The Audinst HUD-mx1 compact DAC/headphone amp was a great little unit. It also happened to be one of the very first reviews that I did at HeadFi. In the subsequent years I've gone on to review dozens of other products from amps to DACs to headphones and more, and I also went on to become something of a "real" reviewer at InnerFidelity. Through all that, I've never stopped recommending the Audinst mx1 as a cheap, reliable unit that sounds good and doesn't take up much space.
Audinst later went on to release a portable amp (the AMP-HP) and another DAC/amp unit (the HUD-mini). Both of these were strong competitors in their respective categories and the HUD-mini came dangerously close to being superior to the mx1, despite being a bit cheaper. Now Audinst launches a refreshed version called the HUD-mx2. It looks nearly identical to the mx1 but adds several important features and upgrades a lot of other things too. With a price increase to $248, a roughly $70 increase over the original, is the mx2 a worthy successor?
The mx2 looks almost exactly like the mx1. The case is slightly larger but for the most part they look very close. Both have the same silver faceplate with a slight curve to it. Both have the same black chassis. Both use the same dual headphone jacks (1/4" and 1/8"), the same RCA jacks, etc. Aside from added features, the only thing that is really changed is the volume knob. Gone is the rubbery, slightly tapered knob of the mx1, replaced with a larger aluminum knob that looks and feels superior. I never had issues with the original but I can see how this new design allows for more precise volume tracking due to the larger circumference.
Despite their similar looks, those familiar with the mx1 will notice a few new things. A Toslink input joins the Toslink output, meaning this is now a dual source DAC. A front panel switch allows the user to choose from Toslink or USB connections. The output still works the same as it did with the mx1, essentially making the unit act as a USB to SPDIF converter in addition to the DAC and headphone amp capabilities. It also keeps the same Dolby Digital pass through function for those using surround sound receivers.
The rest of the changes reside on the interior of the unit. And those changes are numerous. The DAC has been switched from a Wolfson WM8740 to a theoretically superior TI PCM1796. The USB receiver, formerly the Tenor TE7022L, is now a VIA Envy VT1728 which means the unit now supports all sample rates up to 96kHz, including 88.2kHz (which the mx1 did not). The mx2 uses dual LME49860 opamps for I/V conversion instead of just one like the original. Following the I/V stage is a differential amplifier based around an OPA2227. From there the signal passes to the line-out stage which appears very similar to the mx1 and uses the same OPA2134 opamp. The OPA2227 and OPA2134 are socketed and can be swapped. The LME49860 opamps can not.
The headphone amp section is changed as well - where the mx1 had an AD8397 opamp handling all amp duties, the mx2 uses a TPA6120A2 driver chip. This allows for significantly more powerful output. The specs on the mx2:
1.5W at 32 ohm, 5Vrms
300mW at 300 ohm, 7Vrms
160mW at 600 ohm, 7Vrms
Those are some high numbers. The TPA6120 is certainly capable of that much output (and even more according to the datasheet) but the problem is heat: it gets very warm, and requires a thermal pad underneath to help with dissipation. That's why it can be difficult to implement in a DIY design. That probably also explains the venting holes on the rear of the mx2 which were not present on the mx1. I'm doubtful these are the actual measurements, especially on USB power, but are probably more of a "best case scenario" type of deal. Either way, there's no shortage of juice here.
Output impedance is a mere 2 ohms, down from 5 on the mx1. This means very little chance of impedance related interactions even with the crazy impedance swings of multi-driver balanced armature IEMs. It also means sufficient power for difficult planar headphones. The mx1 was released before planars became prominent, and could only deliver a few hundred mW into low impedance loads. That was plenty for most dynamic designs but headphones like the LCD-2 and HE-500 thrive on high current and the mx2 is far better suited in that area. I'm not sure how many people will be using expensive LCD-2s and HE-500s with this budget DAC/amp unit, but it's nice to know they can if they want to.
As some astute readers may already know, the TI TPA6120 datasheet specifies a 10 ohm output impedance for stability. And we normally see this followed, in designs ranging from the budget Fiio E9 to the high end Kao Audio UD2C. But there are ways around it. For example, NwAvGuy shows one method HERE
where he drops output impedance below 1 ohm by adding some SMD ferrites into the circuit, on top of the 10 ohm resistors. Looking at the mx2, I believe Audinst used a similar method - but it's difficult to tell for sure with these tiny SMD parts. NwAvGuy noted a penalty paid in terms of measurable distortion but it was arguably below the audible threshold and overall a worthwhile trade-off for the superior output impedance.
Like the original model, the mx2 has an internal jumper to adjust gain. In this case it has 2 jumpers but the function remains the same - output impedance doesn't change but the gain factor does. Audinst recommends the "normal" gain setting for loads up to and including 300 ohms, and the high gain setting for 300 ohm through 600 ohm loads. In practice, I ended up leaving my unit on low gain all the time. High gain may be useful for certain 600 ohm headphones like the AKG K240DF but I no longer own anything like that. 300 ohm Sennheisers work fine on normal gain so I see no reason to switch for most users.
The last feature has to do with functionality. The USB input is now able to interface with devices such as the iPad through the Camera Connection Kit, or Android phones such as the Samsung Galaxy S3 through an OTG cable. For some, this feature won't mean much, but I think it's a good thing based on the target market for a device like this. I can totally see a person using it at work, for example, where an increasing amount of companies lock down their computers for any external peripherals. That means adding the mx2 as a USB DAC is not an option, and likely neither is adding an external hard drive or SD card full of music. But the mx2 could still work by using a phone or iPad as the transport. Same situation in a college dorm, a bedside setup, or many other scenarios. Bottom line is that the added functionality is very welcome.
There is slightly more free space here compared to the mx1, due to the widening of the board,
required to fit that extra front panel switch to select USB or Toslink:
The new headphone section:
The new differential amplifier for the DAC section:
The actual DAC chip:
The AKM receiver, TXCO clock, and VIA USB chip
The HUD-mx2 builds on the Audinst "house sound" originally established by the HUD-mx1 and continued with the HUD-mini. It's a relatively neutral sound, not overly dry or boring, and it works with a variety of equipment. I started by using the unit as a USB DAC with my desktop PC, also acting as a preamp to feed my powered Serene Audio Talisman
speakers. The mx2 does a good job of controlling volume - balance is essentially perfect across the range and there's no static or undesirable noise during adjustment. I normally use a more expensive Audiotrak DR DAC 2 DX Muses Edition in this setup, which is actually very similar to the HUD-mx2. But more on that later. I played some of my favorites tracks from Marta Gomez, Livingston Taylor, and Eric Clapton, and all sounded excellent. The detail level seems noticeably improved over the original mx1 model - it isn't a complete night and day difference, but enough to move the mx2 one step higher on the food chain. I also played my 24-bit/88.2kHz version of Classic Brubeck
and appreciated the native 88.2 support which was not present on the original model.
Next I tried a dedicated headphone amp, using the mx2 as a DAC. I plugged in my Violectric V200, which is an amp I know very well, and listened to the Audeze LCD-2.2 with various hi-res test tracks. Again I noticed a neutral and fairly transparent sound, with very little character of its own. Bass had nice depth and realism, and timbre was accurate. Soundstage was nicely sized too, with believable imaging. This combo sounded pretty darn good.
I normally use the matching V800 DAC with my V200, which is far more expensive than the little mx2. The Audinst sounded more closed in, with less of a clear window into the performance. It had less extension on both ends but particularly in the lowest lows - which is exactly what I'd expect given the price discrepancy. Yet taken on its own, the mx2 was a capable performer and didn't represent a massive bottleneck. Sure, one could spend more and get more, but nothing about the mx2 is completely objectionable.
I tried the Toslink output from my motherboard and got a sound which was very similar to USB. I thought perhaps USB was more "crisp" and had more defined leading edges, while optical was ever so slightly more relaxed. But this could reflect the optical signal from my motherboard just as much as the Audinst itself. In a computer situation I'd choose USB just out of convenience anyway but Toslink is nice to have around since it opens up new possibilities.
Next, I got rid of the V200 amp and started using the headphone jacks on the mx2 directly. This is where I noticed the single largest difference as compared to the original model. The mx2 simply does soundstage, drive, clarity, and pretty much everything else, better than the mx1. The sonic signature shifts a little bit from slightly forgiving to bold and energetic. Vocals sound especially good - from sultry singers like Jacintha, to the powerful and soulful Beverly Knight, to Annetet Olzon (formerly) of symphonic metal band Nightwish, vocals seem more present and up front. More "believable" for lack of a better word. Not that the mx1 was bad in this area but the mx2 just goes farther. The only potential drawback is with brighter headphones with peaky highs. If someone found a set of Grados barely tolerable on the mx1, the mx2 might push them over the edge. That's not really the fault of the Audinst though - it's just giving you more of a true representation of what the headphone is all about.
I really like the LCD-2.2 straight from the headphone jack. Again, it isn't as good as the dedicated and far more expensive Violectric unit, but it does a very credible job. It also sounds great with my HE-500, HE-400, and Thunderpants planars. I don't care how efficient HiFiMAN makes them, planar magnetic headphones seem to love high current, so the redesigned amp is very welcome here. All of these models should be seeing 1W or more from the mx2 and it really works well, especially compared to the original model which wasn't very satisfying with those models.
I also tried some more affordable headphones which are far more likely to be paired with the mx2 by regular users. The NuForce HP-800 and the VMODA M80 both sounded very nice. Both have a somewhat darker presentation, the former more so than the latter, and both benefit from the slightly energetic Audinst. It's not enough to turn them into a K701, but it helps even things out a little compared to a completely neutral amp. I can't decide which one I like more - the HP-800 has thunderous bass that sounds very good for the price, but the M80 has sweeter and more engaging midrange. Either way, this would make a great starter system for someone just getting into the hobby and not wanting to spend a ton of cash.
Next I switched to IEMs which can be a good test as they can reveal issues with the noise floor in a lot of amps. The Audinst is thankfully very quiet, with no buzz or hiss or anything like that. It's a small thing, but I appreciate having a dedicated 1/8" jack on the front panel. Not that using an adapter is a big deal, but the dedicated jack just feels better. I really enjoyed it with the Heir Audio 8.A and particularly the 1964 Ears V3, which is a relatively affordable CIEM that is more likely to be used with the mx2 as compared to the more expensive 8.A. The V3/mx2 combo produced a detailed sound that remained smooth and grain free, with excellent dynamics and deep bass extension. I'm spoiled by my high-end gear but even I am impressed with this simple and reasonably priced combination. The only thing to watch for is with brighter IEMs - I'm not sure I love the mx2 with my Lear LCM-5 or even with my JH13 FreqPhase. It seems to accent the upper mids and highs just a little too much, making them seem overly aggressive. It's listenable but not ideal. This would probably apply similarly to other universal IEMs like the DBA-02. Interestingly, I didn't have a problem with the HiFiMAN RE-400 at all. It sounded clear and present but not overly so. So I guess it's worth trying everything to see what works best.
All of the above comparisons were done straight from USB power. In that case the device runs at 5V internally. Adding the bundled external power supply brings operating voltage up to 12V. The benefit in terms of DAC sound quality is limited - I sometimes thought it sounded slightly better, but it might be my imagination, and I really doubt I could reliably tell them apart in an A/B comparison. The downside is when running from the external PSU - the enclosure of the mx2 gets fairly warm, which doesn't happen with regular USB power. It's not a huge concern of mine but I figured I'd mention it.
It is worth mentioning that this higher operating voltage determines which opamps are suitable for use. So opamp rollers need to take note. Something like a Muses01 needs at least 9V which wouldn't be satisfied when running in USB mode, but would be fine with the external PSU. Not that Audinst users will likely be lining up to try that particular opamp which is around $50 a pop, but you get the idea.
For headphone use, I do notice a difference when using external power, depending on the headphones being used. It does seem to help with clarity and drive on the HE-500 and to a lesser extent the LCD-2. With most dynamic headphones I notice less of a difference if any difference at all. I'd say that 90% of the time or more the external power is not necessary, but it's sure nice to have the option. I do have a NuForce LPS-1 linear power supply which should theoretically work with the mx2 - I need to give that a try one of these days and see how it does.
The main comparison that came up for me was with the Audiotrak DR DAC 2 DX Muses Edition. The Audiotrak sold for $329 (but was often available for less) and has many similarities to the Audinst in terms of features and appearance. Unfortunately it appears that Audiotrak has upgraded yet again to what they call the "Top Edition" which uses Muses 8920 opamps in place of the 8820 in my unit, as well as some upgraded capacitors and other parts. The new TE sells for $329 and I can't find it any cheaper. So the comparison I'm doing might not be all that relevant at this stage but here it is anyway.
The Audinst is nearly a match for the more expensive Audiotrak unit in almost every way. The key difference is the sound signature of the headphone output - while Audiotrak is slightly warm and smooth, the mx2 is more energetic. It has more sparkle, and thus creates a more lively feel. The result is dependent on headphone pairing - mx2 sounds better with LCD-2 and Audiotrak works better with bright headphones like K701.
As a DAC both units are very similar. Audiotrak has a slightly more laid back, relaxed midrange, while Audinst is more forward and "snappy". But the differences are small. Overall the mx2 is impressive here - it competes with the $329 Audiotrak which is a nice unit in its own right. And subjectively I like the mx2 appearance much better.
The Grant Fidelity TubeDAC-11 ($350) is very stiff competition, and the Audinst doesn't quite beat it. The TubeDAC-11 is more resolving, and more natural, though the margin is not huge. Audinst has some functionality that the TubeDAC doesn't - Dolby Digital passthrough via optical out, USB to SPDIF conversion, support for 88.2kHz sample rates, a dedicated 1/8" headphone jack. In return, the TubeDAC has some unique options of its own - more inputs, a tube headphone output (by way of an adapter), and multiple options for DAC output (tube, SS variable, SS fixed). It has a more robust power supply and looks like a more "serious" audio component if that matters to you. Obviously the Audinst is far better suited to be used as a transportable solution, so each unit has its place. For ultimate fidelity the TubeDAC is king, but the Audinst is not completely blown away and has some advantages, including its low price.
In a surprise comeback, the Matrix Audio Cube DAC
has dropped from $299 to $199. That's a great price for a quality unit, and it becomes serious competition to the Audinst HUD-mx2. Ultimately I believe the mx2 is the better sounding option, with more refined highs and superior transparency. The Cube is a compelling buy for the price though. Companies are competing heavily for this market segment, and there's no way the Audinst would walk away with the category without some tough competition. The current version of the Cube has swappable opamps (mine doesn't) and could possibly be tweaked to extract further performance. But the same applies for Audinst.
Audinst has shown itself to be an established audio company by consistently releasing one good design after another. The HUD-mx2 is the best of the bunch so far - it takes the established Audinst "house sound" which first debuted in the original HUD-mx1, and turns it up a notch or two. In this rapidly progressing field, where many good options exist, and there's simply no way the mx2 could be a "giant killer" or anything of the sort. It lies squarely in an area where spending more can certainly get you more - unlike high end multi-thousand dollar DACs where spending more often nets a "change" but not necessarily an improvement.
But don't let this dissuade you - the HUD-mx2 is one of the best values currently available. For a lot of users it will surpass the "good enough" mark where more spending will simply not be necessary. With its powerful and refined headphone stage, it makes a good argument for throwing the bulk of your cash at the best headphone you can afford. The HiFiMAN HE-400 makes a particularly good combo, though others also work well. IEMs are also fair game thanks to the low noise, low output impedance, and improved volume control. All in all the Audinst HUD-mx2 just "works" and I can easily recommend it.