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By the end of March 2014, Nagra's first production run was in full swing. And, we'd settled into our new digs above Chardonne, with Matthieu Latour visiting to "sign off". Time to reenter this story. This (not exactly but close enough) also coincided with PS Audio's announcement for their new DirectStream DAC. It arrived as a salacious press release claiming a truly revolutionary breakthrough which finally fixed what had been wrong with PCM all along. "The problem is the PCM decoding process itself. Whether a classic ladder DAC or more modern multi-bit Sigma-Delta type, PCM processors universally mask some of the subtle cues in music. No amount of upgrading, expenditure, tweaking or improvement can fix this fundamentally flawed system. In order to extract everything hidden in PCM recordings, a completely new processing method is needed. DirectStream converts all digital inputs including PCM to pure 1-bit DSD in an elegantly simple path. In the process the PCM feed becomes more linear and less edgy. Never-before-heard musical details are released from all digital audio recordings. Billions of CDs and high-resolution downloads worldwide will gain new life, be saved from obsolescence and recycling bins or landfills." Apparently PS Audio hadn't gotten the memo. Meitner and Playback Designs had done exactly this for years already if without such boisterous claims. Marketing speak aside, PS Audio's following illustration is perfectly relevant to the subject.
The big deal of this simpler circuit approach is the necessity for endless lines of custom code writing. Since true one-bit converter chips have gone the way of the dodo, one must program an FPGA to operate as one. Chord in the UK support the PCM format but have used custom FPGA replacement algorithms for commercial chips for years. Ditto Andreas Koch. As a major authority on DSD he simply clones one-bit conversion with its associated filtering. Now code writer Ted Smith had licensed his direct-stream code to PS Audio to follow suit. As we already know, Nagra went with Koch's code.

After researching all commercial USB transceiver options, Nagra then picked Italian Amanero Technologies for their 32bit/384kHz PCM and I²S-output DSD128 solution (up to DSD512 on Windows). But the HD DAC wouldn't be Nagra if it ran on off-the-shelf tech. Whilst Amanero offers turn-key modules just like M2Tech, XMOS & Co do., they also offer their basic chip around which more resourceful makers can build their own modules with the exact parts they want*. That's what's in the HD DAC. Filter/sampling options are accessible in the software menu.

* Whilst Nagra hold those cards close to their chest, Metrum Acoustics' new USB interface at left gives some insight on how Amanero can be custom implemented. "Until now our Octave MKII and Hex used an OEM USB board from M2tech. With Mavericks OS their driver became outdated. Though M2tech have undertaken development of a new driver, several months without news became too long a wait. We now use Amanero with additional glue logic to get all necessary data for our DACs. We also adapted the new board's form to become a drop-in replacement. Unlike the stock two-layer designs from  M2tech and Amanero, we use four levels. Two layers are assigned to data handling. This creates lower induction from shorter traces and less radiation. A third layer is used as ground plane, the fourth is dedicated to power. A power breakout can implement passive power filtering for the Octave MKII or a separate power source for the Hex. In addition very high-quality clocks get jitter to the lowest possible level."

As Matthieu explained for overall featurization gestation, once their resident PCB layout wizard had given his green light that there indeed was sufficient internal real estate to incorporate the high-quality headphone module, a volume control for it became an obvious necessity. With that aboard, looping it via a fully software-controlled relay-switched path also into the main outputs so it could be categorically bypassed when a preamp was used became a logical final step. (This too is set with the software controller knob. Likewise for max output voltage and more.) However there was no room to add a buffer for the main output. Given overall concept, this would have meant another tube. Hence the physical setting of the analog pot determines output impedance which changes with position. Whether an external preamp like my Nagra Jazz would be preferable to going amp-direct would thus depend on the amplifier and standard listening levels both for its Ω impact and overall gain needs.

To wrap our intro, Nagra's documented expertise in the analog domain—remember how their founding project was a tape recorder that made history with journalists and location producers in the movie industry—promises the next chapter on digital conversion tech as it's been previewed by Meitner and Playback Design. Though PS Audio's DirectStream DAC campaign meant to promote just itself, it inadvertently also included Nagra's HD DAC by exploiting the same concept for optimized playback of PCM files as the vast majority of our current music. As I've written elsewhere, my 2-year+ focus on USB-enabled DACs has centred on the €3-5K range with occasional stretches to more as with Meitner's $7'500 deck. I'd posited that to demonstrably eclipse the best in that field—my picks include AURALiC's Vega, Metrum's Hex, the Resonessence Invicta and Aqua Hifi La Voce S2—would likely mandate about 4 x their coin. With Nagra's HD DAC I'd finally have my most promising example to test that hypothesis. Not only is a quad-box dCS statement far too rich for my blood, it's also too complex and complicated a proposition. The compact full-featured Nagra would be my go-for-broke reach for the stars.
... to be continued in probably late July if a loaner manifests by then...
Audio Technology Switzerland website