Reviewer: Srajan Ebaen
Financial interests: click here
Sources: 27" iMac with 5K Retina display, 4GHz quad-core engine with 4.4GHz turbo boost, 3TB Fusion Drive, 16GB SDRAM, OSX Yosemite, PureMusic 3.01, Tidal & Qobuz lossless streaming, COS Engineering D1 & H1, AURALiC Vega, Aqua Hifi Formula, Fore Audio DAISy 1, Apple iPod Classic 160GB (AIFF), Astell& Kern AK100 modified by Red Wine Audio, Cambridge Audio iD100, Pro-Ject Dock Box S Digital, Pure i20, Questyle QP1R
Preamplifier: Nagra Jazz, Vinnie Rossi LIO (AVC module), COS Engineering D1, Wyred4Sound STP-SE Stage II
Power & integrated amplifiers: Pass Labs XA30.8; FirstWatt SIT1, F5, F6, F7; Crayon Audio CFA-1.2; Goldmund Job 225; Aura Note Premier; Wyred4Sound mINT; Nord Acoustics One SE Up NC500MB monos; LinnenberG Allego monos
Loudspeakers: Audio Physic Codex; Albedo Audio Aptica; EnigmAcoustics Mythology 1; Boenicke Audio W5se; Zu Audio Druid V & Submission; German Physiks HRS-120; Eversound Essence
Cables: Complete loom of Zu Event; Ocellia OCC loom: KingRex uArt, Zu and LightHarmonic LightSpeed double-header USB cables; Tombo Trøn S/PDIF; van den Hul AES/EBU; AudioQuest Diamond glass-fibre Toslink; Black Cat Cable redlevel Lupo
Power delivery: Vibex Granada/Alhambra and Three 11R, Titan Audio Eros cords between wall and conditioners and on the amp/s
Equipment rack: Artesania Audio Exoteryc double-wide 3-tier with optional glass shelves, Exoteryc Krion and glass amp stands [on loan]
Sundry accessories: Acoustic System resonators
Room: 4x6m open to 4x8m kitchen and 5x8m living room with tall gabled beam ceiling
Review component retail: n/a

DIY vs. commercial. With FirstWatt, that line blurs. By eventually giving away schematics for most of his FirstWatt amplifiers to the do-it-yourself community, Nelson Pass encourages solder slingers to get rolling. This routinely prompts group-buy PCB runs. One more experienced forum participant will draw up the necessary files to get a circuit board printed. Others experiment with sundry parts substitutions or circuit variations. Often Nelson injects a suggestion. And so each model launch at FirstWatt becomes a DIYaudio party. Each crumb of intel Nelson spills prior to the eventual full reveal prompts an immediate response. It guesses at further circuit details and simulations. This invariably leads to critiques by other members to sort out errors or improve things. Once soldering starts, builders share their feedback and the community grows its library of ideas and projects. In such open-source spirit, each formal FirstWatt production model becomes but the first paragraph in a new chapter of this good book. The Friends of Pass write out the rest with many takes on the theme like J.S. Bach did with his famous Goldberg Variations.

Then one rainy December day, N.P. Bach rang my bell. Had I heard a preamp with a Korg Nutube in it? I sadly had not. But with Google in Dublin being everyone's favourite uncle, I woke up. Shortly afterwards I had at least read up on it. Now I understood that a Korg Nutube not keyboard looks like a larger 17-legged opamp but is actually a device with an anode grid filament that operates like a triode and exhibits actual twin-triode curves. Yet it is built with vacuum fluorescent display aka VFD tech from Noritake Itron Corp. It can run off just 5 volts to produce 8dB gain whilst consuming a mere 12mW/ch. A 30V supply coaxes out 17dB gain with an S/NR of 110dB. A Nutube runs far cooler than traditional bottles, is a lot smaller, promises a life expectancy of 30'000 hours of continuous use and may be powered by a battery. And yes, it is a direct-heated triode device. How had I missed that? Time to crawl out from under my mossy rock. A new day for this leprechaun?

As Nelson put it, "serious triode effect. I'm not making such a preamp commercially but DIYers are having a good time with it. As part of a larger talk, I did a short bit about it at Burning Amp Fest this year. I might make a PCB on available and I am building up ten FirstWatt B1 units which uses up all the Nutube parts I've been given. No plans to manufacture with it but I wouldn't be at all surprised if some company didn't launch product." Nelson found Noritake's application notes for a circuit "very suitable and I made only small adjustments… I also experimented with the bias settings, noting the quantity of 2nd harmonic distortion. Like other triodes, it can be adjusted for positive or negative-phase second harmonic; or at the point where 2nd harmonic has been nulled out. After listening tests I settled on a value which gives a negative-phase second harmonic at 1.5% with 1V output… to emulate the 2nd harmonic of the SIT-1 and 2 on my speakers and amp…"

Here is his full BAF talk excerpt with distortion graphs; and here are Noritake's application notes. Attentive reader have already grasped subtext. Nelson used listening not bench tests to arrive at the final tuning. And there he preferred a very specific THD shape (negative-phase 2nd harmonic) at 1.5%/1V which those who believe in 0.000x% distortion would find unseemly high. But then, they rarely believe their own ears. They need lab tests for proof that something actually sounds good. And didn't this particular experiment call for the serious triode effect rather than nulling it out? It's all horses for courses and having fun. And then there's this. "The Korg part has been around for a couple of years. I believe it was designed to give Korg the option of triode sound for their synthesizers." Those who've always thought of synthesizer sound as – well, synthetic; they might revise their thinking when certain Korgs no longer use DSP-based tube emulators but actual direct-heated Nutube triodes.

When I submitted the above to John Darko as the 52nd installment of my KIH column, our man in Berlin remembered. In 2016, he'd come across a Korg headphone amp with Nutube at the Tokyo Fujiya Avic show. Overwhelmed by product announcements and speaking no Japanese, he'd simply not fully understood their explanations to miss the Nutube's potential significance. Digesting this narrative, the reader has grasped another thing. Today's B1 with Nutube inside sticker isn't a formal FirstWatt product. It's why the equipment listening paragraph shows no retail price. As such, this product doesn't really exist. But I thought that the opportunity of hearing and commenting on this part in a Nelson Pass-built Jfet-buffered passive volume control with two inputs and zero feedback overrode more mundane concerns over buying one. After all, you might just feel compelled to go DIY; or petition your favourite designer to launch a Nutube model. And yes, that does break with our usual rules. But the part itself would seem to as well.

One day after posting the above, reader KS had this: "Took in a hybrid Nutube/solid-state headphone amp last month and found the sound I’ve been shopping for - a little bit of candlelight glow and rosy cheeks without the need to fully commit to the stereotypical flesh, dynamic exaggeration or wetness of the breed. Not usually one to pursue tubes, the Stax hybrid amps having too much flavouring for my taste, I jumped on this one after a review described it as graceful transistor sound with subtle overtones. The machine in question is an Apex Sangaku. It's been commercially available for most of this year though seems not to have penetrated the European news cycle. The man behind the brand, Peter Millett, has put in some leg work to introduce the Nutube to US audiences. Not having the means to tolerate hot glass or beastly amplifiers, I’d say job done on introducing the formerly niche DHT sound into my more 'modern' demographic."