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April 2021

Best Of 2021 Blue Note Equipment Awards

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World Premiere Review!
AGD The Audion MKII Monoblock Amplifier Review
The Audion MkII takes this prized monoblock to the next level.
Review By Rick Becker


AGD The Audion MKII Monoblock Amplifier Review


  To be a successful manufacturer it is imperative to know when to draw the line on product development and call it ready for production. The problem is most designers know the creative process can go on indefinitely. Technology rarely stands still. What is cleverly called "passion" often can be arguably described as an obsessive-compulsive addiction to perfection. Those who cannot draw the line and commit to even small-scale production become artisans of serial, unique products, no two of which are alike. A clever way to avoid this trap is modular construction.

Alberto Guerra was thinking several chess moves in advance when he came out with his stunning Vivace monoblock Class D amplifiers with GaN MOSFET transistors. His earlier career in electronics involved the development of the Gallium Nitride power MOSFET which led him to transition to high-end audio. Being Italian, he couldn't help but take a bold artistic approach wherein the power stage was encapsulated in the glass shell of a vacuum tube complete with LEDs to simulate the glow. It has a contemporary chassis of uncommon proportions and modern design topped with either tinted or clear beveled Plexiglas to reveal the inner workings of the design. It will stop you in your tracks the first time you see it. While the tube containing the solid-state power stage might first seem a gimmick, it makes more sense when you learn it is a modular unit that allows for future refinement and power upgrades. You just remove the old tube and plug in the new one — if and when it becomes available.

While I admired the contemporary design of the Vivace at a show back in 2019, Alberto's second design, The Audion, a diminutive monoblock that actually looked like a tube amplifier, grabbed my immediate attention. A review sample arrived shortly, followed by a rave review and a Blue Note Award. It took my music to a new height and it put a smile on my face every time I bent down to turn it on or off. Perhaps what speaks most highly of The Audion is that it replaced a pair of 18-Watt tube monoblocks with parallel 300B tubes. As I've asked before, how often do you hear of a guy giving up Class A, 300B SET amps for Class D? This is a very special amplifier.


AGD The Audion MKII Monoblock Amplifier Review


2020, A Year For Development
Over the following year, I enjoyed The Audion so much that I began feeling guilty for not heaping enough praise on it. My pride of ownership grew steadily. This is a factor that is often short-changed or even completely overlooked when looking for a new amp. It shouldn't be. If you take pride in your car, your home, your clothes, why should you settle for something less in your audio system? Quite unexpectedly, a year after the review, I heard from Alberto that he had a new power tube for The Audion. Would I like to have a listen and give him some feedback? I immediately sensed this was a loaded question and envisioned hundred-dollar bills drifting endlessly out of my wallet. It's not like I was feeling any deficiency in The Audion but how could I say no? I'm a victim of my curiosity.

The new tubes, all calibrated and sealed, arrived encased in the same cylindrical bamboo boxes as the originals, cushioned inside by a bag of bubble wrap. Each cylinder was carefully labeled with tube type, serial number, and QR code with an identical yellow sticker on the base of the tube. A glance at the architecture beneath the large red AGD logo revealed a considerably different village of components inside. In fact, it was looking more like a small city inside the new tubes. The original tube weighed 108 grams, the new one 135 grams. Alberto explained the difference:


Important changes are related to the Analog power supply board with new type of decoupling capacitors and improved thermal performance. Then an improved layout of the GaN MOSFET board that has allowed to increase the switching frequency to a nominal value of 800kHz. Several additional parts have been included in the new design (you can feel the different weight), as well as more capacitors.

In the photos below, the new MkII tube is shown below the original tube.


AGD The Audion MKII Monoblock Amplifier Review


But it is not quite as simple as just adding more parts. Unlike more conventional components, you cannot simply alter the circuit on a breadboard and test the results. Due to the layout constraints of putting the circuits into the glass tube, each time you alter the circuit you must do it as a final product — and this adds considerably to the cost. Alberto continued:


"It took me hundreds of hours to nail down all the little details, the material/component improvements, on top of lengthy and sometime frustrating listening sessions before freezing the design... not to mention the significant amount of money in prototypes of course."


AGD The Audion MKII Monoblock Amplifier Review


The new tube has been included in The Audion and Vivace monoblocks since the start of 2021 at no additional cost. For earlier production, the new tube is available for $2000 per pair. Since it is no different than changing a vacuum tube, it is a simple DIY project that takes five minutes per tube. First, I hunted down a white cotton glove to keep my skin oil off the tube. This is an aesthetic consideration, since it is not a vacuum tube in the purist sense. I was lucky to have the right Allen wrench to remove part of the tube cage. It is an uncommon size. None of my small Allen wrenches from either the English or Metric sets worked. But I had one odd-ball colored wrench that probably came with some furniture from China... and that did the trick. For what these wrenches cost in bulk, it should have been taped to the outside of the new tube box. That's my only complaint about the whole experience.


Notes from The First Hour
After listening to some familiar cuts on my compilation CD and then installing the new tubes I listened to the same (and more) music for over an hour. My notes read:

"I liked most of what I heard but felt it still needed more air on top and more transparency. Hopefully, it will open up more as it burns in. Great gains in resolution and sense of depth, as well as control of sibilance. No overshoot on the attacks of notes, and much better decay. Supremely smooth on long sustained notes. The pacing seemed a little slow, probably because the attack of notes was so tightly controlled. This will probably loosen up with time. And probably more air at the top end, too. It's looking very promising."

You can forget all the above shortcomings because we all know things get better as the equipment burns in. I wrote Alberto a week later and suggested he advise customers to reserve judgment for 100 hours. He replied, "All electronic components should get up to 168hrs before reaching the plateau of performances (this is true for any components)." I expect you will be so into the groove by 100 hours that the next 68 will blow by unnoticed.

Alberto shared some feedback he received from Sigurd Groven in Norway after receiving his new Audion with the new tubes. With Sigurd's permission I share it here:

The term "look endlessly into the music" really comes into its own with these amps! The midrange is really crisp and makes instruments and voices appear crystal clear and not the least; lifelike and realistic! The bass is (by my ears) perfectly balanced tonally - super-precise, fast, punchy, and tight, with a really high-class P.R.A.T-factor! Actually, if I had not been paralyzed in my legs and sitting in a wheelchair, my foot-tapping would have gone bananas.


New Versus Old
Sigurd's observations were spot on — especially the subconscious emotional toe-tapping he felt and shared with us. That frees me to focus on the improvements the new GaNTube KT88 MKII tube brings to the music. I discussed the use of GaN MOSFET Class D technology within my original review of The Audion, and you can read more about it on their website. Briefly, it has a very fast rise time and can reproduce a near-perfect square wave on an oscilloscope. With a switching rate of 800kHz, it is capable of producing a very smooth analog-like sound. With the right implementation, it can sound holographic and tube-like. In retrospect, I'm glad I didn't go overboard with praise in my initial review, or I would now be painted into a corner.

The improvements span the entire audible range but what initially becomes apparent lies in the bass and treble. These are the areas of loudspeaker design where compromises are often made in the interest of cost control. After all, most of the music lies in the midrange as we all know. My Kharma Ceramique 2.2 speakers are no exception. They are not shabby at the extremes, but a 9" woofer in a 6000 cubic foot room is a challenging proposition for an amplifier. And loudspeaker and driver designs have improved considerably over the past 18 years. Nonetheless, I've updated the old girls with Synergistic Research HFT tuning dots, Bybee Quantum Purifiers on the bass and midrange drivers, and Soundeck Damping Feet under the spikes. Each of these tweaks brought an improvement and together they've kept this vintage speaker close to current standards without breaking out large bills.

With the original power tubes, The Audion, with 85 Watts into 8 Ohms, 170 into 4, was a very good match with the benign impedance of the Kharma, rated at 89dB efficiency, even in my large room and using a roughly 9' equilateral triangle mid-field listening configuration. I'm not a headbanger, preferring to listen in an 88 to 93dB range at the listening chair. That said, with the new KT88 MkII tube, The Audion seems a lot stronger.

New GaNTube KT88 MkII tubes, at $2000/pair, keep me from having to spend another $20,000 on new speakers. The improvement in The Audion is that significant. The bass is noticeably fuller with more slam that I can now feel in my chest (like with the drum kit in Paul Simon's "Diamonds on the Soles of Her Shoes"). More important (to me, at least) is the improved timbre in the bass. I can more fully taste the drum heads on the deepest of drums on my Chinese Drum cut. More resolution down there, if you will, and no gradual blur as it descends into deepest notes. It just drops off into the silence of the –130dB noise floor. Even the 20Hz organ notes, which are below the 33Hz rating of the speaker were merely subdued, not muddy. Credit the 30A current capability of The Audion and the high damping factor of this Class D amp. The extra capacitors added to the circuitry inside the tube, combined with Alberto's tuning was clearly doing the job.

In comparison with the original Vivace monoblocks that visited last year in conjunction with the new AGD Andante Preamplifer /  DAC / Phono Stage / Streamer that I reviewed, The Audion now achieves as much prominence in the bass as that larger amp did with its greater power supply. And if recollection serves me right, the new Audion has even better resolution than the original Vivace. If you've got an older Vivace, it would serve you well to upgrade to the new Mk II tube as well. It will feel like you've upgraded to a brand-new amp. The current production of the Vivace gets the new tube as standard equipment if you're thinking about an upgrade in this price range. If you want to go extreme, there is the new Gran Vivace, conservatively rated at 400W into 4ohm with a peak output of 50A from the new, similarly designed GaNTube KT120. But that's a story for another review — and be prepared to spend large.

Moving up to the treble, the music here was also more refined as evidenced by cymbals and brushes on the drum heads. Violins were sweeter and more nuanced. And Paul Simon's sibilance was more controlled. Equally important, there was more air in the treble, which is to say the –130dB noise floor again became apparent. While this lower noise floor might not rationally equate, there was a greater sense of height in the soundscape. Maybe it is the natural flow of overtones into this region, made more noticeable by the lower noise floor, that contributed to this sense of height. Even with my compromised hearing in the upper treble, the greater height was both noticeable and appreciated. (Disclosure: my room has a vaulted ceiling, low behind the speakers, high behind the listening position.)

With these noted improvements to the bass and treble, you can pretty much guess the midrange was noticeably improved, too. My attention was first drawn to the bass and treble because the differences were more obvious. What then struck me was the seamlessness from top to bottom. Yes, it was seamless with the old tube, but the bass and treble kind of drooped at each end in both volume and resolution. Now, it was flat from one extreme to the other — at least as far out as I could hear, without any boominess in the bass or shrillness in the treble.

The resolution was improved across the board, leading to many other attributes becoming apparent. One, the soundstage was more transparent. There was better lighting. Individual musicians were more clearly defined and you could more easily follow musicians placed upstage behind the musicians or singers in front of them. The "wall of sound" behind Wilson Phillips in their cover of Elton John's "Daniel" actually became civilized. Not only were the individual voices of the three-part harmony more easily discerned, but so was the orchestration behind them, particularly the violins.

The soundstage was lighted from side to side and front to back. Back-up singers were no longer in the dark in Lyle Lovett's "Church". The horn at the opening of Bob Seeger's "Turn the Page" had much more presence at stage left, far to the listener's right. With my speakers positioned on the very long wall, virtually all speakers I've auditioned have had terrific soundstage width. What the new tubes in The Audion brought to the party was to make the extreme edges more visible — give them more presence in the performance. This was particularly obvious with orchestral music where the stage is filled with musicians, left to right and front to back. Bravo!

Not only were the dynamics improved, but micro-dynamics and tonal nuances with the fingering of stringed instruments, plus the sustained notes of solos all also became impeccably reproduced. Tonal color, was already great with the original tube and I didn't notice any further or excessive saturation.

One of the big issues for me, particularly with rock and pop music, is the recognition of lyrics and here the GaNTube KT88 MkII took it up a notch once again, making it much easier to relax and enjoy the music. Not nearly as much leaning into the music, asking "What was that he sang?"Of course, there will always be singers who have to invent their dialect to keep you on your toes. But I had to use a lot less brainpower to sort out lyrics and take in the music with the KT88 Mk II.

Likewise, it became a lot easier to imagine being in the venue of live recordings, given the greater transparency and all the improvements it wrought. It also became a lot easier to turn the volume up to "live performance" levels without listening fatigue, which meant I sometimes had to reach for the SPL meter when going from one LP or CD to another to make sure I wasn't inadvertently punishing myself.


Part of the value equation is how much music factors into your life and how much sound quality you need to satisfy your listening experience. Another part of the value equation is how expensive a piece of gear is compared to other gear offering similar sound quality. Sure, The Audion would cost a lot less if it were in an ordinary chassis like... well, most everybody else. And sure, there are a few brands out there that cost a whole lot more money, that might sound better than these... and you know who they are. My hot-rodded front ends are respectable, but modest compared to a lot of high-priced gear. Swap them out for something at or near world-class and who knows how good The Audion with the new tubes might be? But that's way above my pay grade and deep into Lotto territory.

What I can say is that given the sound quality and the unique aesthetic design of The Audion, it is an outstanding value that offers immense pride of ownership. Alberto Guerra may be relatively new to the high-end scene, but his products that I've reviewed and these new tubes, in particular, indicate that he will likely become a major player in this field. He already has representation in Great Britain, France, Sweden, Hong Kong, and Singapore. When audio shows resume, his room should be a "Must See" on your list.


Don't it always seem to go.
That you don't know what you've got.
Till it's gone.
--Joni Mitchell, "Big Yellow Taxi"


Well, yes, but turn the record over and it should also tell you that you don't know what you've got till you've got something better. And that's the story of the new AGD GaNTube KT88 MKII. The MkII power tube takes the already acclaimed Audion monoblock and raises it to the next level. The GaN MOSFET transistor has already proven its superiority over traditional silicon-based transistors and now we have a further refinement of its application in a Class D topology that sings with many of the very best amplifiers out there. Only time will tell if this faux tube design will be celebrated in the pantheon of the greatest amplifiers. I expect one of my grandkids will treasure it someday. It's already a classic in my view. Elvis was over for lunch the other day and I let him sing without a mask. He loved it, too.





Sub–bass (10Hz – 60Hz)

Mid–bass (80Hz – 200Hz)

Midrange (200Hz – 3,000Hz)

High Frequencies (3,000Hz On Up)



Inner Resolution

Soundscape Width Front

Soundscape Width Rear
Soundscape Depth

Soundscape Extension Into Room


Fit And Finish

Self Noise

Value For The Money




Type: GaNTube monoblock audio amplifier
Output Power: 85 Watts @ 8 Ohms, 170 Watts @ 4 Ohms (>30A max current capability)
Frequency Response: 5Hz to 100kHz (+/-3dB)
THD+N: <0.005%
Input Impedance: 40kOhm (600 Ohm upon request)
Noise: < -130dB (A-weighted)
Gallium Nitride MOSFET based Power stage
Audio Power Stage Fully integrated in Glass Tube
GaNTube PWM Frequency: 800kHz
Size: 5.5 x 9" x 7.675" (WxDxH) 
Weight: 6 lbs. 5 oz. each
Warranty: Three years parts and labor
Price: $7500 matte finishing, $8400 mirror polished finishing




Company Information
AGD Productions
Los Angeles, California
Website: AGDproduction.com
















































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