Abbingdon Music Research DP-777 D/A processor
Computer audio is more than just a pleasant distraction. For the jaded reviewer, USB digital converters and the like are an escape from that humdrum, if only because they bring with them so many variables: myriad combinations of different platforms, storage devices, operating systems, device drivers, media players, codecs, word lengths, sampling rates, connection protocols, and more. Challenging though they may be, computer-audio products are a tonic for reviewers inclined toward apathy.
A new digital-to-analog converter from Abbingdon Music Research, the DP-777 ($4995), serves as a good exampledoubly so, one might say. The DP-777 is literally two digital converters in one, incorporating separate processor chips to handle high-resolution files and "Red Book" CDs (and music files derived therefrom). The DP-777, being a clever thing, automatically chooses between the two, depending on the nature of the incoming datastream.
From there, more variables abound. The AMR converter offers the user choices of: five digital filters (two "Red Book," three hi-rez); six sampling rates; two jitter-reduction settings; word lengths up to 32 bits; and two modes of overall operation, to automate (or not) the preceding. There are five types of digital input jacks, two types of analog output jacks, and four levels of panel-light brightness. It also has analog inputs, to allow it to be used as a conventional preamplifier, and there is an optional volume control.
My wife and I have a standing joke: Whenever we find ourselves in a retail store with an overwhelming number of choices10 varieties of Colgate toothpaste, six formulations of Advil, Gap jeans that hug the waist and thighs in a dozen different wayswe complain that life would have been easier in the old Soviet Union. We're kidding, of course.
The AMR DP-777 is built into a generously sized aluminum chassis constructed from various extrusions, interior braces, and well-finished exterior panels. A digital display and five touch-sensitive control buttons take up the front panelas one would expect, far more control options are available on the included remote handsetand the converter's top surface has two rows of Perspex windows that offer a glimpse of the various capacitors, oscillators, regulators, transformers, and tubes within.
Yes, tubes: Each channel of the DP-777 uses a Russian 6H1n-EV dual-triode as an output amplifier, one half configured for gain, the other half as a buffer stage. More significant, the DP-777 uses a Russian 6H11P dual-triode as an S/PDIF input amplifier on two of its digital inputs. As AMR sees it, S/PDIF is essentially an analog transmission protocol, traditional implementations of which use a positive-feedback mechanism that adds considerable distortion. Their tube-based signal amplifier, they say, which operates into a range of several hundred megahertz, is the best way to maintain the clean squarewave of the digital input.
This being 2012 and all, one might expect the buyer of an outboard D/A converter to have more interest in computer-audio sources than disc transports and the like; the engineering effort that AMR has brought to bear on the DP-777's USB input is thus unsurprising. And that effort has been directed by the notion that optimum playback quality is possible only when a recording's original sampling rate and word length have been carefully preserved and maintained. I can't help seeing a parallel between that point of view and the prevailing attitude among hardcore mono enthusiasts such as myself, who believe in matching stylus dimensions and playback EQ curves to different vinyl and shellac discs. Then again, given that AMR's chief designer is the well-known DIY guru and analog maven Thorsten Loesch, I believe I can be forgiven.
The heart of the DP-777 is its Gemini Digital Engine board, which is home to the twin processor chips mentioned earlier: a Wolfson 32-bit processor optimized for high-resolution files, and a Philips UDA1305 for more plebeian inputs. The proprietary AMR software that detects the incoming signal and selects the appropriate D/A processor resides in another chip on the same board (the one labeled V2.5, visible in the interior photo).
The DP-777 also has a Zero-Jitter Mode, which can be left on at all times or manually selected from the remote handset. AMR says that the Zero-Jitter Mode system determines the long-term average of the incoming signal clock, then uses a high-capacity memory module to dynamically match it to an accuracy of 0.001Hz. Separate from this, the AMR also boasts an asynchronous USB input, which keeps the converter from relying on the clock in the datastream supplied by the computer.
Construction and parts are all first rate. Tubes are surrounded by copper-foil shields, electrically grounded and physically damped with polymer rings. Transformers and even capacitorsthe latter including some proprietary silver-mica film typesare custom-made for AMR. Layouts are clean and sensible, cooling is provided for, mass is sufficient to the task and not a gram more. Even the rubber feet, which make intentionally limited but stable contact with the surface below, are well thought out and, apparently, effective.
Installation and setup
The AMR DP-777 was no more difficult to install than any other USB DAC I've triedby which token, one can also assume that it was easier to work with than even the least challenging wireless or Ethernet processor I've tried. On connecting the DP-777 to one of the USB ports on my Apple iMac (running OS X 10.6.7) and opening the System Preferences and Sound windows, I was presented with a device selection labeled "XMOS Processor"not quite "AMR DP-777," but neither was it anything I'd seen on that list before. I clicked on it, and that did the trick. (I'm told that a dedicated device driver, which can be downloaded from the AMR website without charge, is required for use with Windows 7.)