When you think of pro-audio gear, you may be like me and think of those at the low end of scale that have a reputation for having a rather harsh sound-signature. Even the more expensive and capable DACs on Head-Fi from pro-audio manufacturers like Benchmark, Lavry and Apogee have a sound signature that could best be described as more "analytical". I discovered this buying an Apogee Duet on its reputation as a great source, but it didn't mate well with the K701s I had at the time. I have also owned a Lavry DA-10, which had a rather "nothing but the facts ma'am" signature. Though I suspect now it might have been considerably better had I used better than just the optical input.
Fast-forward to now and my interest was piqued in a particular piece of pro-audio digital gear that is primarily designed for recording and digital mixing but, according to a few people, was a great-sounding DAC, the Metric Halo ULN-2.
Unfortunately, the chance of it becoming popular is seriously limited by a couple of factors. First, unlike a normal DAC which you plug in and it just works, the ULN-2 must be programmed via a Firewire-connected computer before it will output audio. While it has a crapload of inputs and outputs (Firewire, S/PDIF, AES/EBU, mic pre-amps, fixed and adjustable analogue output as well as a headphone socket) it requires that a route for the audio through the device be set up.
When mine arrived, I went through all sorts of confusion trying to make head or tail of the software and, only AFTER realising that mine was a "legacy" version and I didn't need to touch the mixing console or anything like that did I finally get audio running through it.
As a pro-audio device, it is rather fascinating. Imagine you are given a mixing console + (if you pay for the software) a virtual blank slate in how single or multi-channel audio is routed or processed, the possibilities are infinite. Indeed, the difference between the cost of the hardware itself, and the hardware with all the software features is something like double, or triple. Since I was primarily interested in it as a DAC, I shan't be taking advantage of any of these features unfortunately.
The one feature I did use was the pre-configured states, switchable from the front panel, which I set up to allow me to change the output as I couldn't find a way to have multiple outputs running at the same time (possibly without paying for a $500 add-on card that would allow me to use the virtual mixing console). I rigged everything up so that I could switch between digitally passing through from the ULN-2 to my Reference 1 or just sending the signal via the analogue outputs.
Inside the ULN-2 is a AKM DAC. While not the top-of-the-line chip currently doing the rounds in everything from the up-coming Schiit Bifrost to the Esoteric K-01, it is still very capable. While a DAC isn't just its chip by any means, the AKM chips have a good reputation for being more "analogue-sounding". Being that one of the things that annoyed me about so many DACs that I used was instruments sounding like poor digital reproductions when they weren't on the recording, this is something I welcome. More recently, I'm leaning towards believing that this has more to do with the quality and design of the digital input circuitry and the accuracy of the clock crystals than the DA chip itself. This makes thoughts about the DA chip itself somewhat moot as a result.
One relevant point about the design of the ULN-2 is to be found in its name: ULN stands for Ultra Low Noise. At the very least, the premise that low noise = good sound quality is a great concept. Arguably that premise falls apart in equipment that has purely been designed to have a measurably low floor, but is otherwise poor-sounding for other reasons. The Benchmark DAC-1 is lauded by people who favour measurements for this reason, but from experience, I understand why it isn't popular with audiophiles, as, compared to a good, genuinely high-quality (and well-measuring) audiophile DAC, it gives the impression of being bright and harsh. The ULN-2, thankfully, delivers music in a manner that is neither bright nor harsh, but more akin to something that might be described as "audiophile", yet without any obvious coloration. It comes across very much as the kind of device a recording engineer and also audiophile could both have their cake and eat it with too.
I have been comparing the ULN-2 with my Audio-gd Reference 1 for a while now, both connected using Van Den Hul The Orchid cables from both DACs (via RCA to TRS or XLR adaptors from the ULN-2) to a Stax SRM-007t and SR-009s. Being able to switch on the fly between the DACs has been very handy.
I'm pretty sure if I had been tested on the spot, it would have been very hard to tell both DACs apart, but after spending some time comparing, even just through the RCA outputs, which have the lower signal-to-noise ratio, however, the Reference 1 has more apprent clarity, with instruments more precisely placed and the image less blurred. However, considering I picked the ULN-2 up for AU$700, considerably less than cost of the Reference 1 (or 7) I have no complaints. As a dedicated DAC, once configured and using custom-built cables that are TRS-terminated, it's a fantastic bargain.
It also has a headphone socket with volume control. However, when I tried this with LCD-2s and AD-2000s, I found it was quite unpleasant-sounding, which was a great pity, as the unit is small enough to be transportable and it only requires Firewire bus power. There is also the option of using it as a pre-amp, as there are both fixed and variable outputs. As for the inputs, I might try seeing if I can pick up any difference between my cables, since the noise floor is -130dB.
