British high-fidelity brand Meridian has long forged its own path to hi-fi. When much of the UK industry was grimly adhering to analogue as the only true path to high fidelity, Meridian embraced digital, improving it as well as using it. The company developed systems with digital signal paths all the way to active loudspeakers, and created such things as Meridian Lossless Packing, a compression system for digital audio that not only preserved the entire signal but included integrity measures to ensure that each byte matched that of the originally encoded signal. 
So good things might be anticipated from Meridian’s portable Explorer2 DAC (digital-to-analogue converter), which is designed to take the USB output from your computer and deliver from it far better sound than any built-in DAC would offer. And thanks to a bonus bit of socketry, it can undertake such duties at home as well.  
There are two faces, if you like, to this DAC. One is its operation as a traditional (if such a word is appropriate) DAC. The other is its support for the new format called MQA — Master Quality Authenticated. See below for info on MQA; mostly the main review will concern those traditional DAC functions.
The Explorer2 DAC (the '2' is raised, like a squared number, but sadly our particular slice of the internet doesn't allow us to serve you with such typographical complexity) is just a touch larger than some competing models, but unlike most you get two outputs, separately for headphone and line-level. This is quite right; it’s not clear that an output designed for one would necessarily work for the other. Both are 3.5mm mini-jack sockets. 
The line-level output is rated at a fixed 2V RMS (presumably with a 0dBFS signal). The headphone output (with an internal impedance 0.47 ohms, says Meridian) has a variable output level. The adjustable level is achieved by analogue means, but under digital control. When you change the system volume level (both Windows and Mac), the Explorer2 adjusts the output level.
The two 3.5mm outputs are on one end of the unit, which is housed in an oval-section 102mm extruded aluminium cylinder. It comes with a plastic plug in the line output, making sure you don’t inadvertently plug your headphones into it. (Indeed, you are specifically warned not to do so in the instructions, so some care should be taken. It’s possible that damage may result to the unit if it’s faced with a low-impedance headphone load. Remember that a line-level input represents a 47kohm resistive load, so the maximum current the line output expects to be called upon to deliver is around 40μA.)
At the other end is a mini-USB type B socket. A short USB cable is provided for connecting this to your computer.
On one of the broader sides of the unit there’s a rubber base, which seems to imply that the unit is supposed to rest upon it. In fact unless your analogue plug is very heavy, the USB cable tends to hold the unit askew — not surprising given that the unit weighs only 50 grams.
Meridian doesn’t mention what DAC chip it uses inside. Being Meridian, it probably doesn’t use an off-the-shelf one, as is the common practise. Meridian does say that it employs a dual-tile XMOS DSP with 16 cores and 1000 MIPS of processing. Meridian further says that it upsamples all inputs to 192kHz or 176.4kHz (as appropriate) before performing analogue conversion, and rather than use the conventional steep ‘brick wall’ filter it employs its ‘apodising’ filter. This design eliminates ‘pre-ringing’, to which Meridian attributes the claimed harshness in sound quality of early digital recordings.
There are three indicator lights on the unit. The colour of the first light indicates the MQA status. The second one glows white if the signal employs 88.2kHz or 96kHz sampling, while the second and third glow white if the signal is 176.4kHz or 192kHz.
The current low availability of MQA files (at least with an equivalent non-MQA version to compare with) made testing that side of performance impossible. But a little background - the ‘A’ in MQA is the crux of what MQA is about. MQA’s literature talks about how much better MQA is than MP3 (which loses 90 per cent of the recording), but the real comparison should be with FLAC and other lossless schemes. It appears that in fact MQA uses extra metadata that can be added to FLAC and ALAC files, metadata which will be ignored if played back on a non-MQA device.
The ‘A’ stands for ‘Authenticated’, and Meridian intends that it be an integrity guarantee system for the music file. The Mode indicator light glows white for 44.1kHz or 48kHz audio, green for MQA mode, and blue for ‘MQA Studio’ mode. Standard MQA mode guarantees that the signal is identical to that originally encoded. ‘MQA Studio’ mode means that the studio has signed off on the quality of the source as well. 

We heard MQA files in the Meridian/Wavetrain room at the recent Australian Hi-Fi & AV Show, but they were not being demonstrated against anything, so their relative quality was impossible to judge.
Will MQA take off as a useful thing? After a slow start there is growing industry support from both hardware and software vendors. 2L, 7digital, Onkyo Music, Technics Tracks, UNAMAS (Japan) and High Res Audio all now have some MQA tracks in their high-res download catalogues, while NAD and Bluesound (both part of Canada’s Lenbrook Group) have confirmed they will deliver MQA-compatible hardware — Bluesound’s soundbar may be the first actual product, not quite the type for critical evaluation, but there you go. Only time will tell, and we are not bold enough to do a John C. Dvorak (who famously predicted the failure of the iPhone before its launch). Being adopted by Tidal (already testing) or Apple (a long shot) would go a long way to ensuring MQA’s long-term importance.
But be it MQA or high-resolution FLAC or ALAC, in a sense it doesn’t really matter. What the unit feeds on is PCM at anything from 44.1kHz to 192kHz, and in either 16 or 24 bits. Plug it into a Mac and it’s ready to go, having communicated to the Mac its precise capabilities (some DACs seem to mislead Macs, suggesting capabilities they do not have). Plug it into a Windows PC and it won’t work at all until you download and install the necessary USB Audio 2.0 driver. This is painless and it seems to install easily and cleanly. A rumoured Windows 10 upgrade in 2017 may fix this anomaly forever (please).
We played mainly FLAC in various high-resolution flavours, along with a stack of CD resolution music. It’s hard to see how MQA could significantly improve the results. The Explorer2 delivered music with authoritative bass, and a marked smoothness and clarity into all the headphones and earphones used, with impedances from 300 down to 16 ohms. Despite the limitations of being powered by USB, the Meridian delivered plenty of power — enough for high-impedance cans, and with the ability to deafen when using low impedance ones — all the while remaining serenely clean and coherent.
We conducted measurements using a Windows computer and the WASAPI driver to ensure bit-perfect signal delivery, and of course not using the volume control at all for the line output.
The headphone output remained unclipped up to a setting of 94% into an open circuit and a 552-ohm load (both channels driven, as were all these tests). Into a 32.6-ohm test load the output was unclipped at 88% (for 100Hz, 89% and 91% for 1002 and 1000Hz). Those delivered outputs of up to 6mW into the 552-ohm load, or about 8dB above the sensitivity rating of the headphones.
For the lower-impedance load the output was 39mW, 48mW and 58mW (100Hz, 1002Hz, 10kHz) for at least 16dB above the headphone sensitivity rating. The headphone output level adjusted in increments or decrements of about one decibel, so fine adjustments of volume are available.
As for noise — what noise? With 24 bits the noise level (including the measurement rig) was at –108.9dBA, THD at 0.0023% and IMD+noise at 0.0030%. With 16-bit/44.1kHz signals, the noise was around the expected –97.5dBA, and distortion still at vanishingly low levels. 
The headphone noise level with 24-bit sound was around –98.8dBA irrespective of impedance, and THD and IMD a touch more than via the line output, but still well under 0.005%. At 16-bit 44.1kHz, noise was –95.2dBA and distortion less than 0.006%
We are highly enthusiastic about the Meridian Explorer2. Despite it having been on the market for a few years now, the MQA ability gives it a lifespan-lengthening boost, and it’s a neat DAC to slip into your luggage for high-quality audio on the go, plus the line-level duties when at home. Even without evaluating the MQA, Sound+Image magazine awarded it as their DAC of the year, with good reason. So long as you don’t insist on DSD capability, Meridian’s Explorer2 is a first-class portable DAC. 

Meridian Explorer2 digital-to-analogue converter
Price: $449.95

+ Excellent performance; Separate line-level & headphone outputs; MQA compatibility
- No DSD support

Inputs: USB mini-B
Outputs: 3.5mm analogue line out, fixed 2V RMS; 3.5mm headphone jack with variable-level output, impedance 0.47 ohms
Power: via USB, nominal 5V at <500mA.
Type: asynchronous USB 2.0 HS
Dimensions (wdh): 102 x 32 x 18mm
Weight: 50g