March 2024

World Premiere Review!
AGD DUET GaN-Power Monoblock Amplifier Review
Defining your artistic sensibilities.
Review By Dr. Michael Bump

"The objective isn't to make art, it's to be in that wonderful state which makes art inevitable." – Robert Henri

Professional musicians have a first love, and it's usually not their mother or high school sweetheart, but rather the instruments they surround themselves with throughout their lives. Sometimes this serves a nostalgic purpose, though in most cases, it is because the instrument's sound defines them (The Harry Potter analogy of "the wand chooses you" comes to mind). Theirs is a uniquely personal relationship. We all identify John "Bonzo" Bonham through his style of drumming and the sound of his Ludwig drum kit. The size, tuning, shell material, cymbals, use of timpani, etc. were his calling card, inspiring his touch / gesture, helping define him, and in turn, helping define Led Zeppelin.

For almost his entire career, Pablo Casals performed on but a single instrument – a cello crafted around 1700 by the Venetian luthier, Matteo Gofriller. And of course, who can imagine listening to Willy Nelson's music without the voice of "Trigger," his faithful Martin N-20 nylon-string guitar, on which he's been performing and recording for over 55 years.

Ultimately, a musical instrument serves as a conduit to the musician's soul. Assessing its worthiness comes down to how deeply and truthfully it speaks on their behalf. As a professional musician/audiophile who is asked to present his impressions of high-end audio pieces, I naturally gravitate to that which is most important to me – the music. I respect the hard science and engineering behind the incredible designs that make up reference home audio pieces. I acknowledge the visual aesthetic of performance art in reference systems, just as it exists in live performance (instrument-as-art-sculpture, performance gesture, et al).

At their core, however, it is about how they artistically move vibration through air and time, serving to nurture the soul. In many ways, this ability is congruent with hard science. And then there is the percentage that transcends science, the undefined ingredients that music strives to define. The end-game for both – the live performance – is what truly matters. As one who lives in both worlds, I've always considered my audio reference system one of my primary musical instruments, as my reliance on it to express (and assess) my musical voice is just as relevant as my acoustic instruments. Indeed, I posit we are all 'musicians' in this audio art, as we each have a valid artistic sensibility that defines us, forged over time, and channeled via the conduit that is our reference system.

In my years as an audiophile, I have met many artisan audio designers who I count among my most respected 'musician' friends, whose mission is to create not just fine audio equipment, but true musical instruments. Some of these artisans are performing musicians (and quite accomplished). Regardless, all have one key trait in common, and that is they allow live music to be the key informant in the development of their audio designs, and ultimately the guiding beacon of their company's direction. One of these artisan friends is Alberto Guerra, founder and chief design engineer of AGD Productions, Inc., based in Palos Verdes, California.

A brilliant engineer and visionary, he has emerged as the torchbearer for innovative Class D designs, largely through his unique applications of Gallium nitride (GaN) transistor technology specific to high-end audio amplification. Each of his product examples utilizing this technology (Tempo di GaN, Audion MkIII, Vivace, Gran Vivace, and the Solo) has significantly and uniquely shortened the bridge between live music and the recorded medium. AGD's most recent addition to this objective, the DUET, narrows the divide all the more, offering, among several new applications, the first compact monoblock amplifier utilizing 100% Gallium Nitride power MOSFET for both the output stage and the power supply.

Class D: The Forbidden Fruit
A few years ago, I was introduced to contemporary high-end class D technology at the 2015 AXPONA, where I had the pleasure of meeting Merrill Wattasinghe, founder and chief designer of Merrill Audio Advance Technology Labs, located in Bernardsville, New Jersey. At the time, Merrill's room was featuring a pair of his Veritas monoblocks and the new Cara line stage preamplifier, all driving a pair of Reference 3A Taksim speakers. Debuted in 2011, the Veritas was considered a breakthrough design, bringing Class D applications to high-end audio through the use of Hypex's Ncore NC1200 power module.

I was smitten with the level of speed, clarity, and presence that these amplifiers brought to the system, and spent a great deal of time visiting with Merrill to learn more about this technology. He maintained that class D amplification, when properly designed from input to output, has the highest potential of delivering the most accurate, holistically convincing reproduction of live music.

A few years later, I reviewed and compared his new Merrill Audio Class D amplifier design, the Element 116, with its predecessor, Veritas monoblock. Unlike the Veritas, the Element 116 took advantage of the latest, faster transistor material technology using Gallium Nitride (GaN). Additionally, Merrill had developed a zero feedback, open loop proprietary design within the Element series (Code named ZXOL), that provided a quantum leap forward in both detail and immediacy. Though I was colored impressed with this improvement to what were already revelatory levels of speed, resolution, and soundstage, there was admittedly something nagging me suggesting an ever-so-slight leanness, perhaps sterility, to the presentation, leaving me for want of a more fleshed, human character to the music, a visceral sense of place and time.

Although I was familiar with AGD Productions through online readings and word-of-mouth from colleagues, my first direct exposure was at the 2019 AXPONA. In 2023, I was covering the show for Enjoy the Music (only my second such coverage of an international show), and had no strategy, beyond simply moving floor to floor, starting at the top. Casually walking by room 722, my feet unexpectedly stopped moving, for there was this music emanating as someone opened the door. It was only a few seconds, but the quality of sound wasn't just different, it was compelling - a what-is-that-live-band-doing-in-a-hotel-room kind of compelling. Upon entering, the components were indeed deceiving, as what appeared to be examples of small, low output SETs were offering up 200 to 400 Watts... and to many a raised eyebrow! On active display were a pair of AGD Gran Vivace monoblocks, a pair of Audion MkII SE monoblocks, the AGD Tempo di GaN stereo amplifier, and the Andante pre / DAC / streamer / phono. My quote from the Enjoy the Music.com 2023 Show Report speaks for itself:

"Paired with the superb Ocean Way Eureka speakers by Allen Sides (via Ocean Way Recording Studios in Santa Monica), the presentation never ceased to amaze and impress. Within and beyond their price points, AGD offers some true giant-killers in high-end audio. Gorgeous, three-dimensional imaging extending well beyond the room, macro and microdynamics in spades, no noticeable sonic artifacts, all with lightning transient speed and tautness. Always an engaging listening experience."

There was something truly innovative happening here that was beyond just breaking with Class D stereotypes, which moved past discussions of topology and other technical distinctions. Above all else, 'live' music was being reproduced in as authentic a manner as I had yet experienced, regardless of price point. This AGD / Ocean Way system wasn't just serving the music, it was serving the recorded moment, regenerating every visceral characteristic of place and time.

So now that I've experienced "the moment" and we've had our "cigarette," just how did AGD Productions do that?

GaN: The New Frontier
Class D technology employed for audio is not new. There have been numerous essays, tutorials, and reviews written, particularly in recent years, as to the evolution of this topology in high-end audio (For an informative summary of Class D history, I refer the reader to Greg Weaver's excellent video review of the AGD Productions, Gran Vivace, Episode 108:

With the debate between linear and switching advantages/disadvantages ongoing now for some decades, the employment of GaN MOSFET transistor technology in various guises beginning in 2010 has contributed more linear characteristics to Class D sound quality, similar to SET or Class A designs, adding girth, stage dimensionality, and lower frequency definition to what had already been significant improvements in transient speed, upper-frequency extension, and overall clarity.

Historically, switching speed and timing inaccuracies have been the greatest limiting factors of Class D amplifiers. Though GaN technology generally relies on Class D topology, it operates at double the switching frequency (equating to a higher signal accuracy) of traditional Class D amps and tends to be amazingly efficient, with AGD products setting the pace on this point for the entire audiophile industry. Higher performance and greater efficiency equate to more energy devoted to better music reproduction.

Specifically, it has been the patented GaN circuit whose design Alberto was involved with while at International Rectifier, optimized specifically for high-end audio applications and no other, that has altered the evolution curve. Because this patented circuit is exclusive to AGD Productions, it offers Alberto and his company a key that no other audio designers have access to, unlocking significant revelations in the presentation of recorded music at levels of fidelity that are shockingly realistic.

The New AGD DUET GaN Power Supply Main Board

DUET Overview
Like the AGD Productions Tempo di GaN and Solo, the DUET represents a more traditional, box design amplifier with a small footprint for those who need such for space or general aesthetic considerations (Tempo di GaN is the only amplifier currently in the product line-up that is single-chassis). Like the tube envelope output stage of the Vivace and Audion, the DUET provides easy accessibility for the owner to upgrade the output stage via a traditional pin connection if and when improved circuitry becomes available. However, unlike the faux KT88/KT120 glass envelope, the DUET instead contains an internal, easily accessible 'daughter' circuit board inside the chassis which attaches to the power supply 'mother' board by means of a similarly designed multi-pin connection. An ingeniously clever and simple plug-n-play upgrade process, uniquely Alberto's creation, allowing the owner to maintain both the value of investment as well as maximum instrument output as the youthful GaN technology continues to evolve.

The new AGD DUET GaN Output Stage

AAt $11,500 per pair, the AGD Productions DUET (300 Watt @ 4 Ohm) is postured in their amplifier offerings between the Audion MkIII ($7,850 per pair, 200 Watt @ 4 Ohm) and the Vivace MkII ($15,499 per pair, 250 Watt @ 4 Ohm). It offers a roughly 50% increase in power compared to the Audion MKIII, however requiring slightly less production / machining / chassis costs than the Vivace MkII, due to its smaller footprint. To achieve these performance levels in such a small footprint, the DUET incorporates the first such Class D amplifier utilizing 100% Gallium nitride MOSFET circuitry for both the output power stage AND the power supply.

The motherboard, in particular, is a four-layer PCB, redesigned with low-noise linear supply rails and a superlative analog buffer stage based on AD797 op-amps. With 20-ounce copper traces, the new board presents an ultra-low resistive path and minimized stray-inductance. It can accommodate a large bulk capacitor stack (up to 30,000 uF), supporting the most challenging load demands. The new power supply has also doubled the size of the front-end capacitors and improved the RF filter section.

The incremental improvements resulting in this GaN-based SMPS circuitry over the previous generation silicon-based Cool-MOS power supply technology have resulted in significantly lower noise and higher efficiency. The newly-designed GaN output stage is based on a new set of discrete high voltage GaN MOSFETs, which can operate up to 800kHz and at elevated slew rates delivering exceptional sound, largely due to its well-measured switching performances.

GaN Sound Module

The visual presence of the AGD Productions DUET monoblocks within a reference system is something of an enigma: They are at once solidly commanding pieces of art, distinguished and refined in appearance. Understated, yet beautiful, exuding great pride of ownership. And yet, their demure footprint can be overlooked as an accouterment among a family of audio pieces that, by size alone, steal the limelight. Measuring 10" x 5.6" x 4" and a mere 8.25 lbs. each, the DUET's small footprint, combined with their light dispersion of heat, lend incredible versatility to their placement. They are as comfortable center stage in an audio rack as they are as bookends on a bookshelf. This is indeed an amplifier that can hold its own on any stage.

The front and rear plates are of a solid mirror-polished finish, while the chassis body is available in either black anodized or silver mirror-polished finish. The business side of the DUET is simply and conveniently organized, including the IEC grounded power cable jack, unit On / Off toggle switch, fuse access panel, a 12V remote trigger input (using a 3.5 mm mono jack), hefty WBT speaker binding posts, and a pair of unbalanced RCA or balanced XLR balanced analog inputs with an analog input selector toggle switch. On the front panel exists a push button that will illuminate the LED logo. Pressing the button a second time will place the unit on stand-by without the need to toggle the main power switch. Four conical feet mounted on metal isolation discs insulate the chassis from foundational vibration.

It's all in the presentation, as AGD Productions has tended to every detail of both product and packaging. The DUETs arrived in two bomb-proof Pelican containers, perfectly preserving the DUET chassis, as well as including an excellent proprietary 1.5m power cord, a detailed and well-presented manual, and a pair of handling gloves. One cannot help but be immediately taken aback by the level of attention and panache typically reserved for audio pieces many levels above the DUET price point. Indeed, this was an impression that transcended my entire experience with these marvelous amplifiers at every turn.

Artistic Sensibility
In one of our earlier conversations, Alberto shared this thought with me:

"For me, as the same for you I am certain, audio electronics at their most basic purpose, like musical instruments, are an apparatus to transfer a signal from one level to another level by the most efficient means possible. Still, there is a soul, there is something in the apparatus that is more than science and engineering. It requires an artistic sensibility to create this. My passion to merge perfection and beauty in the art of audio reproduction will always be driven by this sensibility."

As I enjoyed several weeks of reference listening with the DUETS powering my audio system, I was continually reminded of Alberto's comment and how closely it aligns with my own sensibilities, both as an audiophile and as a professional musician. The truthfulness of a reference audio system serving as a conduit between the recorded musician and the listener could not be made plainer.

I found the technology inherent within the DUET to be in complete harmony with its intended outcome. Music was served respectfully, wholistically, and above all, realistically. Overall sonic quality was on par with the best amplification I have experienced in my system, regardless of price. Every reference example was rendered a characteristically honest presentation. Neutral, yet engaging, with a palpable layering of instrumental voices.A holistic and fully-rendered presentation with an ability to retrieve an extraordinary amount of musical detail, yet with no unintended sonic artifacts.

Unlike prior Class D experiences, my house sound powered by the DUETs was given a sense of physicality. Sound took shape, giving music a true presence between the ears and behind the eyes. My body was compelled to assume a posture, occupying space in the performance venue, with a sense of authentic scale fully revealed in this newly created, corporeal reality that had been created.

There was no over-emphasis on upper frequencies, stridency, or stretch. Transient string and metallic percussion articulations were strikingly beautiful, consistent across the frequency and dynamic spectra, devoid of any sense of artificial exploitations. There was a tangible, organic presence about the lower frequencies – Firm, well-fleshed, and true to character. Definition, speed, and tautness were equally accurate and impressive. For such an unassuming amplifier, the DUETS made their presence known with real weight and punch – plenty of heft behind them.

In addition to outstanding transient speed and agility, macro-, and to a lesser extent, micro-level details were very convincing. Internal lines were distinct, yet kept in perspective within the texture. I could clearly hear through and around the sonic image. There was a warmth and fullness in the mids that I had not experienced with prior iterations of Class D amplification. I found this trait to be particularly enjoyable.

A Few Reference Examples
Bruce Hornsby – Harbor Nights [Vinyl / RCA]
This 1993 release was the first without "The Range," and, with the exception perhaps to the fifth track, "Fields of Gray," really broke loose of the pop shoestrings that, IMHO, tended to tie down Hornsby's amazing virtuosity, both in performance as well as compositions. His choice of guest bandmates had as much to do with this, guests such as Pat Metheny, Bonnie Raitt, and Jerry Garcia, who collectively were given ample musical room to stretch out and feed off of one another in an otherwise improvisational, jazz-like setting (For further study, I highly recommend his 2007 project, Camp Meeting, with Jack DeJohnette and Christian McBride).

The overall mix of this recording is quite hot, with a fairly forward balance, but otherwise good dispersion side-side. The opening title track, "Harbor Lights," in particular, really tests the DUETs' ability to tame saturation in the mid-bass and bass frequencies, which they do exquisitely. There's enough groove, attitude, and personality in these tunes to keep any crowd in perpetual motion for days on end, and the DUETs integrated into my system didn't miss a step. They expanded the walls of my listening room and opened the ceiling without issue. Quite an aural illusion for such little monoblocks!

Even within the thickest of tracks, the DUETs revealed a clean, discernable layering, bringing out a balanced, live-event mix, allowing transient speed to infuse clarity and air between both instrumental and vocal sources. Timbral characters were crystal clear and consistent, regardless of position in the mix. "China Doll," in particular, was fully represented as every bit in the recorded moment, with all the virtuosic displays of the band brought to bear against the emotional frustration of Bruce's lyrics, expertly synergized and removed all constraints.

AKA Trio – Joy. [CD / Bendigedig]
Antonio Forcione (acoustic guitars), Seckou Keita (koras), Adriano Adewale (percussion) come from three different continents, speaking three different languages, formed by three different cultures and musical traditions. The resulting amalgam has brought great color and spirit to this recording. Though Adriano has performed with Antonio for some years (Tears of Joy, Sketches of Africa, et al), Seckou's presence is a wonderful new voice, with all three artists communicating an eclectic musical language. Bob Schult (of the former Ridge Street Audio Designs) first enlightened me many years ago to Antonio's music via one of his compilation "test" CDs, for which I am grateful.

The extensive collection of acoustic instrumental colors in Joy offers a sonic playground for reference listening. Powered by the DUETs, I am reminded of the correct balance among these musicians. Nothing is emphasized in a manner unintended by artists or engineers. The tactile presence of fingers, nails, and hands in contact with all manner of strings and hand drums raises goosebumps. Leading edges rise unfettered in an experience that is amazingly reflective of a top-shelf pair of headphones. It is that nearfield-yet-spatial-surround-sound-head-to-toe sonic experience. All I can say is, wow.

Jeff Hamilton Trio – The Best Things Happen. [CD / Azica]
"Moonbird" & "C Jam Blues" are two great reference pieces for Jeff's brushwork artistry and effective drum kit mic'ing techniques. If you really want to know what your audio system can do at low amplitude listening, check these tracks out. The DUETs know how to retain body and dimensionality at these lower listening levels. Above all, Jeff knows how to swing a band. These wonderful amplifiers let you feel every sensual element of the swing, from the feathered bass, to the placement of the stick on the ride, to that just right laid-back walking line in the dbl bass. I also love this recording overall for its spot-on stage placement and imagery.

Skywalk – The Bohemians. [LP / MCA-Zebra]
Though I spun other LPs with the AGD DUETs in my reference system, this one remains one of my eldest go-to's. Back in the 1980s, I was obsessed with this little-known Canadian fusion group (many may recognize one of the tunes, "Jesse James," as an early background for the Weather Channel). Killer talent in every position of the band with outstanding recording and engineering skills behind the board. Kat Hendrikse, in particular, masterfully dialed in an excellent electro-acoustic synthesis of drum kit and early experimentation with loop patches and real-time samplings via Linn Drum programming. "Grandstand" and the title track stand out as spotlights to Hendrikse's virtuosity. But again, talent abounds in all directions within the group, with this recording really giving your system a ride.

The DUETs demonstrated the frequency extremes of "Grandstand" with ease, opening the basement frequency doors with incredible firm clarity and control. The sense of stage within this recording was something not every reference system 'gets.' With the DUETS dialed into mine, I was not disappointed. Crystal clear source placement, space, and depth were outstanding. The closing track, "The Torchbearer" (so appropriate to my title bestowed on Alberto!), had all the subtle, nostalgic late-night 80s jazz seductions that one might hope for. Again, and without sounding too cliché, just try closing your eyes and not feeling the band in the room....

Education
Before wrapping up this review, I would be remiss if I did not reference a very important ingredient to AGD Productions' success – That is Alberto's exceptional marketing and promotional skills. Being the torchbearer of such significant and innovative technology carries sizeable responsibilities toward educating the masses. This Alberto does with aplomb.

A brief perusal of AGD's website will reveal a comprehensive library of company history, technical descriptions and specifications of the entire product line, a listing of international distributors, current news, and an extensive listing of reviews, testimonials, and YouTube videos, all presented in an impressive and user-friendly layout.

Adding to this strategy has been AGD's steady presence at trade shows. Those who know, know. That is to say, visiting/experiencing a room with AGD components is not only wonderfully enjoyable, but it is a learning opportunity – a classroom if you will. As a university music professor, I can speak to Alberto's engaging and educational method of working the room. Everyone he meets is a friend, and an ongoing opportunity to teach all who will listen about his ongoing work and employment of GaN technology in his designs. An extensive list of reviews has added significant support to this educational process, offering enthusiastic endorsements of the quality of the product and function of Alberto's work.

Indeed, credit is well-deserved to Alberto and his marketing team for their successful and persistent publicity, providing audio enthusiasts every opportunity to learn about and appreciate the innovative work of AGD Productions. The marketing strategy and long-term vision of its designs and contributions to the art and industry have made all the difference.

Coda
Spending quality time with AGD Productions' DUET GaN-Power monoblocks has been an absolute pleasure, and was difficult to part with. Listening to music with them in the reference chain was not a passive experience. Rather, it solicited my undivided attention. Conversation or extraneous noise was not an option, as doing so risked the possibility of vandalizing an artistic moment. The concept of a review was often lost on me – considering the experience as a task never entered the mix. Rather, every listening session was refreshingly engaging, non-fatiguing, and a genuine reluctance to end.

The AGD DUETs anchored a truly stunning aural experience, retaining perfect control of image and musical energy regardless of amplitude through continued listening sessions. In essence, and with every reference example, I found the DUETs to be a key conduit to the soul of the recorded musician.

Of particular significance to the presentation was the absence of any mediating presence between the musical experience and myself. There was never a sense of 'vehicle' that was transporting me. To be entirely cliché, I was there at the live event, with all the trimmings of space, dimensionality, organicity, transiency, and placement that come with the experience. Music lifted by the DUETs was not only a re-creation of the performance event, but wholistically an authentification of the moment, in a manner only a true re-generator of live music can offer.

Alberto and AGD Productions have something very special here. GaN technology as applied to high-end musical reproduction isn't a novelty, sidebar, or passing fancy. Rather, we are in the infancy of a new technology that will influence reference listening in profoundly positive ways. For the present, however, Alberto and AGD Productions are leading the way into that future with exclusive rights and development of the only GaN circuitry truly designed/devoted to high-end audio applications, and they are running with the advantage! As their first design to represent fully-powered Gallium Nitride amplification through both the output and power supply stages of design, all eyes and ears will be on the DUET, and deservedly so.

Rest assured, the AGD Productions DUET monoblocks will bring you a level of reality in recorded music that will be very difficult to better, and whose value will be well beyond the measure of their relatively modest financial investment.

When the opportunity arises to audition, I would ultimately suggest simply sitting back, close your eyes, connect, and just listen. It's really all that matters. With the AGD Productions DUET monoblocks driving the bus, you're in for a whole new level of reality in the presentation of recorded music.

Enjoy the Music!

Reference System
Preamp: VAC Signature mkII SE (with phono)
Amplifier: VAC Statement 450S iQ
Digital Source: Esoteric K-03
Analog Source: Kronos Sparta / Kronoscope RS tonearm
Dynavector XV-1s cartridge
Loudspeakers: Von Schweikert Audio Endeavor Special Edition
Subwoofer: Velodyne DD10+

Cables: Silversmith Fidelium Speaker cables, JPS Aluminata power cords & interconnects (XLR & RCA), WyWires Platinum (phono)

Power Conditioning: Audience adeptResponse aR6-T4 (w/ Audience frontRow power cord)

Vibration Control, et al: Critical Mass Systems CenterStage 2M footers, Butcher Block Acoustics

Isolation Platform, Furutech NCF Cable Boosters, Nitty Gritty 2.5Fi LP Cleaner

Specifications
Type: Class-D solid-state monoblock amplifier
Output Power: 300 Watt @ 4 Ohm
THD+N: <0.005% @ 10 Watt / 1 kHz
Frequency Response: 5 Hz to 80 kHz (+/-0.3dB)
Input Impedance: 40 kOhm (600 Ohm upon request)
Efficiency: >94%
Gain: 23dB
GaN Power Module PWM Frequency: ~800 kHz
Dynamic Range: >120dB
Dimensions: 10" x 5.6" x 4"
Weight: 8.25 lbs. each
Price: $11,500 per pair

Company Information
AGD Production
2644 Via Valdes
Palos Verdes Estates, CA 90274

Voice: (888) 652-0104
E-mail: adgsupport@agdproduction.com 
Website: AGDProduction.com