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Positive Feedback ISSUE 23


art audio

Jota High Current Amplifier - the Emission Labs 520B-V2 tube est arrivé

as reviewed by Marshall Nack






Kharma CRM 3.2 and Kharma Ce-Sb-10 sub woofer.

von Gaylord Audio (formerly Legend Audio Design) LAD-L2 Signature line preamplifier, von Gaylord Audio Nirvana Mk II monoblock amplifiers.

Linn LP12, LINGO, EKOS tonearm, ARKIV II cartridge, AHT Non-Signature phono preamp modified by Walker Audio, Extremephono Limited Edition and Graham IC-30 arm cables. CEC TL1X transport, von Gaylord Audio 2 chassis DAC.

Interconnects by Kubala-Sosna, Harmonic Technology, Kondo, Kharma Enigma. Digital Cables by Kubala-Sosna, and Kondo. Speaker wire by Kubala-Sosna, Harmonic Technology, Harmonix, and Kondo. Power Cords by Kubala-Sosna, Harmonix, Golden Sound, and Shunyata.

TAOC Rack and TITE-35S component footers, Golden Sound Intelligent Chip and Magic Rings, Walker Audio Tuning Discs and Valid Points for LP turntable, Harmonix RFA-78i Room Tuning Discs, RFS-66ZX Tuning Feet, RF-900 and RF-999MT Tuning Spike Bases, Acoustic System Resonators, Argent Room Lenses, Echo Buster & Sonex acoustic panels, Reimyo ALS-777 and Shunyata Hydra power conditioners, and Ensemble Mega PowerPoint outlet strips. ERaudio Space Harmonizer component platforms. Shun Mook Tube Resonators.

A new year may have begun, but I am traveling back in time on a journey to that seminal winter I spent working on the Survey of Select SET Amplification. It was cold outside then, but nice and toasty in my living room, aglow as it was with the dull wattage of single-digit 300B type valves.

What is jogging these memories is the sweet, round, liquid, dancing line of ART Audio's aptly named Jota amp (Jota is the name of a regional folk dance from Spain).

Let us see, I believe we left off last time at the beginning of the Britten: Violin Concerto (EMI 5 57510 2, with violinist Maxim Vengerov and Rostropovich conducting the London Symphony Orchestra).

The piece begins with distant timpani rolls, soon joined by soft brushwork on the cymbal. Massed strings come in, and I do mean a mass—the entire left side of the stage and part of the right is occupied. While I can't claim to count the string players (did you ever hear of anything so ridiculous, so patently an artifact?), I do know there's a lot of them because of the volume the mass occupies, and by the slight pitch and texture variations across the expanse that gives it inner life, and how it recedes back as the instruments would in conventional orchestral seating—first violins from center to just inside the left speaker and up front, seconds behind them (and moving right from the microphone position), violas on the other side, just right of center and deeper than the cellos, which are center-right and at the speaker plane. Massed string tone is something to behold: sweet, silken and smooth, full of body, articulate and textured, but not overdone. We are off on a winning note already. The strings quiet down, allowing the bassoon and flute to recall the opening timpani motif, each woodwind's timbre quite fully realized and each image spatially separated—no guesswork is required on your part. Then Vengerov makes his entrance. The violin has lovely tone and focus and the image has appropriate shape. The low strings come back with percussive thrusts. Soon we are off on a merry symphonic roller coaster.

Ah, 'twas delightfully done. All of the instruments exhibited splendid timbre, abundant musical weight, and the message was communicated succinctly. I am thinking, "I could live with this one", but there is a job to be done—so I don the green eyeshade.

Through the Green Eyeshade

There is not a lot of loose fluff hanging about this soundscape—everything there seems to have a purpose and to be contributing to the musical message. Images are large objects, full-bodied and dense, with slightly amorphous boundaries, but still good separation. There is none of the tubey, acoustic aura you often get with valve amps. Vengerov's violin has lots of SET filigree, the minute details and micro dynamics that make reproduction seem less mechanical (not to be confused with the acoustic aura), and unmistakably, it has a bit of bite. This last is important to verisimilitude—strings naturally have some bite and you want that—but you do not always get it. Nowadays, we have become ultra sensitive to the slightest roughness up top (as soon as it is heard, the red flag goes up, and the listening panel dismisses the product). Consequently, most designers will do anything to avoid offense in the treble, so they over-compensate, erring on the side of excess smoothness. This is how we wind up with beautiful, but dull and unrealistic sound. The HC Jota has treble edginess under control, but not smothered—there is enough edge up there to be reassuring without it turning into something aggressive and irritating. (In the real world, I know most of us have treble issues; if you have to pick your poison, overly smooth is certainly preferred to abrasive, but at least be aware that it is doing Band-Aid duty.) The soundstage is smooth across its expanse even with that little bit of edge—have no fear of attack by treble projectiles.

I have to keep reminding myself I am not listening to an 8-watt SET. Coming off the solid-state Kharma MP-150 amps I had in directly before, or the tubed Air Tight ATM-211 SET mono amps before that, there was no question as to this machine's voicing. Power-wise, the HC Jota resides in the mid-ground between the single-digit wattage SETs and push-pull tube amps, but where the similarly powered ATM-211 leaned towards push-pull, the HC Jota's voicing makes it kindred with the low-powered amps.

Up to a point.

I have not played the Lincoln Center Jazz Orchestra's Big Train (SRCS 8980, Japanese import) since the Hørning Agathon Ultimate loudspeakers and the TRON amps were here. The memory of that last go-round lingers. It was the first (and only) time I experienced the sound room's atmosphere pressurized, the air in motion, sympathetically active, almost like it would actually happen live, along with the huge low-end wave fronts this speaker can generate. Playing it now, yes, I miss that sense of immersion and some of the weight. I am using moderately sensitive speakers (the Kharma CRM 3.2s, along with the Kharma Sb-Ce powered sub). The amp providing the experience is the High Current version of ART Audio's Jota. With 24 watts of SET power into 89 dB sensitive speakers, I was not exactly re-experiencing the eight-watt TRON Cantata monos into the 100dB Hørning Agathon loudspeakers. Not exactly, but there were many similarities, more than ever since the Agathon's departure.

Like the best of those amps, you will find the treble less prominent, the midrange a little full, the bass warm but proportional. Bass performance is very good for this type of amp, but falls short of the benchmark Air Tight ATM-211 22 watt SET monos I had in a couple of months before in the areas of heft and definition. Ah, but the upper register and midrange of the HC Jota, c'est magnifique. Following the SET model, the treble is completely subservient to the whole. The midrange is full of joie de vive. Timbre across the spectrum is highly credible—there is a whole potpourri of instrumental textures. This combo is very clean. Instrumental lines are soooo easy to follow.

Stage width is as good as it gets here—surprising, because the HC Jota is a single chassis stereo amp. There is a lot of soundstage layering, if not to the furthest reaches attainable with some other amps. Sometimes I wondered if the HC Jota could do the 3-D palpability thing like the single-digit SETs.

Kharma Exquisite-Midi

A fantastic thing happened in the course of this review. My ultimate speaker fantasy came true—the Kharma Exquisite-Midi was made available (review to come). I know the E-M from trade shows, where it unfailingly places among the Best of Show contenders, and from the importer's showroom. This is the elder brother by three to my CRM 3.2s, with many design features in common, just that everything is more mature, more fully developed, 'mo better.

Of course, it is not likely you would find the HC Jota paired with the E-M in the real world—someone who could afford a $75,000 speaker would not have it harnessed to a $9600 amp. What the speaker did, in addition to considerably upping the bliss factor, was allow more insight into the operation of everything upstream. And the HC Jota did not embarrass itself. On occasion, some critical ears came by. You know the type—they sit down and diagnose your system, and in short order leave you a quivering puddle of raw nerve endings. Well, they came by, and they tried, but the only thing mentioned vis-à-vis the HC Jota was along the lines of it being "the little amp that could". If a shortfall was apparent, fingers pointed elsewhere. Everybody liked this amp. I will describe the HC Jota / E-M for you. (Note: the sub was unplugged while the E-M was in line).

Put on something like Besame Mucho, with the Gonzalo Rubalcaba piano trio (The Blessing, Blue Note CDP 7 97197 2). Rubalcaba's rendition is slow, soft, and swooning—highly romantic. The keyboard is caressed, each note suspends time until it becomes the following one—there are no "black silences between the notes" because the resonating body of the instrument is always present to some degree in that space. The piano's verisimilitude is startling in its completeness, in the correctness of its overtones. This is a very difficult instrument to get right—beyond its complex amalgam of harmonics, an acoustic piano has the second widest octave range (only the organ has more); hence, it has the potential of generating the extremes of sound wave lengths simultaneously. Even when only playing treble notes, there must be sympathetic low frequency strings resonating. There are many facets to savor in the opening melodic statement on this track. What strikes me most after the piano's timbral fulfillment are the step-less micro dynamics and frequency transitions. And, in the background, the little thumping sounds of hammers making contact with the low strings. It is almost a sub-vocé event, a very low frequency impulse you feel more than you hear.

While Rubalcaba's demonstrating his excellent touch and sensitivity, Jack DeJohnette comes along, speaking in his subtle, magisterial, cymbal dialect—restrained, as is his usual trio modus operandi. In the past, it has always been fun to try to resolve each of DeJohnette's Sabian cymbals distinctly (spread out as they are by multiple miking). Success was variable. At this moment, it is child's play. Each piece of his drum kit, with all the minor components and mechanical workings, resolves effortlessly. The CD has ended, and I never bothered to mention Charlie Haden's double bass. That is because it never called attention to itself. Slam was not of grandiose proportion, but the HC Jota's low-end was satisfying, articulate, and realistic. Yes, I might do a little tweaking to get some more, but right now, it works.

However, on something like the title tune on "Send in the Clowns", with Sarah Vaughan and the Count Basie Orchestra (XRCD VICJ - 60246), we are sailing along beautifully, until the final refrain. Sarah hits her solo crescendo and the orchestra slams in en mass, kick drum leading the way, and it really wallops you—except it doesn't quite, not like I've heard it on other occasions. On a speaker with magnificent dynamic capabilities like the E-M, you are missing out.

Not a big surprise, really—no one expects a 24-watt SET amp driving a medium efficiency speaker to have slam (I just wish I had an easier load / more sensitive speaker of this quality on hand. The E-M is not the easiest load to drive). Moreover, it will not be noticeable with most fare, until the going gets boisterous. However, when pressed via forceful jazz programming or heavy symphonic demands, the amp did not have the heft, or the speed, that the Air Tight ATM-211 was able to marshal. The 22-watt ATM-211 SET monos mated to the Kharma 3.2s had a very firm grasp over their abundant low-end, rivaling good push-pull tube amps with a lot more power. (It has been a couple of years, but if memory serves, the 40-watt Mastersound 845 SET mono amps could do it, too.) When the HC Jota runs out of headroom, you get soft clipping—dynamics avoid breakup or distortion and just flatten out. (For that matter, it did not have the ATM-211s amazing resolution, which created a densely packed virtual stage—you do not have that sense of the troops (images) at attention.) However, the ATM-211 did not come close to the HC Jota's midrange or treble quality, or its nuance and emotional involvement.


There is none. The effort required to get the HC Jota up and running, and voiced was the least I can remember since the TRON Cantata spent time here. It went right onto the Harmonix TU-888 System Tuning Board (now discontinued) and Harmonix accessories—RF-66ZX footers and a Studio Master power cord plugged into a Shunyata Hydra conditioner. Interconnects and speaker cables were the robust Kubala-Sosna Emotion. Later on, I switched to TARA Labs The One power cord and PM/2 passive AC conditioner—and liked it better. (The TARA Labs power products came out of nowhere and now occupy first place—stay tuned for more). The HC Jota's tonal balance was also a little lighter than these amps. You tube it up, flip the switch, and let it cook for a couple of minutes. As there is no countdown timer to mute it on startup, begin with the pre-amp volume off. The manual suggests a 20-minute warm up before serious listening—I found the HC Jota got better and better as the day wore on. (Also, note the HC Jota's input sensitivity is unusually high, much higher than the Kharma Ce-Sb-1 powered sub. I had to up the sub's volume to max, and it still could have used more—output levels between sub and main were mismatched).

The HC Jota runs almost silently—no tube rush, no intermittent gurgling, nothing unusual to note in terms of noise through the speakers. There is a residual transformer hum if you get physically close and listen for it, but nothing from the listening seat. It does run hot—keep your distance from the 520B-V2s and that middle transformer. Operation was flawless over the audition period of several months. One option is worth mentioning: if you already possess your desert isle speakers, the HC Jota can be ordered with the transformers' impedance optimized to match their particular load.

From the website:

The HC Jota is a transformer-coupled, zero-feedback, pure Class A, single ended stereo amp. It is a dual-mono design on a single chassis. Tube biasing is automatic. When installing the big output tubes, don't get nervous if they move around a lot—it's because they're mounted on a floating isolation plate assembled with vibration dampers to acoustically isolate them from any chassis vibration, not because they're loose. Just make sure they are all the way in, and do not apply too much force.

There is a lot of verbiage on the importance of output transformer design. The ones used here are a novel split-core type, not normally associated with single ended amps. They are custom-designed and hand-wound by a reclusive artisan residing in a secret place in the wilds of the English countryside. This hermit/craftsman has been with Art Audio since the firm's inception, but has only been seen once in public (so legend has it). His people have contracted with Art Audio's and negotiated the exclusive use of his skills.

über Tubes

The heart of the HC Jota, its soul, is the power tube. The standard model (MSRP $8800) yields 20 watts and comes with a choice of tubes, including the KR 32B. The amp I have been describing is the Jota with High Current option (MSRP $9600), yielding 24 watts and 30% more current. Mine came outfitted with the brand new Emission Labs 520B-V2 power tube, a 300B derivative, hand made in the Czech Republic, and it is something to behold: about the same girth as an the big 845 tube, it has a ceramic socket, gold-plated pins, hand-blown glass bulb, even the printing on it is done in real gold.


I also got to try the KR 300BXLS power tube. Swapping between the KR 300BXLS and the Emission Labs 520B-V2 is painless (the auto biasing circuit handles the differences). Remember: we are talking HC Jota here. DO NOT USE THESE TUBES IN THE STANDARD JOTA. Modifications are necessary for the standard model to become HC, principally a transformer upgrade. Also, note that the KR 52BX tube—previously offered with the HC Jota—is now discontinued.

In the old days, we used to refer to a certain female physique as duck-like, one with a disproportionate amount of rear-end. The KR 300BXLS power tube was like this, at first. The bottom was plump, warm, and loose, and there was a whole lot of it. It remained this way 'til it got past the fifty-hour mark, and then evened out and became tonally lighter than the 520B-V2. Its exaggerated decay was getting annoying. This was the case for another ten or twenty hours. Finally, somewhere around the 100-hour mark, it settled down to about the same tonal balance as the 520B-V2, and lost the emphatic decay. This faked me out good—do not let it fool you, too.

Lovers of seductive SET midrange take note. The 300BXLS has even more shimmering warmth, smoothness, and relaxed ease. It was goose-bump time with any kind of vocal music. Anne Sofie von Otter (Berlioz: Melodies, DG D206 306) was remarkably evocative of the evening I saw her at Carnegie Hall last year. The tube is more forgiving, too, in the classic way SETs cover-up unpleasant sounds with stuff that sits better on our ears, a mild form of euphony. You will suffer less glare, grain or edge with the 300BXLS (not that there was any to speak of with the 520B-V2).

A Word on Soundstage Imaging

The 300BXLS is the antithesis of audiophile soundstaging, the kind that High-End mags go on about, with decisively inscribed instruments. Instead, the 300BXLS had "cohesiveness" across the span, and was part way towards becoming a wide-mono presentation. Images were hazy, no longer specifically demarcated, more like covering a general area, but still relatively stable. In this way the stage filled in and became one—the time/space continuum became continuous (Wow—too much gas in that phrase). However, one man's cohesiveness can be congestion or muddiness to someone else. This cohesiveness affected inner detail. The violin had life, but a little less sparkle. Moreover, macro dynamics dropped off—the Kharma Exquisite-Midis sounded more underpowered. Where the 300BXLS is tactile, soft and caressing, with more color, the 520B-V2 has more clarity and is more articulate, faster, and dynamic. No question the 520B-V2 has punchier, and tighter, low-end. It has more instrumental texture and less "air" floating around the upper-mid. Its presentation is big and more forward, just as it is physically larger.

The Emission Labs 520B-V2 and the KR 300BXLS have different personalities—one tube is pointed: the other is rounded. Both are terrific sounding valves, and I recommend both. In this iteration, the 520B-V2 tube did more of all the things I look for and handled the Kharma speakers better. The 300BXLS might be perfect with a more efficient speaker and, of course, a listener attuned to lower powered SET amps.

Über Tubes

The lust factor with both of these valves is high. Closet tube-o-phile or not, take the time to inspect the quality of manufacture. They are beautifully hand-made like fine jewelry. But the price is like sterling silver: if you have to replace them, the KR 300BXLS runs $439/ea, while the Emission Labs 520B-V2 is $599/each. This is not a lot in the world of single-ended power tubes: I am told a Western Electric reproduction 300B runs about $500, while the real thing from the fifties or sixties, in good shape, will set you back $1,000 to $2,000 each—that is, if you can find one on Ebay. Once acquired, you are good to go for many thousands of hours. (The tubes I am using are too new to have a true benchmark shelf life. I am told the consumer can expect between 2,000 and 2,500 hours in this amp). To the best of my knowledge, the HC Jota is the USA premiere of a production amplifier using the 520B-V2—gentlemen, raise your glasses, please, and quaff a little bubbly, if you would, for the arrival of a contender.


The big Emission Labs 520B-V2 tubes and the mid-size CV 378 do not mind being out-rigged with Shun Mook tube resonators. With these S M hats on, you would not know the speakers were in the room, so complete was their integration and coherence. The S M hats seem to enforce a more coherent transient—the instruments involved arrive closer together, time-wise, and the demarcation between silence and transient is cleaner. As they let more atmosphere flood in, everything got softer and moved further in the direction of SET voicing. The beauty of this sound is enough to bring tears.

Various Harmonix footers also worked well. With the Kharma E-M speakers, the S M tube resonators were overkill, since there was so much built-in sweetness of tone and color.


The HC Jota sports pared down, classic good looks—functional and shiny, but no frills. The version I am looking at is an expanse of monochromatic, polished, non-magnetic stainless steel, with a touch of satin black on the sides of the transformers. Very fine metalwork is in evidence—except for the cap nuts on the transformers, nothing mars the expanse of polished steel. The bulk of its 70 lbs is borne on the transformer end—be careful placing footers under it. Light-blue indicator LEDs on top tell you all-is-well with each channel. There is evidence of intelligent design (not to be confused with Intelligent Design Theory) around back. From the IEC power jack and the on/off toggle switch in the center, moving off symmetrically to either side, first you have small black heat sinks, then speaker binding posts (my sample had 4 and 8 ohm taps), then RCA type inputs. The Kharma CRM 3.2s played off the 8-ohm tap; the E-M sounded best off the 4 ohm.


The ART Audio Jota SET Stereo Amplifier, High Current version, brings a goodly taste of the quality SET experience to the realm of medium efficiency speakers. Its treble is a model of proper voicing, always subservient to the whole and never calling attention to itself. The midrange is loaded with life and a little fulsome. The bass is actually very good for this type of amp. If you were to walk in off the street, on first listen you would be in love. Speaker selection deserves careful planning. My two available transducers were great mates, and certainly provided exceptionally refined levels of playback. For the 89dB sensitive, 8 ohm Kharma CRM 3.2s, the only thing less than excellent, apart from bass quality, was the dynamic oomph more power would have brought to crescendos. With the 90dB, 4 ohm Kharma Exquisite-Midis, again headroom was the issue. (The amp should have smooth sailing with an easier load or more efficient speaker). Never the less, the HC Jota got so much right with the speakers that I am still recommending these matches. I would not tinker with the voicing of this amp, or change a thing—well done, guys! I challenge you (the consumer) to find a better treble or midrange.

The HC Jota sounds quiet and clean, with great musical clarity. It is softer than the Air Tight ATM-211 SET, but not as soft as the single-digit wattage SETs, and while much more emotionally engaging than the ATM-211s, it fell short of the low-wattage amps. Grain is non-existent and breakup was minimal—at normal listening levels, there was never an incident of audible distortion—when dynamics got too much they just flattened out. Speed was unremarkable (that means it was good, if not great).

The power tube determines the HC Jota's voice, and you have some flexibility here. I found the Emission Labs 520B-V2 best for general, all-around listening, but if you love classical vocal, the KR 300BXLS will stimulate involuntary, potentially embarrassing, reflexes, like hair standing on end and goose bumps, as it positions you closer to the classic low-wattage SET experience. I would suggest buying, and enjoying, both. Swapping between them is effortless (but let the amp discharge for ten minutes after you turn it off before re-tubing).

I do hate to be a joiner. Most of my life I have stuck to my guns and relished the status of non-conformist. When the subject is the Art Audio Jota High Current amplifier, however, I am afraid I must concur with the long list of previous Jota reviewers, and add to the chorus of accolades. Given its topology and MSRP (medium powered SET tube amps, in the neighborhood of $9600), if you're mostly into classical or small jazz combos, and not hard driving, dynamic jazz or rock, and have a medium sensitivity speaker, the HC Jota is at the top of the heap.


I was able to audition briefly the HC Jota hooked up to the 102 dB efficient Hørning Alkabiades speakers and the Audio Note Japan M77 pre-amp. This sound had balls. It was big, dynamic and powerful, tonally in the zone, and still sweet. However, there was no denying it was also thinner, and more forward, with less atmosphere, or air than some high quality, 8-watt amps on hand. The noise floor, unfortunately, was elevated and slightly audible from the listening seat. This makes me think the HC Jota would be ideally mated with an 8- or 4-ohm easy load speaker of 90 to 98dB efficiency.


  • Type: transformer-coupled, zero-feedback, pure Class A

  • Output Power: 24 wpc

  • Input Sensitivity: 400mV

  • Input Impedance: 380Kohm

  • Output Impedance: 4 and 8 ohm

  • Frequency Response: 20Hz to 20 KHz -0.5dB

  • Tube Complement: (2 x 6DJ8), (2 x 12BH7), (2 x CV 378),

  • (2 x Emission Labs 520B-V2) or (2 x KR 300BXLS)

  • Dimensions 18.25" W x 13.75" D x 10" H

  • Weight 70 lb.

ART Audio Jota SET Stereo Amplifier, High Current version
Retail: $9600

Art Audio USA
34 Briarwood Road, Cranston, Rhode Island 02920
TEL: 401. 826. 8286
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