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By the end of March 2014, Nagra's first production run was readying. We'd settled into our new digs above Chardonne, with Matthieu Latour visiting to "sign off". The time had come to reenter the assignment. This (not exactly but close enough) also coincided with PS Audio's announcement for their new DirectStream DAC. That 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 many 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. He simply clones one-bit conversion with its associated filtering requirements. Subsequently code writer Ted Smith licensed his direct-stream code to PS Audio to follow suit. When I met him in July 2014, ex Sony engineer Alex Peychev of APL Hifi too claimed to have his own proprietary PCM-to-DSD conversion tech which he exploits in a tube-buffered interstage-transformer model called the DSD-M DAC (right). 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 off-the-shelf tech. Whilst Amanero do sell 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 Latour explained for overall featurization gestation, once their resident PCB layout wizard had given the green light that there indeed was sufficient internal real estate to incorporate the high-quality headphone module on Nagra's wish list, 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 with the HD DAC would thus depend on the amplifier and standard listening levels both for its Ω impact and overall gain needs.

To wrap introductions, Nagra's documented expertise in the analog domain—their founding project was a tape recorder that made history with journalists and location producers in the movie industry—promised the next chapter on the digital conversion tech which has been widely previewed by various EMM Labs, Meitner and Playback Design converters. 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 how our current music is formatted. As I've written elsewhere, my 2-year+ focus on USB-enabled DACs had 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 the Aqua Hifi La Voce S2—would likely mandate about 4 x their coin. With Nagra's HD DAC I had my most promising example to test that hypothesis. Not only is a quad-box dCS statement far too rich for my blood, it's too complex and complicated a proposition. The compact full-featured Nagra would be my go-for-broke reach for the stars. Well more than a year after I'd first heard a prototype in Nagra's Lausanne facility, a production review loaner finally became available by August 2014. Matthieu proposed to bring both the stock power supplies and the optional quad supply for comparison purposes. I agreed. I wanted to run the HD DAC amp-direct; through my Nagra Jazz preamp; and each with and without the MPS. The final wrinkle would be the MPS's battery output to determine where it might reap the most audible rewards - on the Jazz; or as the HD DAC's analog or digital supply. I also had Audeze LCD-XC, HifiMan HE-560 and Sennheiser HD800 headphones on hand to take Nagra's 6.3mm jack for a spin.