Questyle CMA800 amplifier

This review is one of the most difficult I've had in quite some time. Not because the product had flaws, or because I couldn't make sense of what I was hearing (which sometimes happens), but because I struggled to understand the concept behind the design. I'm still not totally convinced that I have a handle on it, but I know some people are waiting to read this so it's time to move forward regardless. The product in question is the CMA800 headphone amplifier from Questyle, which is a new company hoping to take off in a big way. Based on what I've heard so far I think they have a good chance of succeeding. 
 
 

 

 
 
 
Chinese firm Questyle Audio Engineering has been working on this particular design for over 6 years, going through a total of 22 different revisions before achieving what they believe is a ground breaking, reference class headphone amplifier. We'll talk about this particular implementation shortly, but first a bit of background on current drive amplification in general.
 
I'm going to paraphrase Tyll Hertsens because I think he has a knack for explaining complex things in simple terms. In a current drive amplifier (aka transconductance amplifier), the input voltage causes the amplifier to deliver a proportional output current. The output stage of the amplifier will do anything it needs to in order to deliver a current signal that matches that of the audio input, feeding it straight to the voice coils of the connected headphones. It's the current in the voice coils that actually drives the diaphragms, so this type of amplifier creates a very linear relationship between the incoming audio signal and the physical response of the driver. A byproduct of current-mode amplification? The traditional damping factor, which we all know and love, no longer applies. Headphone impedance essentially becomes irrelevant. Or at least that's one theory. Others insist that current drive amplification would only work properly with a headphone that has a completely flat impedance curve. But that doesn't really matter in this case, as the CMA800 actually has a voltage output, despite using current mode operation internally. Confused? I sure was. Still am to some degree, though I can't argue with the results I'm hearing - regardless of the path taken to achieve them.
 
Let's take a closer look at the CMA800 ($1499). The CMA stands for "Current Mode Amplification", and Questyle describes their implementation as a four part process - an input buffer, a current transmitter, a trans-impedance amplifier, and finally an output buffer. These stages collectively form what Questyle calls a TransLinear Loop Circuit. The key benefit of such a design - it avoids transient intermodulation distortion (TIMD) which according to Questyle causes unpleasant, metallic sounding highs (which is historically a complaint leveled at solid state gear, often from lovers of tube amps). For the longest time I was under the impression that Questyle used a true current-mode output, much like the Bakoon HPA-21. Though my contact there did a fine job at explaining things, I somehow misinterpreted what he was saying, or perhaps I had a preconceived notion of what to expect. After much discussion, their engineers broke it down for me with some in-depth information. English is not their first language but they do a much better job than I would do if I tried speaking their language! What follows is directly from Questyle, with a few minor corrections for readability. I don't normally like to post manufacturer info directly but in this case it makes sense to do so. Some of the charts got a little fuzzy during formatting, sorry about that.
 
 
 
1) Super Linear theory
In 1975 Mr. Barrie Gilbert created the “Translinear-TL” theory, and published in ISSCC . This marks the birth of Current Mode theory. A translinear (TL) circuit is a circuit that carries out its function using the translinear principle. In TL principle, ideal BJT Collector Current’s log domain (Lc) and Vbe graph is a straight line. See below graph, BJT Collector Current increase from 1pA (1E-12) to 10mA, which is a billion times increase, while log (lc) and Vbe is a good straight line. Current Mode circuit is using this super linear feature to make up a TL loop. This is the basic theory for Current Mode ultra-high precise, ultra-low distortion function.     .
 

 
 
2) Ultra-high speed
In Current Mode circuit, the inter-electrode capacitors in between transistors which affect speed and band are working in a very low impedance point (usually several ohms or dozens of ohms, which is hundreds or even thousands of times less than a voltage mode amplifier). With large amplitude current wave, the inter-electrode capacitors complete their charge and discharge cycle very fast, so the efficiency is much higher than that of a voltage mode circuit.    
 

 
 
 
CMA800 Work Principle
 
Since 1989 in the 87^th AES forum, when Dr. D.C Wadsworth first point out current mode circuit in audio can achieve ultra-high speed, ultra-low distortion function, people start ceaseless dedication towards current mode amplification. So far some brands created CAST (Current Audio Signal Transmission) technology, SATRI technology based BPM7110 modular, etc. These are pioneers on developing current mode technology application on audio. But they are mainly “non-feedback” mode, mainly for current mode “transmission” rather than amplification. Questyle is different, as we specially focus on “amplification” instead of “transmission”, so CMA800 provides more outstanding specifications and sound performance.     
 
 

 
Above is the CMA800 current mode amplification circuit diagram. The circuit is consist of VCCS (Voltage Control Current Source), A(i) --Current Mode amplification modulator, I/V converter, OPT and Negative Feedback sections. In an electronic circuit, both voltage and current are exist - neither can be missing. CMA800 signal is controlled by current so as to provide high performance, while the input and output are in voltage mode. This is to ensure better compatibility with other audio devices.   
 
From the diagram we can see, input signal will first to to VCCS, voltage mode is converted to current mode here, then goes to Ai for amplification (in current mode), next go to I/V converter to become voltage mode again, and output to OPT stage to drive headphone. 
 
For better performance, in this circuit loop we applied Negative Feedback design. The impedance of the whole Negative Feedback loop is in the hundreds of ohms, meaning amplification is very fast; thus the Negative Feedback process speed can be as much as 100 times faster than that of voltage mode designs. In this way CMA800 is free of problems like TIMD or other which occur in voltage mode designs.
 
CMA800 Full-Power Bandwidth approaches Closed-loop bandwidth, and the converting speed shows a linear increase as input signal amplitude increases. These features are totally different from that in voltage mode, and together they support CMA800 to achieve ultra-low distortion and ultra-high efficiency.    
   
The workmanship
 
  Current mode amplification internal translinear-TL loop has critical requirement on transistors. A Canadian semiconductor manufacturer, Microsystems International Limited, had attempted to make a current mode amplification IC but failed on workmanship.
In a TL loop consisting of 6 transistors, a mismatch as small as 5uV Vbe will create 0.01% harmonic distortion. Precise matching is critical. Questyle engineers studied over 5 years on this, working together with Dr. Charles from California university who is dedicated on transistor workmanship research. By selecting from lots of customized specific components, and designing specific circuit structures, we finally achieved success.      
 

 
 
1KHz@300Ω , THD vs Power
 
 
 
 
 

 
1KHz@300Ω , Po=10mW FFT Spectrum Analysis
 
 
 
 
Intermodulation Distortion (IMD)
 
  Some engineers use Sine waves to test amplifier performance. They think that as long as the THD+N value is not high, it should therefore be a good amplifier. But in actuality each music signal is made up of thousands of harmonic waves - it’s expected they will interfere each other, so IMD value is a more fitting approach to indicate signal accuracy as opposed to THD+N. “IM (Intermodulation)” means two or more signals that come together which will interfere with one another and make up a new signal. IMD decides amplifier’s purify, density and details. CMA800 IMD spectrum is almost as perfect as AP2 signal source - we hardly see any mixtures at all.     
 
 
 

 
TIMD @300ohm 0dBV FFT
 
 
 
 
Full frequency low distortion
 
  Ultra-wide bandwidth provides amplifier with great sound, but ultra-low distortion is also required for high frequencies. Otherwise the sound will be terrible. CMA800 distortion graphs at all frequencies show a straight line; the value is ultra-low.   
 
 

 
Po=25mW @300Ω,  
 
 
 Broadband High-speed Amplification
 
  Only an amplifier with broadband high-speed amplification can properly recreate a burst pulse wave signal, accurately reproducing the necessary speed and clarity. CMA800 frequency response is essentially a straight line, with only a -0.3dB drop at 200KHz.   
 
 

Frequency Response @300ohm, 0dBV 
 
 
 
Specific Power Supply
 
For an analogue amplifier, power supply plays a very important role. We work together with Plitron Manufacture in Canada Their senior designers specially designed many versions of dedicated transformers, evaluated and selected the best one to fit our needs. In addition, CMA800 adopts schottky rectifiers to reduce noise, as well as 22 instances of 35V 1000uF Nichicon FG capacitors providing clean power.  
     

 
 
Components
 
  CMA800R adopts best Hi-end components to ensure best sound. Here “best” means best fit, like OPA627 operational amplification module, DALE military resistor, WIMA customized capacitors, Alps volume potentiometer, Nichicon FG capacitor, etc.     
 

 
 
Sound Performance
 
  Besides good parameters, good components, a good sound performance is the last step but most critical factor. Before production, CMA800 was tested with various headphones, and was sent to many studio engineers and musicians for critical evaluation. Once they were all satisfied with our sound, the CMA800 detailed specifications were then fixed and we started production.   
 

 
 
Chassis  
  
Mechanical resonance will affect performance for any Hi-end device. CMA800 chassis is made up of pure Aluminum with “mutual bit” structure, 10mm thick. Each section weight is equal, for maximum reduction of resonance. Feet are made of solid aluminum as well     
   

 

 
 
 
Wow, lots of good info in there. Worth noting: Questyle has an Audio Precision AP2722 (a $30,000 audio analyzer) with which they measure every amp before shipping it out. Each customer is provided a printout showing the results of their specific unit. Pretty cool. In general terms the amp is listed as having a signal to noise ratio of 120 dB, and a real world THD+N figure of 0.0004% @ 1kHz (20mW into a 300 ohm load). That's impressive no matter how you slice it. Setting the TIMD issue and the current-mode operation aside, this is a very well done amp with excellent performance characteristics. The picture shown below is from Questyle's test bench, featuring two of their new CMA800R monoblocks. 
 
 

 

 
Externally, the CMA800 is very nicely done. The look is simple and classy and vaguely reminiscent of the Luxman P-series headphone amplifiers - not bad company if you ask me. The layout is very straightforward - XLR and RCA inputs, dual 1/4" headphone jacks, switches for power and input toggle... and that's pretty much it. A few status LEDs show the user what's going on without being overly bright (which I appreciate). The enclosure, binding posts, and switchgear seem commensurate for this price class and the whole thing is very well built. There's a definite Ayre vibe to the design - not identical or obviously a clone, but they go in the same direction for sure. I find it very tasteful and clean looking. If I must find something to complain about I'd pick the pointy "feet" on the bottom. They aren't really sharp like the spikes used for speakers, but could still do some damage to shelving as well as other components if you stack the amp on top. But they look pretty nice so I suppose it's not a complete loss. 
 

 
The spiky feet, sitting on some audiophile puck I had laying around:

 
Powerful enough to happily run two headphones at once:

 

 

 
LEDs are not too bright - a good thing!

 
Rear panel with XLR and RCA inputs:

 
 

 
Opening the top cover reveals the CMA800 to be packed full of goodies. The first clue hinting at Questyle's ambitions is the power supply - large Plitron brand toroidal transformer flanked by 10 ultra-fast recovery rectifiers and a massive array of Elna Silmic II capacitors, which are often considered to be a top choice for high-end audio applications. I see some opamps making up part of the "TransLinear Loop Circuit" but they have custom "C800" markings so I didn't know their specifics until Questyle mentioned in the above text (they turned out to be the well regarded OPA627, which is one of the most expensive opamps available). Questyle claims a pure class A operation, with output listed as 170mW at 300 ohms, 700mW at 64 ohms, and 1100mW into 32 ohms. It can swing 20 volts peak to peak and has a bandwidth of DC to 650kHz (-3dB). Output impedance is extremely low, to the point where it is a non-issue. 
 
 
A quick note about availability - Questyle seems very serious about breaking into the market. International sales are currently handled through eBay but aggressive pursuit of distributors is currently underway, including plans to show at CES in January. In the CMA800 thread we had a buyer whose amp was somehow damaged during shipment, and Questyle took care of him immediately - he remains a satisfied customer. This is certainly no fly by night OEM. 
 
Now, about the sound....
 
 
When discussing this review with the folks at Questyle, they seemed very concerned about the quality of the source I'd use while evaluating their amplifier. I assured them I had quite a few very nice options to choose from. Despite that, they insisted on sending along their CAS192 DAC ($1,999) to make sure I got the most from the CMA800. The two make something of a matched pair, rather visually striking, though again I dislike the spikes on bottom from a functional standpoint. I had to use some tweaky audiophile footers to keep them from scratching anything. That aside, their CAS192 is an exceptional DAC. It's not the focus of this article, so I'll just say it's very transparent, open, and natural sounding. Definitely competitive with the best I've encountered in that price range or even beyond. I used it for the majority of the review process, though I also swapped in the Resonessence Labs Invicta, NuForce DAC-100, Anedio D2, and BMC PureDAC. The CMA800 worked well with all of them. My contact at Questyle did mention the idea of utilizing current-mode signal transmission (from component to component) as seen in Krell equipment for example. This is not something they want to do for now, because it would mean their DAC becomes exclusive to their amp and vice versa - neither would work with equipment from other brands. Someday, when Questyle is more well known and can thus justify that kind of behavior, it may be an idea they reconsider. Audio GD offers both their proprietary connection as well as separate, standard XLR or RCA options - though they do point out that their ACSS is the best option to use when possible. So that's an idea Questyle could maybe consider.
 
 
Some shots of the matching CAS192 - the CAS stands for "computer as source":

 
Selectable upsampling uses different methods for 44.1kHz and 48kHz signals:

 
 

 
It can also disable upsampling, plus has multiple filter options:

 
Other DACs I used included the BMC PureDAC:

 
The Yulong DA8 (pictured here playing DSD):

 
Source was my Auraliti PK90 music server with NuForce LPS-1 power supply:

 
I also tried multiple DAC options from Resonessence Labs:

 

 
Questyle tells me the amp was designed with the Sennheiser HD800 in mind; hence the name CMA800. I figured that was a great place to start so I grabbed my HD800 and got to listening. I played some tracks from the Reference Recordings HRx Sampler. Holy smokes! Talk about clarity.... Yikes. You want air up top? Tonal accuracy? Well defined transients? The CMA800 really gets that ring radiator jumping, and the result is one killer combo. I've always thought the HD800 was pretty much the best headphone out there for peering deep into the mix of a recording, and the CMA800 brings out that aspect better than just about any other amp I've tried. It practically begs you to throw your most demanding music at it. The complex tone poems of Strauss? Check. The deceptively elaborate IDM masterpiece "Trace" by Gridlock? No problem. The precision polyrhythms of Meshuggah? Bring it on. The team of CAS192/CMA800/HD800 resolves this stuff like pretty much nothing else I've experienced this side of a megabuck electrostatic setup. If detail is your thing, I can't imagine doing much better than this. I like the term "unforced clarity" - there's a complete lack of grain, and details are abundant, but they come with a sense of ease like it's no big deal. "Hi. I'm Detail. This is my friend Accuracy, and my other friend Resolution - we're sort of a team. We'll just be sitting here doing our thing. You can ignore us if you want, or not. Whatever."
 
Now, I can anticipate what the reader might be thinking. Detail is great and all, but what about emotion? What about musicality, the "feel" of the performance existing as something more than a lifeless reproduction? It's true that the HD800, in stock form, will probably never suit certain listeners and their preferences. Nothing we can do to change that unless we modify the headphones to achieve a different sound. For its part, the CMA800 stays well clear of the cold, analytical presentation the HD800 can have when paired with the wrong amp. It remains spectacularly detailed which I use as both a positive and potentially negative description. But that detail is accompanied by very fast and tight bass reproduction which can hit quite hard when called upon to do so. It's definitely not the same thin sound I've heard from the HD800 so many times in the past. This fullness of note results in probably the best compromise between "accurate" and "fun" that I've yet heard from the HD800. Questyle's current-mode amplification process seems to have an iron grip on the drivers and does an exceptional job of reigning them in to avoid excessive harshness, but I have to be honest - these headphones will still be a bit too bright for some people, no matter what amp is used. 
 
The Audeze LCD-2 makes a perfect counterpoint to the HD800. It has that smooth, somewhat thick sound signature that I can spend hours listening to, just melting away the day. Again the CMA800 proves a worthy match. The LCD-2 can sometimes come across as overly dark or slow, but I hear neither of those issues when using this particular amp. While the cheapest Audeze is still not a soundstage champion, the CMA800 helps it sound more open and expansive than most other amps. And that bass that caused so many of us to fall in love with Audeze in the first place? Yep, it's there. Deep, layered, up there with some of the best I've ever heard. Vocals? Those too are very well done, not distant at all which is a problem when using lesser amps. Everything just seemed to fall into place and I found myself wondering how much better the more expensive Audeze options could possibly be - but I've got an LCD-XC on the way so I'll find out soon enough.
 
To get an idea of how black a background is, my favorite test track is John Cage's 4:33. Sounds ridiculous, but I'm dead serious here. Sure, I could simply crank the volume knob to full blast with no music playing, but that doesn't quite recreate the same conditions as when the system is actually in use. In this case, both the HD800 and LCD-2 show no signs of noise until the volume is almost at full blast. I can't quite call it a completely silent amp because there is a tiny bit of hiss or hash present at full output. But I would never get even remotely close to this level even with a super-demanding headphone and quiet music. So for all practical purposes the amp is essentially silent, even if not quite so under extreme conditions. 
 
Speaking of demanding - I tried out the HiFiMAN HE-6 to get an idea of how the CMA800 performs with difficult headphones. I've got to be honest - with most amplifiers, the HE-6 seems a bit lackluster.... not worthy of flagship status in my mind. It's a little on the tizzy side, somewhat thin sounding, and just kind of bland overall. This applies even to some very good amps such as the Icon Audio HP8 mkII. Fortunately the CMA800 is able to wring some musicality out of them - I don't know if it's the current-mode amplification at work or just a byproduct of the excellent specs in general. Either way, the HE-6 as heard through the CMA800 is something I can actually enjoy. A lot. The bass, which seems anemic at times with many other amps, is solid and punchy. The midrange is nicely open and transparent. Those highs are still prominent, and at times it can become fatiguing depending on the recording. But with this combo I feel more confident in blaming the song itself than the headphones, at least in most cases. Of all the amps I've paired with the HE-6, this is up there in the top three along with the AURALiC Taurus and Violectric V200. Not bad company I'd say. Would more power be welcome? Sure. There's plenty of gain here but I do here more grunt with the beefier AURALiC in the chain. But the differences are not as big as the power specs would suggest. 
 
I kept on going, trying various headphones to see what sort of character I could flush out from this device. I had many highly enjoyable moments along the way, while finding just a few minor points to complain about. The CMA800 made the Grado PS500 sound better than any amp I had previously tried. It did a bang up job with my difficult to drive Smeggy Thunderpants, and energized my beyerdynamic T1 like few others can. With the Ultrasone Signature Pro, the sound quality was outstanding, but I had very limited play in the volume knob. Things got really loud, really fast. Which brings me to one of my complaints - the gain seems a bit higher than it needs to be. 
 
The Signature Pro is a rather sensitive headphone, perhaps more so in practice than on paper, and it didn't take much turning of the dial before I found myself maxed out. And that came at around 11 o'clock on the dial. That's not a lot of play between zero volume and too loud, making it tough at times to dial in a comfortable level. Less sensitive headphones work well enough but even the very insensitive Thunderpants and HE-6 only use maybe 60 to 70 percent of the dial with most recordings. If those beasts don't come close to maxing out the knob, nothing will. I'd like to see the gain drop low enough to where an HE-6 gets crazy loud at 4 o'clock (5 o'clock being roughly the highest the amp will go), and most others get loud around 1 or 2. That way most of the dial is used and volume more easily managed. The CMA800 is not alone in this area (the Apex Peak has very similar behavior) but it's something Questyle could address.
 
My other complaint is a relatively common one with big, powerful amps. Sensitive IEMs bring out some hiss, making the CMA800 not well suited for those types. The actual sound is excellent, and if I use a less sensitive model like the Lear LCM-5 with their 180 ohm "Sound Tuned Adapter", I get a glimpse of what could be - and it sounds mighty fine indeed. But I have very little patience for hiss. Factor in the high gain and this just isn't an amp I'd recommend for IEM users. 
 
However, I would enthusiastically recommend the CMA800 for most everyone else. It's one of the most transparent amps I've experienced, and is extremely useful for hearing minor differences between various DACs or CD players. It's not as focused on leading edges and transients as the Apex Peak/Volcano duo, but reminds me more of the L-2 parafeed tube amplifier from ECP Audio in that it shows a level of resolution that few amps - regardless of price - can hope to match. Yet isn't the least bit edgy or fatiguing which is something a greatly value in my gear. There are amps out there with a more exciting signature (like the AURALiC Taurus, review coming soon), and some that sound more spacious (the Eddie Current Balancing Act for example), but overall the Questyle CMA800 seems worthy of competing with this bunch. Any one of these could legitimately qualify as a "desert island" or "end game" headphone amp, so it's impressive to see the CMA800 play on this level.
 
COMPARISONS
My only potential hesitation is one that I commonly encounter when the gear gets this high end. It has to do with value per dollar. Now, we can argue all day about what constitutes a worthwhile expenditure, but in the end I think most people agree that the value-per-dollar apex comes somewhere significantly lower than $1500. In my view it hits hardest around the $400-600 range. There we find the Lake People G109, NuForce HAP-100, DarkVoice 3322, Little Dot MKIV SE, HiFiMAN EF-5, Burson Soloist SL, and any number of others. Between these various models you can essentially drive any headphone from the most sensitive IEM to the most beefy planar, and everything in between. Once you move up from there you do get improvements, but not keeping in step with the money spent. Even within one brand, it's not possible to achieve: the exceptional Violectric V200 at $999 is not twice as good as its sibling, the Lake People G109 ($500). Is it better? Sure. In many ways. But twice as good? Nope. This discrepancy only continues as we go higher and higher up the food chain. I doubt even Pete Millett himself would describe his own flagship Apex Pinnacle amp as being 5 times better than his Peak amplifier with Volcano PSU. It may be one of the best amps in the world, but 5 times better is simply not possible when the Peak/Volcano is already so good. Yet it's that last bit of extra performance that is so difficult to achieve, and thus people are willing to pay big money to get it. Keep that in mind as I compare the Questyle CMA800 to some of my other high-quality amps in the $1K+ price bracket.
 
Violectric V200
The V200 is the obvious comparison here, as I've long championed it as one of the best amps out there while still keeping a "sane" price. And indeed it does not feel completely outclassed by the CMA800. The two are very different, with the V200 having more grunt and gusto, while the CMA800 has a more delicate, nuanced presentation. I'd call it more balanced, faster, more accurate up top, and generally of a higher resolution overall. The V200, with 8 transistors per channel, does bass like few other amps out there, and the CMA800 does give up a bit of ground in terms of visceral, pants-flapping bass reproduction. That's not to say it doesn't have some thump of its own - the CMA800 lights up the LCD-2, the Denon D7000, the Thunderpants, like few other amps can. But I wouldn't say that aspect is really the focus. On the flip side, the Questyle amp has a distinct advantage in terms of wide open, breathtaking soundstage. If imaging is your thing, this contest is over quickly - Questyle takes the unanimous victory. There's also the matter of transparency - I do love the V200 but the Questyle does seem more faithful to what I imagine the original recording should sound like. The Violectric has a slight but definite signature that it imparts on everything it plays. It's a very enjoyable signature, but nonetheless is not something we would seek out when looking for the ultimate in transparency. In its defense, the V200 is still more enjoyable with the HiFiMAN HE-6. I love the resolving power on display with the CMA800, but the V200 gives the HE-6 a signature more agreeable to my personal tastes. 
 
It may sound like I'm sidestepping the comparison, but these two amps really are very different. The V200 is undoubtedly the better value - lower price, plays better with sensitive IEMs, and has more power for planars. And it has a more fun signature as well. The Questyle does the "HiFi" thing better though, and I suspect would be considered the superior amp by a lot of listeners (but certainly not by all).
 
Yulong A18
Once again, two very different amps. The A18 is warmer, smoother, more forgiving, but does nearly match the CMA800 when it comes to spaciousness. This comparison really hinges on the headphones being used. With something like a Grado RS-1, I'd take the A18 every time. While the Questyle does a great job not adding grain to the presentation, the RS-1 is still an inherently  aggressive headphone. So personally I like to take some of that edge off, and the A18 does it more tastefully than any amp I've heard. But if we switch to a Grado PS-500, things change and I actually prefer the Questyle. The PS-500 is the first Grado I've tried where I actually want to hear everything it has to offer. There's no need to smooth it out, tame it, or anything like that. I've owned the PS-1000, HF-2, RS-1, and GS-1000 (plus some of the Prestige series models like SR-325), and none of them satisfied, but PS-500 actually does. I have yet to experience the vintage HP-1000 models or the rare PS-1, but thus far the PS-500 is by far my favorite. It still sounds great with the Yulong A18 but I prefer the clarity and detail the CMA800 has to offer. 
 
If we keep switching headphones, we keep ending up with different results. LCD-2 definitely sounds better with the Questyle. So does HD800, HE-500, and V-Moda M100. HE-6 is a close call but I believe the A18 takes the victory with the more robust low end and smoother highs - though I can see how someone else might disagree because the CMA800 has more detail. K550, Focal Spirit Pro, and Thunderpants all do better from the Yulong. IEMs are a tossup because neither amp seems ideal for them. The A18 has some type of "class A hash" in the background, which only appears with sensitive IEMs but makes for an annoying experience. The CMA800 has a tiny bit of this but the main problem is gain - there's just too much of it. It's very difficult to dial in the proper volume levels when the slightest touch of the knob results in several decibels worth of increase or decrease. The CMA800 ends up being more usable than the A18 but frankly I'd take the V200 over bother of them in this situation; the adjustable pre-gain option makes a world of difference.
 
AURALiC Taurus mkII
This is a tough one because the Taurus mkII is probably the best solid state amp I've ever heard thus far. It costs a few hundred dollars more than the CMA800, but has more features such as preamp functionality, balanced headphone output, and a lot more power. Both are phenomenally well built and very attractive. I went back and forth many times and the differences between these two are subtle - very much more difficult to spot than in the prior amp comparisons. Both are extremely resolving, neutral, well extended on both ends, and very open sounding. After much comparison I believe I found some small differences, though again these are way more alike than they are different.
 
First of all, the Taurus seems to have two distinct sounds to it. The single ended jack is more direct, more dynamic, more lively. The balanced output seems more nuanced and subtle, with more expansive soundstage and more precise imaging. In comparison, the Questyle is closer to the balanced output of the AURALiC - both are what I'd call neutral, clear, and highly resolving. The CMA800 takes a very slight lead when it comes to timbral accuracy - it goes a bit further in making me believe I'm hearing a real instrument. Especially with the HD800, which I still feel is its best match. I also thought the CMA800 did a little better when handling complex orchestral works. Maybe it's the "zero TIMD" thing, or some other attribute, but it seemed absolutely unfazed by even the most ridiculously complicated performances. From Mussorgsky's Night On Bald Mountain, to Stravinsky's Firebird Suite, to L'Apprenti Sorcier by Paul Dukas, and many many others (not to mention highly technical jazz, electronica, metal.... I could go on) the CMA800 just seems to have more honesty and instrument separation, and thus better layering. 
 
Is it a slam dunk for the Questyle? Sorry, no. The Taurus has a sound of its own and I love it for what it is - I actually prefer the single ended jack to the balanced most of the time, especially with certain headphones. It's like taking the warmth of the V200, and the smoothness of the Yulong A18, mixing in a dash of precision from the CMA800, and ending up with a delicious cocktail of great sound. But more on that when I (finally) review that amp at InnerFidelity (soon). If forced to choose between them, I'd have an extremely difficult time. If my rig was HD800 only I'd choose the Questyle. If it was HE-6 only I'd choose the Taurus. The possibilities branch off from there - don't get me started on DAC pairing.... I'll just conclude by saying both are exceptional, and really are the two best solid state amps I've ever heard. I've got the Questyle CAS192 DAC and I've heard the AURALiC Vega DAC, and hearing those just reinforces the notion that these are two excellent companies at the top of their game. AURALiC has been around a bit longer and seems to have made significant inroads towards becoming a somewhat well known and respected brand here in the USA. I'm sure Questyle would like to do the same, and so far they are on the right track.  
 
 
CONCLUSION
What more can I say? This whole current-mode amplification thing seems to be quite the trick. Or maybe it isn't - judging an entire design concept by a single example probably isn't the best thing to do. All I can say with confidence is that the Questyle CMA800 is an exceptional headphone amplifier. I place it in the upper echelon with some of the best amps I've ever heard regardless of price. The clarity is simply intoxicating - a real open window to the music - and seems limited only by the headphones being used. This level of transparency is usually only heard in top level tube amps, which tend to be larger and more expensive than this little solid state wonder. Recommended? Yes, very much so. 
 
That was supposed to be the end of this review. But Questyle, a very aggressive and forward thinking company, has already gone and replaced the CMA800. But not really.... allow me to clarify. The new CMA800R phases out the original model. It's essentially the same amp but with some added features - the most intriguing of which is a single 3-pin XLR output on front, in addition to the dual 1/4" jacks. What does one do with a single 3-pin output? Don't I mean a 4-pin XLR for balanced operation, as seen on the Yulong A18 and AURALiC Taurus among many others? No, actually I don't. I said 3-pin and I meant it. Questyle intends the CMA800R to be a modular, dual mono, true balanced amp. Buy one at a time (the price remains the same), use it like the original CMA800, and you'll love it. But get the urge to upgrade and you can later add another CMA800R to the chain, each one powering its own individual headphone driver. Output from the XLR is fixed for precision matching, which means an external preamp or a DAC with digital volume control is necessary. Questyle calls it the world's first true balanced dual mono headphone amp. I'm not sure if they are unaware of the new Woo Audio WA-234 monoblocks (which do seem to have launched prior to the CMA800R), or maybe they know something I don't about the internal design of that product... maybe it goes down to single-ended, then converts back to balanced prior to output, like the Bryston BHA-1? I don't know. In any case, that's a $16,000 device, and the pair of CMA800R amps is $3,000, so they aren't really competing anyway. I may be able to get my hands on that set for a separate review, but in the meantime: by adding features (there's also pre-amp outputs, slightly increased power capabilities, lower distortion, etc) and not increasing the price, Questyle again shows how serious they are in becoming a well-known player on the scene. I'm impressed, and hope to try out the monoblocks soon. If just a single amp already sounds this good..... 
 
 
 
Note - this review originally appeared under the "CMA800R" category. My mistake. Now that I'm posting a review of that model, I had to create the proper entry and move this review over.