Set up as a straight DAC/headphone amp, RME's ADI-2 DAC sounds fantastic and is very much on the neutral and clear, perhaps "clinical" side of things. This tonality is exactly what a professional audio engineer needs in order to make educated sound-shaping decisions. Its built-in headphone amp easily drove SPLs as loud as anyone would desire full-sized cans to be, with impedances ranging from 30-ish ohms to 600 ohms. It is no doubt a fantastic value, at about $1000, just for its straight digital-to-analog conversion and headphone amp quality, but there is much more to this thing.
For inputs, the ADI-2 DAC has USB 2/3, coaxial S/PDIF, and optical ADAT/ S/PDIF. Both balanced XLR and unbalanced RCA audio outputs are also on the rear panel. The front panel includes three multi-function pushbutton/rotary controls, four pushbuttons to access to menu-driven options and controls, a power on/off button, and two headphone jacks. The full-sized 1/4-inch TRS headphone output can operate in either high- or low-power modes, depending on the impedance of the headphones and the user's desired output level. The 1/8-inch jack, labeled "IEM" (in-ear monitor) operates in the low-power mode only, but will feed any type of headphone/IEM/earbud plugged into it. The color display screen, about 2 inches wide and 1.5 inches high, has many menu-selectable modes. There is a "blackout" option to have all front panel illumination shut off a few seconds after the last use of the controls, lighting up again the next time the controls are used.
A small remote-control handles power on/off, volume, input selection, mute, and four programmable "setup" keys (for instance, one could be programmed to deal with sound-shaping DSP and a reference volume level, one could be programmed with room-correction EQ for a set of studio monitors, one could be programmed for mono mode, etc.).
As far as the built-in DAC, it's capable of playing back PCM up to 768 kHz/32-bit and DSD up to DSD256 (11.2 MHz). It can handle DSD-direct playback, but with no built-in digital signal processing (DSP), including the all-digital volume control (some competing professional DACs use hybrid analog-digital volume control systems). It can also trans-code DSD to PCM, thus allowing access to the onboard DSP functionality. To be honest, I couldn't hear any difference between "straight" and trans-coded DSD when the EQ/spatial-image adjustments were flat and the volume level was matched to the -3.5 dBfs DSD-Direct level. For convenience's sake, I listened to most of my small collection of DSD files with on-the-fly PCM conversion.
Now let's get into the weeds with RME's 68-page user manual. This little black box comes packed with built-in DSP, allowing extensive tone-shaping and channel-balancing options. There are even front panel bass and treble controls. Suffice to say, there is a very rich menu-driven feature set to explore.
Page 16 of the manual spells out exactly what can be controlled by the built-in DSP: output volume level, bass/treble and loudness, a 5-band parametric equalizer, various phase and polarity switching, and a 5-mode crossfeed option for headphone listening (simulates listening to speakers in a room by feeding some signal from each side to the center, each mode feeds a different amount of signal with a different filter frequency). The built-in brain also controls the 30-band spectrum display and peak-level metering, 5 low-pass filter modes, and DSD to PCM conversion chores. I am old enough to remember pre-digital audio, and old enough to be amazed by this kind of functionality at this price point.
The downside to this kind of functionality is a complex menu-driven control system. To fully use an ADI-2 DAC, one must read the manual. But it's written in clear English, has an intuitive flow, and includes a detailed table of contents. Following the introduction and users' guide sections, there is an extensive technical reference with, for instance, graphs of each low-pass filter's impulse and frequency responses, a graph showing the loudness curves, and a diagram of the signal flow and levels.
You'll want to keep the manual around because there are so many sub-menus and options that it's going to be hard to remember how to do things unless you tweak the sound very regularly. Suffice to say, and not meant in a negative way, this is German engineering: very complete and complex, but logically designed and functioning exactly as specified. If you look at this way, this is a BMW-complex piece of gear priced Chevy-simple, so once again it's a tremendous value.
So where does a little black box that does so much signal processing as well as digital-to-analog conversion fit into a modern studio? Jeff Petersen, Product Specialist at RME's global distribution partner Synthax, told me that the ADI-2 DAC was developed as a follow-up to the ADI-2 Pro ADC/DAC/DSP box, which became popular with the audiophile headphone crowd. According to Petersen, while the ADI-2 DAC is aimed at home audio enthusiasts, "it's also a fantastic way to upgrade the monitoring path in a working studio, where the audio interface connects to the ADI-2 DAC via S/PDIF optical or coax."
Petersen also described a somewhat hidden feature of the ADI-2 DAC – it can grab a DSD stream from your hard drive. I've read some SACD players, I think an older OPPO model and perhaps a model of Sony PlayStation, can send out a data stream out of the S/PDIF connector. According to Petersen, "If you send DSD over PCM (DoP) to the S/PDIF input, the ADI-2 DAC can read the header to recognize this format and automatically reassemble the DSD bit stream inside and pass this DSD stream to software (such as Pyramix, which supports DSD) through USB." Now, this is pretty obscure, but it seems a simpler way to grab DSD to hard drive than some of the methods I've seen described, using a PlayStation and various open-source programs. But then again, Pyramix software ain't cheap.
The ADI-2 DAC makes a very nice monitoring front end in a mostly digital studio, and its powerful DSP EQ could be used to tame some of the problems of a typical project studio's acoustic environment. The phase-reverse/polarity-reverse, mono/stereo, and mid-side processing tools are useful in mixing and mastering. With enough power to drive any headphones out there, it's a good companion to your computer, laptop, iOS device, etc. Its abundance of features makes it useful in many professional settings and the price is aggressively inviting for the quality of DAC and headphone amp in this little black box.