EXOGAL COMET – A COMPACT REFERENCE

Specs. Numbers on a page. So interesting, so important, yet so potentially misleading. Taken at face value, specs can do more harm than good – at least in some instances. Ever look at frequency response listings for various speakers? One gets the idea almost any “bookshelf” speaker should be capable of a clean sweep from about 50Hz all the way to 20kHz, plus or minus 3dB. In a real life, in a real room? Not so much. That single, simplified measurement does have value, but it only goes so far. To the uninitiated, based merely on that spec, PSB’s flagship Synchrony One B will sound nearly identical to their entry-level Alpha B1 but with just a tad more low end extension. In reality the two sound nothing alike – as expected considering the massive price discrepancy.

Beyond mere specs is the propensity for us audio enthusiasts to think we know it all. Thus, silk dome tweeters always sound like this, while metallic tweeters always sound like that. Balanced armature IEM designs? Those always sound a certain way in relation to their dynamic driver counterparts. Ditto electrostatic headphones versus planar magnetic versus traditional dynamic. I’m sure you can think of more examples.

Perhaps the most flagrant use of this generalization takes place in the realm of the D/A converter. Armchair designers on forums routinely prejudge DACs based on nothing more than the brand of chip used…. Wolfson is warmer, Sabre is detailed but has top end glare, etc. I find this state of affairs ridiculous. It causes us to form inaccurate opinions before ever experiencing a device with our own ears. It’s the audio equivalent of likening a Nissan 370Z to a BMW E46 M3 because, well, the specs are similar on paper… Meanwhile, anyone who’s actually driven both can tell you – each has its own distinct personality, and will appeal to different people.

Another great example of this principal at work is the Comet  DAC ($2,500) from Exogal. While technically a fairly “new” company, Exogal is comprised of several industry industry veterans. Chief Designer Jim Kinne was making high-end stuff at Wadia back in the early days, when that brand was among the absolute best available – he designed the Wadia 270/27 combo which sold for nearly $17,000. And that was in 1999 dollars. He was also the man behind the elusive Wadia 790 PowerDAC, which if I recall correctly was behemoth set of monoblock amps priced in Porsche 911 territory. Exogal seems modest in comparison, focusing instead on major bang for the buck gear – with very little compromise necessary, thanks to the experience of the team.

Why is the Comet an example of this principal? Because the designers go about things in a rather unexpected way, and because the device sounds far larger than its compact enclosure would suggest. I intend to explore each of these aspects in due time.

Design

First, design. The Comet itself is a smallish device measuring roughly 12 inches wide, 8 inches deep, and 2 inches high – and I’m rounding up on all three numbers. Weight is around 9 pounds which is rather substantial considering the small footprint. Fit n’ finish and aesthetic appeal are about as nice as you’ll find this side of megabuck dCS/Esoteric gear, and even then we’re talking minimal gains. Seriously – this thing looks fantastic. Even more so in person than the pictures suggest. My review unit came in silver and Exogal recently added black to the catalog.

The key focus on the front panel is a small display. It’s monochromatic and reminds me of an E-Ink display from a Kindle. It shows which output is active (headphone or line-out), lists volume, input, and sample rate. Given the small size and lack of backlighting (which is either a benefit or a drawback depending on your perspective), it isn’t really ideal for across-the-room viewing. Which is fine, as Exogal has something else in store for us on that front. But more on that later. Around the right side we get a 1/4″ headphone jack, and then the array of inputs/output on the rear as usual. Pay close attention and you’ll notice a distinct lack of switchgear – not even for power. Exogal says the device should stay on all the time, and a low-power standby mode automatically kicks in after a short period of inactivity. This should keep the device “warmed up” enough to sound its best at a moment’s notice, without sucking much juice in the process.

For such a small thing, the Comet is fairly generous in connectivity. Outputs come in XLR and RCA flavors plus that headphone jack I already mentioned, while inputs are USB, Toslink, AES/EBU, BNC, and a set of RCA inputs for analog signals. There’s also a 12V trigger for custom installers, a USB power output for charging a tablet or phone, and something called “Exonet” with input/output using HDMI as a format. This is a proprietary link system which will interface with the upcoming Exogal Ion amplifier. It does NOT accept PCM audio over HDMI, so don’t get any crazy ideas.

Exogal touts their slick proprietary digital volume control as being essentially transparent. But with no switches, buttons, or knobs to fiddle with, how do we use it? For that matter, how do we even cycle through inputs? The answer to that question is the one design aspect most likely to polarize opinions. Personally, I dig it, but I can see how it might be slightly obnoxious for some people.

The potentially controversial aspect? The Comet is Bluetooth enabled. It comes with a tiny remote resembling a car alarm key fob, which works using Bluetooth and therefore doesn’t need line-of-sight. But the Comet seems really intended for use with Exogal’s iOS app. The app handles source selection, volume control, muting, standby activation, and also shows the sample rate of your current musical selection. Basically you can’t do much without one remote or the other.

For most folks, this is no big deal. Tablet devices are prevalent and I’d say a large majority of my friends stick with a single input 95% of the time anyway. And most of them use a dedicated preamp or headphone amp in their system. So with no switching of source, and no volume changes, just pure DAC functionality, the Comet is as simple as can be. But if you do end up needing to switch inputs on a regular basis, or take advantage of that volume control option, you’ll need to use a remote every time. Plugging a headphone directly into the Comet, thus sitting right next to it? Doesn’t matter, you’ll have to use a remote. This is mildly annoying at times.

If you can overcome that ideological hurdle, the iOS app really is a pleasure to use. An iPad is already an integral part of my system so it’s nothing new to me anyway. I use the Aurender control app for my X100L music server in the big system, and Monkeymote for a laptop feeding a different system in the other room – not to mention streaming Tidal, OraStream, Rdio, and other services via AirPlay. This is becoming more and more common so it might be worth snagging something like a used iPad 4 if you haven’t yet succumbed to Apple’s inevitability. I’m told an Android version is in beta testing and should release within the next month or two.

Exogal ships the Comet with an external switch-mode power supply. Not a wall-wart, mind you, but a rather significant brick similar to the type used for a laptop. Exogal also offers a linear power supply option for $600 by itself or $500 if purchased together with the Comet. This PSU features 85,000 uF capacitance and is thus said to help “squeeze out that last drop of performance”. I have both power supplies on hand for this review, so we’ll see.

Now, moving inside of the chassis – this is where the magic happens. The Exogal team has an extensive history – not only in high-end audio design but also other disciplines like GPS, broadband networking, and various other non-audio fields. This allows for a different approach, a road less traveled by designers who lack this experience.

Exogal’s Jeff Haagenstad put it like so:

“As you know, PCM at it’s simplest level is really just the creation of an analog wave using a successive approximation created by interpolating the levels of each sample. With a large number of samples clocked using a very fast clock, the errors in the output are extremely small – hopefully small enough that the ear cannot discern them. However, the jitter from clock asynchronicity (is that a word?) and the subtle errors in the successive approximation process can unavoidably be heard by some people. If not heard, then perceived. And different people perceive it in different ways. We do our digital to analog conversion in a proprietary 6-core DSP chip where the successive approximation is only the first step of a multi-pass process. We don’t try to match the encoding clock with a decoding clock (somebody calls it a “femtoclock”). We use the original signal to create a mathematical representation of the encoded signal. Then we run it into the DSP where we do things to it (sorry, proprietary) so that a recreated analog signal (we call it re-rendered) comes out of the DAC. There is virtually no jitter because the source clock drives the conversion and there are virtually no successive approximation errors because the signal is smoothed and recurved so that even if it’s not a 100% faithful recreation of the original signal, it is so close and so smooth that your ear cannot perceive the effects. See, the four of us have been in and out of audio throughout our careers and we all have signal processing experience on other technologies and applications like network transmission signal processing, video image processing, and processing GPS signals from satellites, as well as other similar experience. We brought in the techniques from some way more challenging and much higher-speed processing problems and applied those techniques to audio. Believe me: once you’ve dug a tiny GPS signal out of a 10x higher noise floor, a relatively clean audio file is a piece of cake!”

Clearly, the Comet does things quite differently. They call their proprietary conversion process the “Exogal DSP Platform” and plan to use it for various other products going forward. Comet does use a traditional DAC chip as well – two of them actually (one for the line out and one for the headphone section) – but not in a conventional way. The Exogal DSP Platform does all the heavy lifting in advance, and merely hands the signal over to a Texas Instruments PCM5122 (or PCM4104 in the case of headphone output) to make use of only the final output stage of those chips. Therefore we can rightly lump the Comet in with the rather small group of DACs which handle D/A conversion through a proprietary method – other members of this club include the PS Audio DirectStream, various models from dCS, and Chord’s Hugo family.

All outputs (including the headphone out) pass through LME49600 buffers, which lines up with my impression of the outputs sounding basically identical. I’ve had DACs where the XLR and RCA outs have completely different character – not so on the Comet.

The headphone stage is relatively tame in terms of output power. It was designed to drive low impedance headphones of at least moderate sensitivity. It can deliver 276 mW into a 16 ohm load, 138 mW at 32 ohms, and 79 mW at 56 ohms. At 300 ohms we are down to 15 mW which is modest even by portable amp standards. I’ll discuss the ramifications of this situation shortly.

Sound

When the Comet first arrived, I spent my first few minutes with it marveling at its external charm. Did I mention how handsome the thing is? Once I pulled myself away from that, I threw together a little system for it using an HP Elitebook streaming lossless tracks from Tidal, the compact Apex Butte headphone amp, and a pair of Audeze LCD-2 headphones. Right off the bat I could tell the Comet was of very high caliber. Even in this hodgepodge system, hastily assembled with no real accounting for synergy, I still loved what I heard. McCoy Tyner’s Expansions sounded spot on, capturing the phenomenal interplay between band members on what I have to say is my favorite Tyner album ever. Soundstage was slightly lacking but that’s to be expected with the LCD-2. That model is high on tonal density and impact, at the expense of some nuances. I switched to Sennheiser’s more spacious HD800 and was greeted by excellent layering and separation, not to mention lightning fast transients – proving the Comet is no slouch in those areas.

Ok, first test passed, time to put the Comet through its paces more thoroughly. I swapped it into my main system – Aurender X100L music server feeding the Comet via USB, a KGSShv electrostatic amp, and either the Stax SR-007 mkII or else the closed back Stax SR-4070. I used an Equitech unit for power conditioning along with a complete set of AC, digital, and interconnect cables from the Cabledyne Silver Reference series. My second test is always to see if a DAC works comfortably with the Linux-based Aurender music server. Some devices still have issues with Linux for whatever reason. The Comet aced it, up to and including DSD128 using the DoP method.

A quick side note – I discussed with Exogal the merits of DoP versus native DSD decoding. Without rehashing our rather lengthy correspondence, I’ll summarize their approach like so: before launching any actual products, Exogal spent significant money on market research. The response indicated native DSD support was not something their potential customers seemed to care much about. DoP works quite well, so the choice was made, and customer feedback seems to support that choice. I’ve seen a few rumblings on the forums about DoP being inferior, and I’m not sure I buy it – how exactly would we even know that for sure? Seems to me it would take a company like Exogal releasing two versions of their Comet – one using DoP, the other optimized for Native DSD – and then we compare the results. To my knowledge, that has never happened, so I’m not sure how we really judge based on what we have available to us. The fact that DAC A sounds better doing native DSD, compared to DAC B which uses DoP, tells us more about those specific products than it does about DSD processing in general.

Anyway, back to my listening impressions from the main rig. I played lots of DSD, hi-res PCM, and of course the classic Redbook material we all know and love (plus some you’ve probably never heard of). This system, being quite a bit more resolving than the first one, helped show what the Comet was really capable of. And it was impressive. I heard abundant treble detail, richly textured mids, and a convincing low-end with excellent extension. I also heard massive soundstage and accurate imaging, not unlike what I’m used to from much more expensive DACs. This Stax rig can quickly become fatiguing when a DAC is voiced overly bright or has etched treble. Thankfully the Comet avoids both, with a very pleasing top end extension that never grates on my ears – even with my extremely neutral SR-4070 studio monitors.

That particular earspeaker is becoming very difficult to find since being discontinued some years back (and they never sold many to begin with). It’s among the most neutral and revealing I’ve yet heard from any transducer – note I didn’t say the most enjoyable for actual listening, but it certainly has a time and place. Think HD800 but even more accurate and resolving. Thing is, the 4070 is revealing enough to show many otherwise well-regarded DACs as downright offensive. I’ve heard highly respected and fairly expensive devices from Benchmark, Mytek, Arcam, Cambridge Audio, Bel Canto, Wyred 4 Sound, and others which fell flat on their faces under this particular sonic microscope. Not saying I necessarily dislike those brands in general, but some of their source gear has issues that my system clearly shows.

A gut reaction would be to go the other direction, for something like a Conrad Johnson type source, maybe some NOS DAC or a Cary DAC-100t – all dark and mysterious, full of tube bloom to offset that brutal Stax accuracy. I think that’s the wrong approach. Why squander something so rare by glossing it over or transforming it into something else? The Exogal Comet delicately walks that line between accuracy and musicality, generally neutral but with just the right touch of warmth to help offset its incisive abilities.

I swapped in my other pair of Stax, the SR-007 mkII, which is an altogether different character compared its sealed sibling. It has a more “all-purpose” sound to it, making it suitable for a wider variety of music. It also seems a little more forgiving of source quality, though this is only in relation to the brutally honest SR-4070 – the 007 still doesn’t put up with edgy treble. Unsurprisingly, the Comet once again made an ideal pairing, with a slightly warmer and less intense experience this time around. This would be more up my alley for broad music consumption.

Jesca Hoop’s The House That Jack Built (B&W 24-bit/48kHz), though somewhat hindered by dynamic compression, still sounds nicely open and layered. Jesca’s dulcet soprano has a sort of “girl next door” appeal making it sound more convincing and natural compared to the typical pop chaff. And when some tracks meander into surprisingly complex synth-balladry, the Comet never loses the plot. Even with material such as this, not spectacular by audiophile standards, I could tell Comet was something special – particularly as it pertained to soundstage.

More brutal fare such as Obscura’s Cosmogenesis or Misanthropy Pure from Shai Hulud, showed the Comet isn’t one of those trailer queen DACs which only sing when fed audiophile gems. These albums ain’t Norah Jones or Diana Krall, but they do require a level of grunt, speed, and overall refinement in order to get the job done. Exogal’s little Comet has those in spades. Both albums have their share of grit and grain, inherent in the recording, and the key is to avoid adding more through your gear. The Comet certainly doesn’t.

I had a blast playing Kraftwerk’s Trans Europa Express. Switching between the Kling Klang CD release from ’94 and the remaster from the fairly recent Der Catalogue box set, the Comet quite clearly revealed the strengths and weaknesses of each. While the old-school version is very impressive and indeed one of the most breathtaking recordings I’ve experienced in terms of sheer dynamic contrast, the remaster is the one I’d rather listen to. It’s overall punchier, more weighty sounding, and somehow also more clear, without losing sight of the intent established by the original. Unlike many remasters which deviate sharply from the source, this really does seem to be the release Kraftwerk would have given us in 1977, had they the resources to do so. Many DACs I’ve had come through this system – even very enjoyable ones – did not have enough insight to make these distinctions quite so clear. Spacial accuracy through the Comet was off the charts on this one – obviously these aren’t “real” instruments so there’s no performance venue to be recreated. Yet there most certainly IS a consistent and well defined sense of space, which leads to palpable depth and layering if your system can resolve it. This is one of the few areas where the DSD version (which I ripped from my SACD, thank you Playstation 3!) clearly surpasses the CD releases. I’ve had many very nice DACs in my system lately that did a lot of things right, but very few can match what Exogal has going in this area. I can count them on one hand, and each costs significantly more than the little Comet.

There’s a track that’s been stuck in my head off and on for about a year now. I use it as a stand in for the usual generic audiophile demo tracks, as it has similar properties yet is to my ears just far more enjoyable. The artist is Inger Marie Gundersen, the album is Make This Moment, the track a cover of Carole King’s “Will You Love Me Tomorrow”. I picked up the limited edition JVC XRCD24 release a while back, when I was on a buying spree hunting down every XCRD release I could get my hands on. I played it a few times and thought it was quite good, but for some reason I shelved it and never fully appreciated it – until recently. Having ripped all those discs to my server, I revisited that particular album and found that particular track, and have been playing it rather often ever since. The entire album is great but that one stands out as something special. Using the Comet in my Stax system allows the mesmerizing drummer/bassist collaboration to shine clearly, like few other DAC’s I’ve heard. Is this the slight low end emphasis at play? Perhaps. All I can say is that I enjoy it immensely, and I think most people would agree. I threw in the Calyx Femto (~$7,000) and later the Esoteric D-07x (~$5,000) and neither had the same grace as the smaller, less expensive Exogal unit. Does that make the Comet a giant-killer? I’m not sure it’s a big enough sample size, but thus far signs point to yes – depending on the particular giant in question.

Headphone Amp

Having established the Comet as a top-shelf DAC, I ventured away from my beloved electrostats to give it’s integrated headphone amp a workout. Exogal tells me the output impedance is roughly 8 ohms, so that combined with the rather low power on tap shows this is clearly not a universal solution. And my testing confirmed that. The Mr Speakers Alpha Dogs seemed uncharacteristically slow and muddy – a symptom I recognize as being related to a deficiency of current. My HD800 responded the opposite way, sounding too wiry and lit up. Low end thump was present and accurate but not impactful enough for my taste. Sennheiser’s prior flagship, the still-popular HD650, was rather enjoyable but again lacked dynamic swing as well as total volume on certain quiet recordings.

My various custom in-ear monitors, while sounding great in many ways, also sounded somewhat “off” due to the output impedance being higher than desired. They all have complex crossover networks and some dip into the single-digit impedance range at certain frequencies. These are the ones which vary most with the Comet, as compared to being driven with a <1 ohm output impedance. They can still be enjoyable – sometimes very much so – but being intimately familiar with their true character, I know the Comet isn’t driving them the way I would like. If I spent many hours researching my choices and then spent $1k or more on a top custom IEM, I’m obviously looking for that certain signature. For the Comet to change it in some unpredictable way goes against what I was shooting for when I chose that particular IEM. Which is too bad, as the device has everything else going for it – black background, excellent volume range, and perfect channel matching even at very low levels. If Exogal tweaked their design to lower the output impedance, this might become an ideal partner for IEMs.

As it stands, the Comet isn’t a great match with many of my favorite headphones, nor with most of my IEMs. So is the headphone section a total failure? Not so fast. Perhaps we just need to find a good match. The 8 ohm output impedance means ideal damping factor will be achieved on headphones with a nominal impedance of 64 ohms or above. Yet the design is generally intended for low impedance headphones, thus many of the higher impedance models from Sennheiser and beyerdynamic are off the table. So what’s left?

I believe I found a perfect dance partner in AKG’s K7XX. An exclusive offering from Massdrop.com, the K7XX is supposedly nearly identical to the K702 65th Anniversary Edition, and has only very subtle differences with the K712. I’ll go out on a limb and say most of the K7 variants will mate well with the Comet, and probably the K612 too. My K7XX sounded open, clear, and detailed without being harsh. Bass was very solid – among the best I’ve heard from these headphones, even compared to expensive dedicated amps. The K7XX might be considered “mid-fi” due to the $200 price but don’t be fooled – when paired with the Comet, it’s a breathtaking experience, bettered only by significantly more expensive headphones.

I revisited Audeze’s LCD-2, this time without a separate dedicated amp. The result was just a tiny step down compared to before, when I was using the outboard amp from Apex Audio. This was still a very convincing performance and I could happily recommend the pairing. Interestingly, swapping out this newer set of LCD-2 with the Fazor technology for an older pre-Fazor model, the Comet drove them just as well. The Fazor is higher impedance so I thought perhaps it would not be a good match, but its higher sensitivity seems to make up for that difference. So basically any LCD-2 you find will sound quite good from the Comet, if not quite the best I’ve ever heard.

Sony’s long-discontinued SA5000 actually sounded quite respectable from the Comet. Bright, of course, as is the character of that headphone, but well controlled and rather enjoyable in its own way. This is a picky headphone which causes many amps to struggle; it has gobs of treble extension and almost behaves like an electrostatic earspeaker rather than a standard headphone with dynamic driver. For that reason it can expose flaws in your signal chain you didn’t know existed. The Comet drives it marvelously. If I wanted to recreate the detail retrieval and imaging qualities of my Stax rig but didn’t have the budget or the room for an outboard amp, this might be the way I’d go. It’s absolutely NOT a great all-rounder, being too thin to satisfy with rock and other genres. But for acoustic singer/songwriter type stuff, classical, and most jazz, it does quite well.

Lastly, the Ultrasone Edition 12 was a good pairing, despite not exactly matching so well on paper. Ultrasone claims use of “hand selected, gold plated transducers”, which is fine, but impedance is the typical 40 ohms we see in most Ultrasone models. The result is perhaps a little underdamped, with a corresponding lack of control, but still very pleasing in general. This is my favorite Ultrasone model yet – I really dislike everything they made prior with the exception of the Edition 8 (decent) and the Signature Pro (good), so I was nervous about this one. Thankfully it doesn’t have the face melting treble of the Edition 10. On looks alone the Edition 12 is an excellent match for Exogal’s little wonder and on sound it’s not far behind either. The combo is cumulatively better than even the K7XX I raved about, though it comes at a drastically higher price. This headphone is new to me so perhaps I just haven’t fully worked out its character yet. But from what I can tell so far the Comet is a very capable partner.

Overall I’d say the headphone stage is hit or miss. It misses often, but when it does hit, the result is something of a home run. Again, if Exogal could tweak the output impedance without causing problems in the circuit, that would be ideal. A lot of DACs have no headphone out at all. Others have outputs which are mediocre at best. Comet gets a leg up on those, and can sound great with the right headphone. However, I do have to mention the existence of other models with even better headphone amps. The B.M.C. PureDAC, when used with balanced headphones, is one such beast. As a DAC it is outclassed by the Comet – their new UltraDAC is priced more in line and potentially competes more strongly, but I haven’t heard it. I reviewed the Asus Essence III and found it’s amp section prodigious – again, the Comet is a better DAC but not as good on the amp side. In the end this situation will inform your decision making process, assuming you even care about the headphone portion to begin with.

Preamp

I spent a little time evaluating the Comet as a preamp. Or rather, a DAC which doesn’t need any intermediate stage between it and an amplifier. This is a common feature on DACs these days but not many do it as well as we’d like. From my admittedly limited testing, the Comet doesn’t falter.

I used it to feed a pair of Ghent Audio monoblocks driving my Sonus Faber Venere 1.5 monitors and I didn’t notice anything missing as compared to the Bel Canto PRe 2 I had been using prior. In fact I think going DAC direct allowed a slight bit more insight into the recording – I won’t say veils were lifted or anything of that nature, but on excellent recordings I heard a small improvement. Besting an aging but well regarded $3,500 preamp is nothing to sneeze at, but that’s as far as my testing could go. I imagine other reviews out there will touch on this more than I can.

Power Supply

Lastly, I have to mention the outboard power supply upgrade. Add that to the picture and you now have what Exogal refers to as a Comet Plus. Word of advice? Just do it. The Comet already sounds exceptional by itself, so it’s not absolutely necessary to upgrade… but the outboard PSU firms up bass impact and slam, and also helps transient response, to the point where I can’t see anyone not finding it worthwhile. The blazing green LED on the PSU will require some electrical tape, and I don’t like how the DAC itself comes in black AND silver but the PSU is black only. And, while I’m at it, the PSU enclosure is nice but doesn’t match the DAC itself. Exogal could have probably made a beautiful matching enclosure to stack under the DAC, similar to what MSB does on their expensive gear. But that theoretical product would surely cost double or perhaps even more, so I think the right path was chosen here.

Again, it’s not absolutely essential to spend the extra cash…. but it DOES bring up the sound by a worthwhile amount. Users can start with the basic Comet and move to Comet Plus as their budget allows. Incremental upgrades are always a good thing, so I’m glad the option exists.

Last Words

Exogal’s Comet DAC is an absolute winner in my book. The sound is richly textured, nuanced, and detailed, and just warm enough to avoid sounding overly clinical. As a DAC it works exceedingly well in just about every system configuration I could dream up. It does well as a preamp too. The one area where I find it picky is the headphone output – many popular headphones just won’t reach their full potential due to power and impedance issues. Yet with a little care in headphone selection, the Comet achieves results approaching that of rather expensive outboard amplifiers. So, it’s not a universal solution, but certainly CAN make a great all-in-one.

As a DAC first, and headphone amp second, I heartily recommend the Exogal Comet, and can’t wait to see what the team will think up next.