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[This article is based on a 04-1996 Stereophile review]
There is an almost amusing casualness that pervades the entire East-Coast Audio Note experience. Well before the subject arose of reviewing one of the Audio Note amplifiers, I had driven out to visit AN USA’s Herb Reichert in his converted firehouse on Staten Island. “Sure, sit anywhere you want! Naww, there’s no listening position or sweet spot, just make yourself comfortable anywhere.” Anywhere turned out to be a low couch against a brick wall, with the AN speakers off in the distance hard up against the facing wall.
I nearly swallowed my tongue as he cleared the stylus of the AN cartridge with his finger and literally dropped the arm onto the LP without having muted the preamp. While the Ongaku amplifier (which had survived) sounded exquisite in the midrange, I wasn’t impressed by its extension top or bottom. I also felt that any sense of imaging was entirely missing.
When I queried Herb about it; he launched into a polemic regarding the importance of the overall musical gestalt which, in his opinion, had nothing whatever to do with imaging. In fact, he was quite disdainful of the entire concept of “looking” at the soundstage.
We brought Herb back to our loft and played our system for him. I think we had the Symphonic Line Kraft 400 monoblocks hooked to the Avalon Ascents at the time, and that combination, apart from anything else, is a champion at imaging. Curiously, Herb left in obvious agitation. I received a startling fax from him soon after which asserted, among other things, that “imaging” in audio reproduction is crude, carnival-like, and distracting! Herb is so refreshingly... direct.
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Single-ended, class-A, dual-mono power amplifier using directly heated triodes. Tube complement input/voltage amplifier, 2x 6072/12AY7: driver stage, 2x 5687; output stage. 4x 3008 in parallel; rectification,2x 5U4GB or CV378. Internal wiring: all Audio Note AN-SPX Silver. Power-supply capacitors: Cerafine and Black Gate. Rated power output 17Wpc (12.3dBWD. Input impedance: 100k ohms. Output impedance: output transformer secondary wired for 8 ohms.
Weight: 90 lbs.
Dimensions: 22”W by 9"H by 13.5"D.
Approximate number of dealers: 12. Price: $52,600. Warranty: lifetime on parts, labor, and tubes. US distributor: Audio Note USA, 60 Hannah Street. Staten Island, NY 10301 Tel: (212) 304-8064. Fax: (212) 304-8064.
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“IT’S AN IMAGE THING...”
Herb soon recovered from his wounds, and we began an interesting dialog regarding the entire issue. He told me he’d run around listening to various systems, investigating “imaging,” and felt somewhat less adamant about its lowness in the firmament of high-end audio reproduction. I, for my part, mused upon the entire matter after a seminal Scotch-soaked evening at the Village Vanguard with Jazz O’Holic club denizen Dan Billet.
He and I sat dead center at the first table in front of the stage and enjoyed an evening of totally terrific jazz with the Jimmy Heath Quartet. (Jimmy’s brother is the legendary Percy Heath of Modern Jazz Quartet fame.) It was a family affair: Jimmy’s brother Albert “Tootie” Heath was on drums, as AbFab and soulful as his brother. Ben Brown did a fine job on bass, with Tony Purrone workmanlike and somewhat self-conscious on guitar. Sitting right in front of the bell of Jimmy’s sax (minimally miked), I closed my eyes and considered what I was hearing.
The first word that popped into my mind was “Bloom” with a capital B. The live imaging was not as precise as I’d previously tuned for in my playback system. I sensed that while the transients were still of utmost importance in defining the acoustic envelope (and imaging in the High-End sense), the bloom that followed not only commingled with the transient but also took on a delicious luminosity of its own, the decay gently completing the musical picture. With my eyes closed, the image was slightly more diffuse than the sharply defined presentation I’d enjoyed and endorsed when listening to push-pull amps on the Avalons.
THE MORE THINGS CHANGE…
In any case, much has changed since these seminal events took place. Herb has acquired a new business associate, Mike Trei (Audio Note USA), and they’ve undertaken importation of the German Avantgarde horn-speaker line, which I am told image quite well, in fact. And, of course, Kathleen and I have become quite experienced with single-ended triode amps, driving the Jadis Eurythmie II hybrid horn speakers.
However, while much has changed, much has stayed the same. Mike and Herb lugged the Kassai over to our place, but when I asked about documentation or a manual, Herb told me . . . there wasn’t any. In fact, here’s all I could find out about the amp, from a fax sent to me by
Herb:
“Okay, here goes... all that I know about the Kassai. It’s a single-ended, class-A, directly heated-triode, dual- mono power amplifier. It’s designed and built by Hiroyasu Kondo in Japan. The Kassai is two separate parallel 300B mono amplifiers on one copper chassis. Power output is 17 watts per channel. Weight is 90 lbs. Each amplifier has its own power transformer and power sup ply choke manufactured by Tango in Japan specifically for this amp. It employs 4N-pure silver primary wire (drawn through custom diamond dies by Audio Note) covered with seven coats of a custom polyurethane. The transformer core is super-Permalloy and the secondary winding is 6N-pure copper wire.
“Tube complement (each amp): input/voltage amplifier NOS (New Old Stock) 6072/12AY7, the driver stage NOS 5687. The output stage employs a paralleled pair of 300Bs. The rectifier tube is either a NOS 5U4GB or a Russian CV378. Input impedance is look ohms. The output-transformer secondary is wired for 8 ohms. Internal wiring is all Audio Note AN-SPX Silver and power-supply capacitors are Cerafine and Black Gate. Warranty is Lifetime: parts, labor, and tubes.”
The amp, or should I say amps (de spite the single, captive, silver-wired power cord with its surprisingly cheap- looking fitting), is quite handsome in an industrial-chic kind of way. The copper top cover is warm and attractive-looking, and counterpoints the glowing tubes and black output transformers at left and right rear. The power transformers, arrayed one in front of the other at the center rear of the amp, are liveried in a curiously dull shade of battleship gray. Japanese understatement, one can only presume. There are two banks of three capacitors each between the output and power transformers, one on each side, and a pair of volume controls up front, between and forward of the input and driver tubes, to correct any small imbalances in the source or tubes.
Speaking of output transformers, it’s time to level a small criticism at the Kassai: The small binding posts (lacking a hex head) are on the top cover just rear of the output ‘formers, pointing straight up. They are a royal pain in the arse to use. One simply cannot find any way whatsoever on this earth to tighten them down on any speaker cable terminated with a spade-lug, other than Audio Note’s own cable —and that only by using the 4mm banana plugs they were supplied with. Thus it was with AN’s own AN-SPX speaker cables, twisted together and plugged into the binding posts on the Eurythmies, that the amplifier was auditioned.
THE MORE THINGS STAY THE SAME …
The amp was set up on a Michael Green Tuning Amplifier Stand spiked with a trio of his all-brass AudioPoints. A single Shakti Stone was perched atop the power transformers, set left to right and centered between the two transformers rather than turned lengthwise, which would have covered both transformers more completely; this orientation was based solely on listening. In fact, best Shakti sound was achieved with the Stone sitting upside down so that the rubber bottom padding of the Stone wasn’t in contact with the transformer. (This was a suggestion by Michael Green who eschews damping of any kind as unproductive in audio.)
Interconnects were either XLO Signature (which sounded terrific) or the new Alpha-Core Goertz silver interconnect. This fiat silver stuff (also available in copper) continues to impress and amaze with its clean, wide-band, fast, yet thoroughly harmonic presentation.
The front-ends revolved around a Jadis JP8OMC up on Shun Mook Super Diamond Resonators. It was fed signals by the following digital systems: all Jadis (clamped JS1 Symmetrical Converter/J1 Drive); all-Forsell (clamped Air Reference D/A converter and the Mk.II CD transport sitting on a combo of Harmonix RF-66 Large Tuning Feet at the back corners and a single AudioPoint front’n’center on a Michael Green rack, with a Shakti on the roof of the transport just above the laser); all- Ensemble (Dichrono Drive/Dichrono DAC on their Honeyplate damping shelves, using their SuprafluxtM line- level interconnects to the preamp).
Digital datalinks in play were the excellent Illuminati Orchid (AES/EBU), its equally impressive stablemate the Illuminati Dataflex Studio (coax), the airy-sounding Marigo Apparition Reference Signature (coax), Kimber AGDL (coax), Ensemble Digiflux 75TM (coax/BNC), XLO (coax), and Aural Symphonics glass treated with ioGel.
Analog signals were generated by the remarkable Clearaudio Insider mounted on a thin arm with a thinnest Teflon spacer on the Forsell Air Force One. Because the phono-stage of the JP8OMC is phase-inverting, I reversed polarity on the Insider.
Power cords included a number of effective The Essence cords by Essential Sound Products —(810) 375-5093 — and Synergistic Research AC Master Couplers. (I put the latest generation of the excellent Marigo cords to one side for the moment.) The Kassai’s captive power cord was plugged into an Audio Power Industries Power Wedge 110, and from there into our 30A isolated hospital-grade sockets. I put a Shakti under the Wedge to good effect.
TUBES
To say that Herb was casual about the tubes in this $52,600 amplifier would be an understatement When I asked him about the AN-branded 300Bs it came with, he really hadn’t a clue. When pressed, he suggested they very probably were Chinese carbon-plate ESTis, which were thought to sound sweeter. (Sweeter than what he didn’t say.) I tried these tubes in the Jadis SE300Bs and a pair in the single-tube Wavelengths, but they wouldn’t settle down and behave — for whatever reason, they’d only work properly in the Kassai.
They did sound pretty good, and were indeed somewhat sweet-sounding. But to me, sweet just isn’t enough for $52k. I didn’t have the Western Electric 300Bs on hand. I substituted the Golden Dragon Supers, which delivered their characteristic lively and punchy musical presentation, but as with the Audio Note tubes, I felt them to sound a bit unrefined and slightly hazy.
Next into the sockets were a quartet of VAIC VV300s. These proved a delightfully synergistic match with the Kassai, their suave and refined voice complementing this expensive-as-hell amp to a fare-thee-well. As I imagine anyone buying one of these expensive Audio Note amps would be willing to shell out the long green for the right tubes, these became my reference 300Bs for the review.
This was in spite of Herb’s initial re action when I told him I’d be trying the new VAIC tubes: “Not in my amps you don’t!” he’d exclaimed, turning an interesting mottled color.
Upon mildly surprised inquiry, it turned out that Herb had imported a number of the more-expensive VAIC VV30 tubes when they were first avail able. These tubes became well-known for their impersonation of diverse fire works on the 4th of July! (I became familiar with this behavior when one of the VV30s supplied with the Wavelength Cardinal XS died.)
I’d been assured by a fast-talking Victor Goldstein, who seems to lose his notable Transylvanian accent under stress, that the factory had solved these teething problems and that all VAIC tubes were quite reliable now. Confirming this, Wavelength’s Gordon Rankin had listened to and measured both the newer VV30 and the less-costly VV300. He’d mentioned in conversation that these VAICs were superior in every way to the earlier tubes he’d been supplied. And indeed, the VV300s proved reliable and trouble-free in operation.
Once I’d settled on the output tubes— it was really no contest — I started evaluating input and driver tubes. Herb had supplied several different 6072 and 5687 drivers, mentioning that the drivers seemed to make quite a difference in sound. When I asked him about the vintage and manufacture of the tubes he’d brought over, he was again short on details: “I dunno, they’re NOS GEs and Sylvanias. Try ‘em!” He also left some Russian CV378 rectifiers, which were auditioned only briefly.
Once more into the breach, Gordon Rankin intervened to excellent effect, as he had with the Jadis amps. “Jonathan, you just have to call Frank Morris at Gold Aero and ask him to send you some black-plate 6072s and those terrific 5687s he has! They’ll really sound great in the Audio Note amp.”
I’ve learned to pay particular attention to Gordon, who must be the most knowledgeable tube guy I know. As he was really insistent — he’s not normally a pushy guy—I didn’t hesitate to do as suggested. And, of course, Gordon was right on the money once again. The Gold Aeros are a must have for these amps: The improvements in refinement, harmonics, tonal color, micro- and macro-dynamics, and imaging were shocking. I heard it literally seconds after I’d slipped them into the sockets and powered up the Kassai.
BLUE NOTES
Wrapping Bluenote Midas Tube Dampers around all four of the Kassai’s small tubes finished things off perfectly. One of my super-tweakie friends from Australia, with whom I correspond on The Audiophile Network,1 became frustrated trying to fit the Dampers on small tubes. Here’s how I do it: Drop the brass cone into the cap, fit one of the “Harmonic Springs” to the bottom pair of support screws, and lower the springy contraption over the intended victim. It’ll settle onto the tube nicely like this, and then you can wrap the top spring around the Damper in situ, so to say.
$52K: HOW DOES IT SOUND?
One lights up the amp by, first, powering up the input and driver tubes with the leftmost of two toggle switches located front and center, then, waiting a tad, then flicking the second toggle, powering the filaments of the 300Bs.
Listening to the Kassai always proved an engaging and enjoyable experience. Interestingly, of all the single-ended in- odes we’ve tried, paralleled or single- tube, the Kassai Silver set its images the farthest back between the Eurythmies. This was particularly noticeable on the new Holly Cole release Temptation (Metro Blue CDP 8 31653 2).
While deep, the presentation was any thing but recessed. The Kassai, like all single-ended amps we have tried, possesses plenty of that attractive inner illumination in the midrange and up that breathes life into the music. The sound-stage the Kassai threw while playing this CD was surprisingly push-pull-like in its size, transparency, and especially focus. At the same time, the sound had many of those special qualities that make the single-ended presentation so approachable.
Actually, pinning down the sound of the Kassai was no easy task. Certainly it was very dynamic and powerful-sounding for an amplifier rated at 17 watts. The power and precision in the accompanying piano and bass on the Holly Cole release, as well as the integrity of her smoky voice and the way the whole construct hung together in space, were impressive, if slightly less pellucid and touchable than was managed by the Wavelength Cardinal XS, or indeed, the Jadis SE300Bs.
I was struck by several things while listening to the simply fabulous Classic Records release of Brubeck’s Time Out (CS 8192). First of all, no, I am not going to compare the Classic with my some what noisy original six-eye ... who cares! This new release has such wonderful musical integrity on its own that my advice is simply to drop your audiophile angst — of which there is far too much around in general—and buy it to enjoy.
The soundstage was enormous, huge, and encompassing. Transients licked off the speakers with particular speed and a lifelike demeanor. In fact, I’d rate the Kassai’s keen sense of pace, rhythm, and timing right at the top of the single- ended heap. The impressive bass was about the best I’ve heard from a single- ended triode: punchy, dynamic, quite deep, and extremely well-differentiated. Look, it was still single-ended bass, but it was really quite kick-butt, considering. Brubeck’s piano sounded powerful, dynamic, and wonderfully ambient, with Paul Desmond’s transcendent horn tying everything together.
In focusing in on the horn sound, we get down to something telling about this amp’s voice. Its mids and upper-mid range were inviting and warm without ever getting smarmy about it — the Kassai always sounded killer attractive. And that’s the heart of this amp—most everything sounds beautiful through it, but hardly in a wimpy manner. Music always sounded punchy, dynamic, trans parent, warm, and colorful . . . what’s not to like, aside from the price of entry?
You really get a sense for the power the amp develops listening to the British vinyl of Dead Can Dance’s Into the Labyrinth (4AD DAD 3013). The Insider delivered its signature enormity of soundstage (in this it is practically unrivaled), and the bass transients and power of the drums, especially in that first cut, “Yulunga, Spirit Dance,” were truly breathtaking.
The Kassai really delivers a superb sense of scale and dynamics, as with Stereophile’s terrific new Festival CD (STPH007-2). My favorite piece is track 3, Darius Milhaud’s La Creation du Monde, of which we must have at least half a dozen other versions on vinyl alone. This recording is incredible — ambient, acoustic, dynamic, and super- musical from the get-go. Midrange textures are marvelous, the bass deep and rich, and the highs clear, extended, and joyous. Importantly, when things get cacophonous, the Kassai keeps every thing in order, never losing its compo sure, and seemingly never falling to its knees and clipping. These are the most powerful 17 watts I’ve ever heard, if not the most expensive as well.
Let’s talk about the midrange for a moment. I urge you all to purchase Opus 3’s new release, Solo Sonatas for Clarinet, featuring Kjell Fagés (CD 19406). You simply have no idea how well-developed digital midrange textures and liquidity can be until you hear what Jan-Eric Persson has done here. And the music is divine, even if mostly from composers I’m not familiar with, such as Mikael Edlund and Erland Von Koch. While not as lush-sounding as the Cardinal XS, or as ohmygawd gorgeous as the Jadis, the Kassai develops plenty of that special midrange magic that makes single-ended triodes sing. And good lord, is this recording ambient!
The overall sense I’d like to bring you regarding the sound of the Kassai is one of profound but refined power. Not as limpid and pellucid as the Wavelength, nor as glamorous, sexy, and rich-sounding as the Jadis, the Kassai is nevertheless possessed of its own special mixture of musical strengths that deliver the Audio Note version of veracity to the source. Let me put it this way: I was less suspicious of the Kassai than of the Jadis. It didn’t put me into shock quite as much as the French amps, but it was no less accommodating when it came to delivering the essence of the music. I found myself more likely to reach for the dynamic material when sitting down for serious listening sessions than I’ve been with any of the other single- ended triodes we have lived with. The Ellington Jazz Party in Stereo (Cs 8127), for example, sounded fantastic in every way — I really didn’t want it to end.
There was an assurance of presentation that belied the Kassai’s single-ended topology. And I guess that’s what we’re all looking for: to transcend the mechanics and get to the music. This the Kassai man ages to do, as it had better for the price.
Although the sound of the Kassai was something special on vocals, I didn’t feel that almost permanent sense of magic and closeness that I felt with the ultra- simple Wavelength or the Jadis, for whatever reason. But this isn’t a put down, really. The Kassai got beyond it and delivered an entire musical construct without focusing attention on any one particular asset or another. And when I was not listening critically, it was always an immensely pleasurable experience.
I’LL WEAR IT HOME, THANKS
Who wants to own an Audio Note Kassai? I see a wealthy guy, probably not a raving audiophile, but one who loves music of all kinds. He’s put his trust in a dealer, and has been set up with a music playback system: sensitive speakers and Audio Note electronics. He’s not a super-picky reviewer-type, nor is he a component-of-the-month victim — one would have to guess he spends most of his time making the considerable monies with which to afford such luxurious audio accoutrements. He’s not plug ’n’ play—he’s more sensitive than that— but he clearly doesn’t want to fuss. I would hope he’s mostly analog. (That’s what sounded best through the Kassai.)
This guy, he just wants to listen to his music and enjoy it. And this the Kassai, as representative of the high-priced, Kon-do-san Audio Note experience, is exquisitely capable of. I’m trying to remember hearing a recording I didn’t enjoy through this amplifier, but I can’t It should be understood that the Kassai doesn’t achieve this great musical beauty by wrapping everything in a gauzy, warm, gooey, and colored manner — far from it. In fact, on balance, the amp seems to have combined the best characteristics of push-pull with the best of single-ended. It’s certainly the least compromised of all the SE triodes we’ve listened to up until now.
If you ask me which I prefer, I’ll choose, on overall balance, the Jadis SE300B, no matter how it measures. But for sheer musical exuberance, it’s hard to beat the Kassai. As I finish up this re view, I’m sitting in the Ribbon Chair hacking away on my laptop, listening to Francis A. and Albert K (Reprise FS 1024). Let me tell you simply... I could stay planted in this chair all night!
If you, too, just wanna have fun, and you can afford it, the Audio Note Kassai Silver sure is one way to have your audiophile cake and eat it too.
MEASUREMENTS:
The Audio Note Kassai was warmed up for one hour at 5.7W into 8 ohms, one- third of its rated maximum power. It ran typically hot for a tube amplifier. Its input impedance measured 76k ohms. Its output impedance ranged between 2.9 ohms and 3.1 ohms, depending on load and frequency. As is usually the case with such a high input impedance, I would expect the Audio Note’s frequency response to be affected strongly by different loudspeaker loads.
The Kassai’s voltage gain into 8 ohms was a high 33dB. Unweighted S/N ratio measured 78dB over a bandwidth of 22Hz—22kHz (ref. 1W into 8 ohms), 76dB over a wider 10Hz—S0OkHz bandwidth. A-weighted, it measured 90dB. DC offset measured 1.4mV in the left channel, 2.5mV in the right. The amplifier inverted polarity.
Fig. 1 shows the frequency response of the Audio Note Kassai Silver. The response into 4 ohms was essentially identical to that into 8 ohms and is not shown. Note the expanded scale to show the wide variation of response into our simulated, real-world loudspeaker, a deviation which will be clearly audible. With a load resembling this one (fairly typical), that would mean a noticeable leanness in the deep bass and lower mid range and a softness at the top end, but with a strong immediacy in the upper midrange and low treble.
Fig 2 shows the Kassai’s small-signal 10kHz squarewave response. It has a moderately slow risetime combined with some ringing on the top of the waveform. The 1kHz response, not shown, is better, though still with the same overshoot and ringing (now less obvious) and the slightly sloping top typical of an amplifier with bass rolloff.
Fig. 1 Audio Note Kassai, frequency response at 1W into 8 ohms (top at 10kHz and into simulated speaker load (right channel dashed, 1 dB/vertical div.); Fig.2 Audio Note Kassai, small-signal 10kHz squarewave into 8 ohms; Fig.3 Audio Note Kassai, crosstalk (from top to bottom): L.-R, R—L (10dB div.)
The Audio Note’s crosstalk is plotted in fig.3. The channels are similar. As you might expect from the dual-mono construction, this is a good result, with only the increase at high frequencies typical of capacitive coupling between channels.
The THD+noise vs frequency performance is plotted in fig.4. It indicates an amplifier which is most comfortable driving an 8 ohm load. Into 4 ohms, the increase in distortion is noticeable, and I would not recommend this amplifier at all to drive a 2 ohm load. There is also a considerable amount of low-frequency distortion, even at the low power level used to generate these results. The subjective “warmth” generated by the high levels of LF distortion here should be audible, even perhaps filling in a bit for the upper-bass dip seen in the simulated real- world load curve in fig.1 (though this will, of course, not be accurate performance). The distortion waveform into an 8 ohm load is shown in fig.5. It is largely second- harmonic plus some noise.
Fig.6 shows the spectrum of the Kassai’s output driving a 50Hz bass frequency into an 8 ohm load (more opt mum for this amp than the 4 ohms typically used). The power level was 11.4W As expected from fig.4, the distortion here is high: —22.5dB (about 7.5%) at 100Hz and —37dB (about 1.5%) at 150Hz. While the second harmonic at 100Hz clearly predominates in this measurement, higher-order harmonics are still distinctly present. A similar measurement made into the simulated loudspeaker (not shown) indicates the same trend but marginally lower second- and third-harmonic distortion.
A combined 19kHz+20kHz signal driving the Kassai Silver at 73W into 8 ohms results in the spectrum shown in fig]. This is a fair result: the primary difference component at 1kHz lies at —36dB (1.5%), and the higher-order difference components at 18kHz and 21kHz each reach —35dB (1.7%).
The THD+noise vs output power measurements (at 1kHz) are shown in fig.8. (Note the scale has been expanded because of the amplifier’s high static distortion and very low power output.) The actual clipping levels are shown in Table 1. The values here are at a 3% THD+noise level, instead of the usual 1%, but the Kassai is clearly most comfortable driving loads of 8 ohms or higher.
The Audio Note Kassai Silver does have higher output power than some single-ended tube designs, though it is still no powerhouse. As with all such amplifiers, however, I continue to be concerned with the presence of distortion and frequency-response errors (into real loads) that we know will be audible. Does the sound many listeners like in such amps outweigh these aberrations, or are they due to these aberrations? I can accept the former, not the latter. We can not yet say with absolute certainty which is the case. It remains up to the listener to let his or her ears, and pocketbook— particularly in this case —decide. It must be said here that nothing in the Kassai’s test-bench performance gives a clue to its high cost. The price/performance ratio here can only be rated as poor. I leave it to __ to comment on the sonic price/performance ratio. I know I would think hard before investing this kind of money in an amplifier that severely restricts the choice of suitable loudspeakers — as do all single-ended tube designs known to me. Apparently even unrestricted funds available to the designer will not change the inherent limitations of this design.
Fig.4 Audio Note Kassai, THD+noise vs frequency at (from top to bottom at I kHz): 4W into 2 ohms, 2W into 4 ohms, 1W into 8 ohms, and 2.83V into simulated speaker load (right channel dashed).
Fig.5 Audio Note Kassai, 1kHz waveform at 1W into 8 ohms (top); distortion and noise waveform with fundamental notched out (bottom, not to scale).
Fig.6 Audio Note Kassai, spectrum of 50Hz sinewave, DC—1kHz, at 11.4W into 8 ohms (linear frequency scale). Note that the second harmonic at 100Hz is the highest in level, at —22.5dB (about 7.5%).
Fig.7 Audio Note Kassai, HF intermodulation spectrum, DC—22kHz, 19+20kHz at 7.3W into 8 ohms (linear frequency scale).
Fig.8 Audio Note Kassai, distortion (%) vs output power into (from bottom to top): 8 ohms, 4 ohms, and 2 ohms.