic review, and gives the impression that I didn’t bother to really figure out the sound of the device. But it actually seems to apply in this case: the Kao DAC doesn’t sweeten your music, it doesn’t soften edges on brittle recordings, it doesn’t deliver lush warmth - it just gets out of the way of the music (another term I despise) and lets it flow.
    I do have to note that Kao was right on with his description of the USB input compared to the SPDIF input. Using USB with several different computers and playback programs, I consistently achieved what I consider very good sound. Despite the limitation to Redbook quality tracks, I heard some absolutely beautiful renditions of my favorite music. When I tried SPDIF, the results seemed more dependant on transport quality. My Pioneer Blu-Ray player, clearly not intended for high end audio, sounded inferior to USB when used as a transport. My Lexicon universal player, fairly expensive when new, sounded about equal to USB. And my low jitter JF Digital music server (basically a computer purpose built for quality audio) sounded even better than USB, but only slightly. Honestly, I would have thought that adaptive USB mode using the older PCM2704 transceiver would offer the worst performance. But that just wasn’t the case. So whatever Kao Audio is doing, it seems to be working. The only consistent benefit seen with SPDIF is the ability to play hi-res tracks.
    Switching to the integrated headphone amplifier, I was met by largely the same transparent sound. After careful listening I determined that the amp section has a hint of flavor to it that seems absent from the line-out. It has what I’ve heard described as a “Japanese Hi-Fi” sound. Compared to something like my Violectric V200, the Kao amp section has a bit less extension at both ends of the frequency extreme. It seems to spotlight the mids just a little, enough to call attention to them but not in an overpowering way, nor enough to make the other aspects seem too recessed. It’s an enjoyable sound and I could see myself living with it as my only amp if I wasn’t such a HeadFi geek. It reminds me of a less refined version of the Blossom Blo-0299, which I thought was a very nice amp (though expensive at nearly $1500). They both have the same slightly mid-centric sound, a little on the “polite” side, but very enjoyable. I went back and forth between the built in amp and some stand-alone units of various prices, and I think this amp is very well matched with the DAC section: unless you are spending a good amount on a nice mid to higher end amp, you likely won’t need to upgrade.
    Another good thing was that it was nice and quiet with sensitive IEMs, and had sufficient drive for most headphones. I don’t have any planar models in house at the moment so I can’t confirm how well it would do. But based on the power ratings and general sound signature, I’m guessing it would pair well with the LCD-2 and HE-500, if not being the last word in performance. The HE-6 is most certainly a no-go. I’m planning on trying the HE-400 soon so I’ll update, but I’m fairly certain that will be a good match.
    I recall the stir caused by the TPA6120A2 when it first came out; some people hailed it as the ultimate headphone driver IC, and others scoffed at it, saying specs alone don’t make for good sound. I remember Kevin Gilmore saying in this old thread that the TPA6120A2 was “mighty good indeed” and that it sounded better than opamp/buffer solutions. The context there was for portable applications, but I’d argue that Kao is working with space restraints closer to portable amps than desktop solutions. Like the DAC section, I really do think the output quality is tied to the supporting circuitry, especially the power supply. That’s why Kao can extract such excellent performance without using exotic parts.
    I recently reviewed the Yulong Sabre D18 DAC, which sports the ESS ES9018 Sabre Reference DAC. I praised it for its ultra smooth, musical tone, and said that nothing I’ve heard really came close for the price ($699). Given the similarities in price and the fact that I’ve spent so much time with the D18 recently, it will be my comparison.
    The Kao UD2C-HP is a very different sounding DAC, both in terms of features and sound. It is single ended, USB oriented, and has a built in headphone section, all of which are opposite of the D18. Sonically, the two are rather different as well. The Kao has a more neutral presentation, without the warmth and smoothness of the D18. The Sabre unit has the edge in soundstage size/accuracy as well as low frequency extension. The UD2C counters with better perceived detail retrieval and that amazing tonal accuracy. Deciding which one sounds better will ultimately come down to the system it is matched with.
    To put it in headphone terms, the D18 is the equivalent to an Audeze LCD-2 R1, while the UD2-C is more like entry level Stax. One is all about rhythm and drive, with a thick, bold sound that is relatively forgiving of flaws. The other is more of a neutral transparent presentation that really nails the timbre and speed of the music. Neither is perfect, but both are quite good, and will appeal to different audiences. The D18 is cheaper but the UD2C-HP has a nice integrated headphone amp, so it roughly evens out. With gear this good, there are no losers in the comparison.
    It’s hard not to stack this Kao Audio DAC up against the new release from former employer Asustek. Yet the Kao Audio UD2C-HP and the Asus Xonar Essence One are about as different as they could possibly be. The Asus boasts an eleven opamp design while the Kao is completely free of opamps. The Asus has a linear power supply with a toroidal transformer while the Kao uses a high speed switching power supply. The Asus has a hi-res asynchronous USB implementation while the Kao sticks to 16-bit/48kHz for maximum compatibility. The Asus uses upsampling while the Kao processes signals at their native rate. Obviously each company is approaching the problem from a different angle, which is why their solutions look so different. Unfortunately I can not draw any conclusions due to not having heard the Essence One for myself. I’ll work on getting one and update if I can.
    Asus aside, the Kao DAC is a very unique and impressive device. With a “less is more” approach, it certainly looks different than most $1,000 DACs, on the inside as well as the outside. But the sound easily justifies its position in that price bracket. It goes to show that even “basic” DAC chips can sound phenomenal when surrounded with the right environment. I really do think that the advanced power supply is key to their accomplishment though I’m sure there are other factors involved. I find their approach refreshing – while it seems like everyone is jumping on the latest features, Kao Audio dares to be different. Not just for the sake of being different, but to reach the goals they set: small size, attractive design, emphasis on iPad compatibility, and a purist approach to sound reproduction. I appreciate this specialist mentality – it seems like they are trying to please a specific type of customer rather than casting a wide net.
    I’m using the UD2C-HP in a bedside rig. Signal comes from my iPad2 with Home Sharing, streaming content from my desktop in the other room. Headphones are the Lawton LA7000, Ultrasone Edition 8 and Heir Audio 8.A, among others. It takes up very little room, looks fantastic, and most importantly, sounds great. None of my other gear could take its place without some compromise in size, sound, appearance, or cost. With that being the case, Kao has obviously accomplished exactly what they set out to do.