This is surely a belated reply… but maybe better late than never?! Since this unit is still on the market in many places, I could as well post it.
The DAC/ADC combination identifiable on the UR44 PCB through visual inspection is as follows:
DAC: Wolfson Microelectronics WM8728
ADC: AKM AK5359VT
(This is for the original UR44 and not for UR44C).
I actually bothered to pull the datasheet for these converters. Looking at the characteristics, what becomes clear is that Steinberg has clearly picked a set of budget converters for this unit. (I am not making a statement about the sound quality, just the price range). In fact, so very `budget’ that some old interfaces from 2005 actually have significantly better converters, in terms of dynamic range, S/N, etc.
The way converter technology goes, the early 2000’s are the first time in history when 24-bit converters with a clean 120 dB dynamic range appeared. Circa 2005, AKM released sufficiently cheap DAC/ADCs that could be used in budget interfaces and offered, well, around 115 dB dynamic range, which, for the time, was quite good. Fast forward another 7-10 years and there comes Steinberg’s UR44 with something inexplicable like a 102 dB dynamic range and something like a 97 dB S/D+N ratio of the converters. While noise characteristics are usually not what I would first look at an ADC/DAC, I do however closely look at the specs of their digital filters, because that does affect the sound.
For reasons better discussed elsewhere, to work properly, an ADC must first filter out anything above 22.05k (or half of the sampling rate, such as 44.1kHz) before quantizing the analog waveform, otherwise you get an ugly-sounding phenomenon called aliasing. To reduce it, the digital filter must attenuate said frequencies enough to put them below the noise floor, while not affecting the audible range (20Hz-20kHz).The extent of such attenuation is critical. The attenuation of UR44 DAC filter is 60 dB, and of the ADC is 72 dB. Convertor filters used in other interfaces, such as AKM5385 and the like, get to around 100 dB, which is adequate, given that you want aliasing distortion to be below the noise floor.
So, while I give my kudos to Steinberg for the overall design of the unit, and I acknowledge that it works like a tank - no scratchy pots, noisy outputs, no poorly designed power supply causing noise, etc. - nonetheless I want to be at least somewhat heavily critical about the ADC/DAC choice. At least for the ADC, I affirm and testify that it is NOT linear at 44.1 kHz. This is not my subjective opinion, but rather the facts from its own datasheet, which says that at this sample rate, the anti-aliasing filter rolloff starts at 17.36 kHz, or 0.3975 of the sample rate, and reaches an attenuation of about 1.5 dB at 20kHz. Audibility-wise, this is not a big deal; a condenser mic would easily have a bigger variation from unit to unit. however, I fail to comprehend why this was necessary in the first place when, years before, considerably more linear ADCs were being available and used in competing interfaces. Anybody who has doubts can google the datasheet of this ADC and see for themselves. the DAC, in terms of linearity, fares better, although to my ears sounds quite harsh (and having seen the digital filter characteristics, I understand why).
Hence, it comes as no surprise that the updated UR44C comes with new converters. The way business acumen goes, I would not be surprised if I received a very polite invitation to spend another 350$ for the interface I already own, so that I can benefit from the better convertor quality of an yet-unspecified chip in the box, supposedly doing 32-bits.However, it still doesn’t explain why Steinberg/Yamaha, after having done a fully decent job on the rest of the box, all of a sudden decided to drop the ball on the conversion. An entire half a decade after the production of truly great converters such as AKM 5385, I can see absolutely no valid cost reason why not to put them in this box, given the speed at which anything digital depreciates in our age. So, of course I am disappointed. Not by the sound quality as much as by the design philosophy: it is okay for the customer to be had as long as they don’t know it. Well, so much for this brand I guess.