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NOS the third: In tube lingo NOS refers to New Old Stock. Think vintage tubes which are no longer made but were never used. A virgin bottle from the 1960s is thus both new and old. In digital lingo NOS came to mean NonOverSampling since Ryohai Kusunoki's original 1996-1997 three-part paper published in MJ Magazine. It described a very simple DAC without digital filter and oversampling. This led to Junji Kimura's 47Lab DAC as the first commercial sample of the breed. Eventually Kazutoshi Yamada of Zanden would apply the same principle at the very luxury end of the market before Thorsten Loesch followed suit with Abbingdon Music Research. Like chip amps or gain clones, the concept became popular in DIY.

In the Metrum Acoustics converter we don't have to stretch much to make NOS also mean No Ordinary Stuff. This implies that it does away with some of the usual building blocks. There are other chips like the Crystal CS4398 which include built-in I/V conversion. Most DAC circuits however involve a separate stage which can be passive (resistor in the Yamamoto YDA-01) or active via either discrete parts (Burson HA160D, Weiss Medea+), tubes (Loit Passeri) or IC opamps (the vast majority).


But what consumer DA converter does away with an output stage/current buffer altogether, either by opamp or with discrete transistors or tubes? The Octave's chips are the output and gain devices. Can it get any simpler? At first glance this concept would seem ideal for a DIY project. Cees Ruijtenberg's low direct pricing and off-the-shelf casings are certainly related. Where that train jumps tracks are two items. First is that 6-layer circuit board. It's wildly beyond DIY ambitions. It's even beyond the ken of most consumer audio designers. The reason why Nagra Audio can design such three-dimensional PCBs is their industrial work for the mothership of the Kudelski Group. The reason Cees can is his prior work for Acelec Engineering.

The second item that should elude DIYers is what Cees calls glue logic. That's engineering speak for custom control firmware which can involve TTL transistor-transistor logic ICs or field-programmable gate arrays aka FPGAs whereby a number of off-the-shelf ICs get interfaced. Here glue logic adapts ultra high-frequency i.e. super fast chips for audio use. While on one hand the Octave does represent a very stripped down essentialized or simple circuit like a Nelson Pass Zen amp, the implementation includes hi-tech ingredients. Cees' external dual-tranny power supply, extensive filtration and regulation show further familiarity with the pro/laboratory sectors where extreme accuracy is key. All this makes the Octave DAC a distinctive combination of the very basic and the quite sophisticated. If the sound measures up, cynics would predict future releases that stick a very similar circuit into a bling enclosure with massively paralleled socketry, overkill power supply and a gold-plated umbilical to charge an arm and a leg for claimed "breakthrough technology". That'd be high-end audio 101. Or 0815.

It likely won't be Cees however. He seems to lack the raw nerve. He's honest that for his industry his DAC is the norm. Germans call that nullachtfuffzehn. Numerically that's 0815. Historically it was a WWI machine gun on which the army had its soldiers train incessantly to utter boredom. The colloquial meaning has become "nothing special". But parts repurposed from one application to another can make for very special results. Nelson Pass' new silicon carbide power JFets called static induction transistors borrow from radar parts technology. While Nelson is still working out how to best exploit the virtues of his new ultra-fast parts for audio, it's predictable that his SIT amplifier circuitry will again be very simple. That's his MO. The Dutch Octave DAC strikes me as a digital FirstWatt.


This is how Cees put it when I vetted the above for facts: "I think you see it right. My background is actually marine electronics like radar and sonar which I still do. Later I designed ultrasonic equipment for industrial use and besides that I also designed audio systems. This meant I had the tools and knowledge for how to combine things from various disciplines. The result is the Mini Dac. I always try to collect good things from different worlds as long as the audible result is a fact. I can thus use complex multi-layer boards and know how to calculate the effects before the design is actually made. In this special case—and as you mentioned already— there is no way to use these components for DIY audio as long as the know-how and tools are lacking to make a reliable and silent converter. But to be honest, there was a huge stroke of luck involved finding this part. Otherwise I'd still be looking."
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