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August 8 2011. Many hours later. The cheat. "Okay, you've had your initial impression so here's the poop. 10 watts/ch into 8 ohms. 47Kohm input impedance, damping factor of about 3, bandwidth is 1Hz to 300KHz. Gain is 19dB. The distortion is 2nd harmonic at about 0.5% at 1 watt and a mix of 2nd and 3rd at about 4% at 10 watts. The amplifier circuit has a single stage consisting of the SIT biased by a Jfet set up as a mu follower. There is no feedback loop and the SIT has no degeneration. It draws slightly less than 200 watts. I hope you are enjoying it!" - np


200 watts at idle for just 10 power watts dissipated by merely one output device per side the size of a postage stamp? That explained toasty and illustrates just how hot these transistors run. Poor of shove but rich in bias just like the working class. And not only was the amp single-ended, it was single stage. Presumably in this introductory circuit pared down to maximal simplicity, the SIT shows off its true colors. More experience and R&D might be required to transpose what has now been revealed as the part's potential to the more complex circuits that are required to increase the power rating. For DIYers hoping to stage an early SIT-in, Nelson had a simple answer. "It costs an enormous amount of money to get a run of these made. My current inventory is only large enough to float the R&D and the FW initial production. That will have to be a success before I can afford the next run after that. If you can raise $300.000 for a group buy, it can happen in 6 months or so, otherwise DIYers will be waiting a while."


SITcom(mentary). My impressions of the first half day went to Nelson as an informal email with zero editorial finesse. What follows is the finessed version. Partners in musical crime were the R/2R Metrum Acoustics NOS Mini DAC Octave fed from my usual Pure Music 1.8 iMac via April Music's U3. Preamp was my Esoteric C-03 set to initially 0dB of voltage gain, subsequently also 12dB and 24dB. Speakers were the brilliant Aries Cerat Gladius 3-ways on loan with their Raal ribbon tweeters, modified Fostex Alnico mids and Acoustic Elegance 12-inch woofers in sealed enclosures with outboard crossovers. Efficiency is a true 91dB, impedance a very benignly distributed 5 ohms without grave phase angles. Even zero preamp gain meant running well below unity gain off the Dutch DAC's standard 2V output.


In keeping with personal tradition—start out with a very simple acoustic instrumental of exceptional recording quality—21 Strings by the Al-Andalus Ensemble of Tarik and Julia Banzi on oud and flamenco guitar respectively here augmented by Charlie Bisharat on violin (Strunz & Farah aficionados will recognize him) kicked things off. The very first reaction was of great relaxation faintly reminiscent of the F4. This quickly found itself forgotten and overwritten by hellaciously fast once Tarik started leaning into his oud to produce some very gnarly transients full of piss and vinegar. Those attacks were plainly wirier, steeper and rather closer than usual to the real thing up close and personal. The overall sensation was extremely lit up, direct, energetic and raw.


The effect was very similar to my friend Dan's system where a superior VT-52 SET like Yamamoto's A-010 'humbucks' a Burson Audio 160 amp in current-buffer mode to drive a quality 100dB widebander like the Voxativ Ampeggio (meaning the power triodes operate in their sweet spot well below getting heavy with THD and additionally see the fixed load of the follower amp). Except the SIT amp was even faster and had lower noise for superior micro² retrieval. Soundstage precision was massive and minute but those are aspects I usually don't pay much initial attention to.


From Al Andalus I veered to Renaud Garcia Fons' Mediterranées with a track of solo 5-string upright surrounded by multi-tracked extremely subdued ostinato tremolo. The latter is so low in level as to approach more subliminal dither whose primary purpose is to create audible space. It makes the solo exploits stage center very contextual. Rather than nude, the giant upright is embedded in prickling space. That's faintly similar to the counterpoint of classical Indian music's tambura accompaniment. Even when Garcia Fons' spiccato bow work takes a breather, it's not against emptiness but vibrantly tacit space. The degree of the latter was ultra keen. Visibility of all the truly tiny fluttering stuff that occurs around the center action was notched up a few clicks.


Curiously the amp didn't really sound lean despite all its very obvious speed. On the Gladius the F5 makes grippier bass (it also exceeds the J2 in that regard). Perhaps that massive sealed 12-inch woofer prefers a bit more raw shove or lower output impedance. Adding gain (12dB, then  24dB in the preamp) usually slows things down. It builds out tone richness, cuddles up the bass and gets a bit fuzzier. With the SIT amp this general tendency remained but was much subdued in magnitude. With Nelson's specifications above it would also seem that whilst the amp runs below or around 1 watt because the preamp handles main gain, the 2nd harmonic becomes dominant to induce slightly greater sweetness. With the Esoteric in hi-gain mode, the sound did get a tad sweeter but not thicker, slower or fuzzier as is usually the case.


The amp also was microdynamically very alive perhaps as a result of its overall speed. This was perfect for maintaining interest at low levels. It occasionally also was a bit shocking at regular room volumes. In short, contrast ratio was unusually high. This operated dynamically as small tone bursts; texturally as even tiny percussive events really sticking out; tonally as very distinctive simultaneity of string and wood; and in the subjective resolution domain where things seemed more raw, unplugged and stripped bare of subtle cotton wadding. It felt like unpeeling a shiny smooth hard object from protective fluff and transparent cling wrap.


All this actually tracked what a really superior no-feedback triode amp like Colotube's 300B monos can sound like if it is mated to the rare speaker that's truly and fully copasetic. There simply aren't many such uncompromised amps. Perfect matches with speakers are even rarer. As a result most people experience SET amps in more or less conformity with their stereotypes well below potential. While the Gladius' sensitivity and bass loading arguably were far less ideal than the Voxativ Ampeggio, the SIT amp's transistors still managed to secure that type of sound - albeit without the usual remnants of minor warmth I hear from triodes.


My first impressions—and in that sequence— thus were of speed, directness, resolution and tone. "My first impressions were similar but the thing that struck me most was the staging. I found myself able to drill down to the details of a single instrument in an ensemble almost to the exclusion of all else, which I found very unusual. These things turned out to be quite sensitive to all sorts of choices and adjustments. By sheer luck I ran into something nearly optimal rather early. It's easy to construct essentially the same circuit and have it come out more ordinary. Because these are subjective qualities it takes much longer to resolve the issues. I didn't dare touch the first amplifier that had accomplished this but started out with 'blueprinted' versions that varied things in isolation. My other piece of luck is to have partners with very sharp hearing. This keeps my own biases in line. They blindly vetted the permutations. 'This one is great, that one not so much.'


"The bottom line is that working with these parts to elicit the best subjective result is quite different from other transistors and it has taken nearly a year to get the hang of doing it with a single stage. Considerable work remains to adapt what we're learning to more complex and powerful amplifiers but we have already made some progress which will show up in future PL product." - np


The above captured the essentials of my first audition. The next page sorts out a few specifics relative to the F5, J2 and a surprise.