December 2024

AGD Productions SOLO Limited Edition GaN-Power Monoblock Amplifiers And Andante Mk II Preamplifier / DAC / Phonostage / Music Streamer Review
The apex of musical reproduction in ways no other Class D design has yet achieved.
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

  As a classically trained percussionist / composer whose forte gravitates towards modern chamber music, I naturally seek out and explore the most current of musical ideas – Those that delve into sonic territory yet undiscovered. Contemporary percussion recognizes the fundamental responsibility of the percussionist-as-inventor. Indeed, we percussionists are creatures donning many hats, an amalgam of scientists, naturalists, engineers, archeologists, et al, compelled to follow the sonic siren of our artistic goals, unconventional or otherwise, that ultimately shapes our art. We are musicians in search of a sound, whose obsession it is to seek the source and its means of actuation (Yes, in the right hands, everything is a potential musical instrument!). Oftentimes, the process of this quest takes a percussionist on the most unexpected journeys. Anyone familiar with the life and music of such composers as John Cage, Harry Partch, or John Luther Adams, will respect the nature of these unorthodox musical odysseys (I was recently refreshed in this mindset while collaborating with colleagues at the University of Iowa in a performance of John Luther Adams, INUKSUIT).

By consequence, my audiophile journey follows a similar curiosity with those audio designs that tend to delve into sonic territory yet undiscovered, often forging past conventionality when seeking authenticity from the live recorded moment. There are of course numerous examples of such designs out there representing varying degrees of success. Contemporary audio artisans credited with moving the needle forward such as Albert von Schweikert, Siegfried Linkwitz, and Louis Desjardins, are but a few who come to mind otherwise guided by this objective, looking beyond the mainstream in favor of a path less traveled.

All have one key trait in common, that is in allowing live music to be the key informant in the development of their audio designs, and ultimately the guiding beacon of their company's direction. No doubt, among these pioneers must be Alberto Guerra, Founder/CEO and principal designer for AGD Productions Inc., located in Palos Verdes, California. An artistic audio designer-inventor of the highest order, Alberto is one driven to regenerate recorded music at the most realistic levels possible, often finding himself in uncharted waters during the journey.

In Search Of A Sound
Established in April 2015, AGD Productions has continually attracted the amazed ears and eyes of the international audiophile community. Driven by a relentless curiosity and quest "to deliver the best possible experience in the reproduction of recorded music," Alberto Guerra has served as the tireless torchbearer of his company's mission, continually raising the bar / re-drawing the map of Class D technology, largely through unique applications of Gallium nitride (GaN) transistor technology specific to high-end audio amplification. Each of AGD's product examples utilizing this technology (Tempo di GaN, Audion MkIII, Vivace, Gran Vivace, DUET, and the SOLO) has significantly and uniquely shortened the bridge between live music and the recorded medium.

Earlier this year, I was privileged to pen the premiere review of AGD's (then) most recent addition to this objective, the AGD Productions DUET GaN monoblock amplifiers (reveiwed here). In my experience, the DUET narrowed 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. While visiting with Alberto at last April's AXPONA in Schaumburg, he asked if I would be willing to compose a comparative review between the DUET monoblocks and his recent, most powerful iteration of this design in the AGD amplification family – the SOLO Limited Edition GaN-Power monoblocks. Two months later, he inquired if I might also add his latest edition of the Andante Mk II reference preamplifier/DAC/Phono Streamer to the conversation. When it comes to quality time with any of the AGD designs and the opportunity to work with Alberto, my response is an immediate and resounding yes to all the above.

I refer the reader to my March DUET review, where much information was provided concerning the specific background of GaN circuitry and whose design Alberto was involved in while at International Rectifier. The objective of this work was to optimize circuitry specific to high-end audio applications. As this patented technology is now exclusive to AGD Productions, it offers Alberto and his company a key that no other designers have access to, unlocking significant revelations in the presentation of recorded music at levels of fidelity that are shockingly realistic.

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 fundamentally 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 exceptionally efficient, with AGD setting the pace on this point for the entire audiophile industry. Higher performance and greater efficiency equates to more energy dedicated to better music reproduction. The patented GaN circuit Alberto helped to develop greatly altered the evolution curve of this technology and its employment in high-end audio, offering a quantum leap forward for those seeking a visceral conduit between the live recorded medium and the listener.

The DUET and SOLO monoblock amplifiers represent the most current applications of Gallium nitride power MOSFET to both the output stage AND the power supply. Having now enjoyed listening through all of the award-winning AGD amplifier offerings, it is my opinion that the DUET and SOLO offer the most organic, full-bodied, linear presentations among them. The incremental improvements resulting from their 100% GaN-based SMPS circuitry over the previous generation silicon-based Cool-MOS power supply technology has resulted in significantly lower noise floors and higher efficiency.

Further, with both operating at up to 800kHz, the consequential elevated slew rates deliver exceptionally crystalline speed without introducing artificiality or stridency. Unlike the faux KT88/KT120 glass envelope which contains the modular output circuitry of the Gran Vivace, Vivace, and the Audion, both the DUET and SOLO instead contain an internal, easily accessible ‘daughter' board inside the chassis which attaches to the power supply ‘mother' board by means of a multi-pin connection. Both board and tube designs provide a convenience of accessibility and exchange, allowing the owner to maintain the value of their investment and assurance of maximum quality output with next-generation GaN-Power modules as the youthful GaN technology continues to evolve. A truly ingenious, consumer-friendly design is exclusive to AGD.

From an appearance standpoint, both the DUET and SOLO represent a more traditional box design, with the DUET providing a much smaller footprint for those needing such space or general aesthetic considerations. Like every AGD instrument I have encountered, both are rock-solid instruments of music, beautifully designed for function and simplicity, while presenting an artistic appearance worthy of a statement piece in high-end audio. Specific to the SOLO, a variety of chassis finish options are available, including oak, cherry, piano black, or glossy red solid wood side panels, as well as faceplate options of anodized black, silver, or a mirror-polished chrome.

The review pair sent to me featured the anodized black faceplate and top, black leather-wrapped solid wood sides, with the AGD logo adorned in red lettering on the top chassis (the push button stand-by on the front panel also illuminates an AGD logo in an attractive red display, eliminating the need to toggle the main power switch on the back panel). I personally loved the appearance of my review sample and can well imagine how easily it would adapt to a host of listening room decors – Very classy.

Similar to the DUET, the business end of the SOLO is simple 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 XLR balanced analog inputs with an analog input selector toggle switch.

Four conical feet mounted on metal isolation discs insulate the chassis from foundational vibration.

All AGD products are provided proprietary power cables in a 1.5m length. Make no mistake, these are well-crafted, substantial, and more than up to the task of channeling quality power. As connectivity is of a standard design, alternative after-market cords can also be used, although I was not compelled to audition others for this review.

Finally, it's all in the presentation, as AGD Productions tends to every detail of both product and packaging. Audio equipment arrives in well-insulated bomb-proof Pelican containers, perfectly preserved, including a detailed, well-presented manual, a pair of handling gloves, and polishing cloth. One cannot help but be immediately taken aback by the level of attention and panache typically reserved for audio pieces far more expensive. Indeed, this was an impression that transcended my entire experience with all my AGD review pieces at every turn.

Power Supply

Main Circuit Board

Upgradeable GaN Power Module

Sound
As I began my formal listening experience with the SOLO Limited Edition amplifiers, I asked Alberto what were the sonic objectives that the SOLO was designed to achieve that other AGD amplifiers do not serve (or serve as well)? He responded with the following:

"When we started the design of this new product, we had a clear objective in mind that was to elevate the performance of the platfom (in common with all our previous designs, and more adjacent to the DUET and GranVivace) to a new uncompromised level. To do so, the first step was to reconsider the entire power supply stage. The second (but only for temporal reasons), the output filter of the power stage.

The first step was naturally required because the target was to deliver 600W. In all our previous designs, we used switched-mode power supply units (designed by AGD). The DUET has the full GaN MOSFET-based SMPS for audio amplifiers, but has a limit in power capability that could not meet the specs for the SOLO. A SMPS redesign is even more challenging than the design of the output stage (the GaN module). It requires a good amount of time and money, something we did not have in the budget. Hence, we decided to take a "brute force" approach and use a more traditional linear topology, based on a classic transformer. This decision was also influenced by another factor in that many customers and enthusiasts had been requesting a fully linear design, citing sonic improvements over the fully regulated SMPS types, although these designs are less efficient and larger in size.

We subsequently chose to pursue a double rectification stage with a smart-rectifier topology that effectively eliminates all unwanted noise generated by standard silicon-based rectifiers, and with much lower power losses. A power supply design based on a transformer with very stringent magnetic and electrical characteristics necessitates sourcing from a highly capable manufacturer. – A challenge we have struggled with for an extended period. We found a world-class specialized manufacturer in Macerata, Marche, Italy, who also serves many well-known brands in the High-End market.

After selecting the transformer, we defined a large capacitor reservoir to meet the peak current requirements of the GaN power stage. We tested and validated parts with low ESR and minimal parasitic elements. To support the power stage, we allocated space on both the PS board and the main analog board for up to ~150,000uF, essentially turning it into a high-current welder unit. To minimize distortion and ensure optimal performance at the 800kHz carrier frequency, we tested various magnetic materials for the inductor and different capacitors.

With the new AGD SOLO, I believe we have succeeded in delivering an amplifier that retains the dynamic and micro-detail capabilities of our smaller models, while providing a substantial, three-dimensional sound stage. This closely replicates live performances in a manner we had envisioned but were not entirely certain we could achieve at the project's inception."

My assessment period with the SOLO was both extensive and varied. Though my home reference evaluations with the DUET and SOLO were separated by a few months, I retained plenty of recall and reference information to compare/contrast. Secondly, while at the '24 AXPONA and otherwise reporting on the show for Enjoy The Music, I spent a significant amount of time in the AGD Productions room, providing many opportunities for me to otherwise directly and immediately compare the DUET and SOLO on a variety of musical examples.

With 200 Watts more output than the Gran Vivace (400W @ 4 Ohms ) and twice the output of the DUET (300W @ 4 Ohms), the SOLO, at 600W into 4 Ohms is characterized by an outstanding >70A peak output driving capability. Indeed, the greatest and immediately obvious attribute demonstrated by the SOLO is pure power. Naturally, an increase of power will yield many sonic advantages, while other technical attributes will prove subservient. My initial comparative impressions between the DUET and SOLO at AXPONA held consistent once the SOLO took its place in my home reference system.

As it was with the DUET, I found music through the SOLO to be served authentically and holistically. Overall sonic quality was on par with some of the finest amplification I have experienced in my system, regardless of price. Every musical reference was rendered a characteristically honest presentation. Neutral, yet engaging, with a palpable layering of instrumental voices.A comprehensive presentation with the ability to retrieve an extraordinary amount of musical detail, yet with no unintended sonic artifacts. The presentations took on a palpable sense of physicality and space. Sound took shape, giving music a true presence between the ears and behind the eyes.

In what ways does the SOLO distinguish itself from the DUET? Though technical specifications would suggest equal footing in almost every parameter but power output between them, the following reflect critical listening versus measurable impressions:

-  Depth and soundstage are expansive in ways that the DUET does not quite reach. It also excels in creating air and space around instrumental sources, defining detailed timbral character in ways the DUET does not.

-  Expansion of discernable frequency spectrum. This is particularly true of lower frequencies where the SOLO appears to reach beyond the DUET. Bass is full-bodied and truthfully textured to the source. Conversely, upper frequencies are refined, but not hyper-extended or rolled off. These are sonic characteristics applicable to the DUET as well, however, there is a more crystalline extension offered through the SOLO that is wonderfully balanced at the top.

-  Though I enjoyed the overall midrange presence as heard with the SOLO, I feel I ‘felt' mids more when experienced through the DUET. There was a bit more of that SET quality. Admittedly, an intangible is difficult to define, as often happens, especially when experienced from a performer's perspective in a live performance. I found mids to be ever so slightly forward with the DUET, with subsequently greater presence. A sonic trait I personally prefer.

-  Transient speed was an exceptional forte of the SOLO that, in my experience, no other Class D amplification, indeed any amplifier at this price point, can generate.

-  Though valid of the DUET, the ambience and the ability to reveal its presence in the recorded venue was exceptional through the SOLO. No doubt, a consequence of more headroom, expanse, and overall control of the recorded information.

-  An immediate and exciting level of musical energy and engagement. Make no mistake, the DUET offers a compelling argument for the musical moment and one that invites truly memorable levels of listening and immersion that, in my opinion, no other amplification designs at its price point can offer. However, the SOLO takes this quality to another level.

-  Consistency and presence of all the above characteristics at lower amplitude listening. Both amplifiers achieve this, however the SOLO is significantly more so. I was not for want of any musical information when assessing any of my references at low amplitudes. For me, this has always been a critical evaluation criterion when reviewing audio pieces. It speaks to the level of thoughtful engineering and craftsmanship devoted to an exceptional audio piece. Indeed, the same can be said of fine-performing musical instruments.

Enter: Andante Mk II Preamplifier / DAC / Phonostage / Music Streamer
I was intrigued by the opportunity to add the MkII version of the Andante to my review conversation. Although I cannot foresee relinquishing my current reference VAC power plant, which consists of the marvelous Master preamp (with built-in phono stage) and the Statement 450S iQ amplifier, I've learned early on that there's much to be said for house/brand synergy. Thus, I welcomed the MkII to my AGD listening pallette. For purposes of clarity, I initially installed only the SOLO monoblocks to assess their effect on my reference system. After some weeks, the VAC Master preamplifier was replaced with the Andante MkII, isolating its effect. For the final two weeks of review, the Andante was removed and the VAC Master returned to the system. Additionally, and for lack of one of the fine AGD factory power cords available, I opted for a JPS Aluminata from my personal inventory, which was routed through my Audience adeptResponse aR6-T4 power conditioner.

The Andante MkII is the result of a two-year upgrade assessment, aimed at optimizing and improving both the digital and analog sections of the MkI Andante, introduced in 2020. The Andante and its smaller sibling, the Alto MkII (with optional full-function DAC/Streamer digital front-end, $4,999 to $7,999), represent AGD's two preamplifier offerings. Both are fully balanced topologies.

A comparative overview indicates the following improvements administered to the MkII:

-  The redesigned analog and digital sections are galvanic utilizing high-end manually-wound isolated transformers. The digital board is now characterized by a 100% discrete-based R2R DAC with substantially upgraded clock stability.

-  Volume control is now based on a high precision, fully-balanced ladder IC coupled with a dedicated high-dynamic/ultra-low noise professional grade instrumentation amplifier buffer stage, delivering smooth and noiseless control of the output signal amplitude. Overall gain has also been increased by an additional 6dB.

-  Redesigned MM/MC phono stage input, with 6dB noise improvement, now has a menu programmable impedance.

-  Full HD streaming capability (DLNA, UPNP, AirPlay, AUDIRVANA) via an ethernet port, and has a new upgraded HD AptX-Bluetooth compliance functionality.

-  State-of-the-art USB module based on the most updated XMOS chipset, able to decode any audio format.

With an emphasis on multi-functionality done at the highest levels, the MkII edition was clearly designed for those seeking to reduce both their audio hardware and their respective footprints. Truth be told, the high degree to which improvements have been bestowed on each of Andante's key functions suggests any one of these functions could serve impressively as stand-alone deserving their own rack space.

The MkII front panel is essentially unchanged, remaining simple and minimalistic, with touch-sensitive keys (no mechanical movement). The option of Apple's handy remote is also included to manage menu functions. Additionally, the Andante now has a new ‘teaching' mode that enables the remote with different types of controllers. A home theater bypass is now standard to enable easy integration with HT systems. The back panel configuration retains the same design of the MkI, with 3+1 analog inputs and 3 analog outputs. The third output can be customized as a subwoofer or high-pass filter output with a cut-off frequency at 120Hz (-3dB, at 6dB octave slope). All connectors are Cardas for all single-ended inputs and professional grade Neutrik for the balanced inputs. The Ethernet port has functional LEDs to monitor the communication stream over the network.

As my interests lay largely in vinyl LPs, my Kronos Sparta table remains my primary reference source, with the Esoteric K-03 available for variety. Tidal, Quobuz or sim.streaming portals are not part of my audio diet as-yet, though perhaps someday. Consequently, my primary evaluation of the Antante MkII focused on its phono stage, as well as a portal for the K-03.

As a conduit for digital listening, I ran my Esoteric K-03 both as a single box player and as a transport utilizing the Andante's internal DAC. In both positions, digital examples retained a silky, yet highly detailed character, with wonderfully articulated transient speed throughout the dynamic spectrum. Used as a DAC, the Andante proved exceptional, demonstrating no over-emphasis of upper frequencies, stridency, or stretch. Transient strings and metallic percussion 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, albeit slightly less so than when the K-03 was used as a single unit player. Nevertheless, firm, well-fleshed, and true to character.

It is through the MkII's phono stage and related versatility that the Andante shown impressive. I thought it was initially unfair to be comparing it to the VAC Master's outstanding internal phono stage. Yet, that reluctance quickly melted away on the first needle drop. Though the Andante phono stage lacked the Master's liquidity and wonderful breadth of soundstage, it held its own in such sonic criteria as transient speed, agility, and spatiality. Vinyl examples demonstrated real weight and punch – nothing inflated, with plenty of heft behind them. Indeed, I was taken aback by the presence and viscerally on most every reference.

The definition, speed, and tautness experienced with the SOLO/VAC Master combo were also accurate and impressive with the Andante MkII, if not an ever-so-slight degree leaner, particularly in the midrange. This I would have anticipated when compared to the likes of the VAC Master.

To be sure, and for such an unassuming preamplifier, the Andante MkII made its presence known with great aplomb serving all functions I had sampled.

Conclusion
In my opinion, the SOLO GaN monoblock, in every sonic attribute, truly represents the apex of realistic musical reproduction in ways no other Class D design amplifier has yet achieved. Power is more than enough to satisfy any speaker load without so much as breaking a sweat. Its rich and bountiful speed and detail bring authenticity to the listening room in ways few amplifiers can, and most certainly at this price point. Adding the Andante MkII to the mix demonstrated the value of synergy with like-intended designs, each audio piece intuitively reading the other, complimenting and expounding on their respective sonic efforts, bringing about authenticity to the music that could not be thoroughly replicated by any other means.

The above being said, I sense the term "Class D" has become a bit of an over-used characterization when referencing the work of Alberto Guerra, the novelty of such among high-end audio designs is now outdated. Granted, audiophiles are cognizant of class topography characteristics, and for compelling reasons. Nevertheless, as a professional musician, I sense the evaluation and enjoyment of high-end audio has reached an intersection where we must shed once and for all the outdated biases and stereotypes that segregate our listening pleasures via Class topographies, and simply listen.

In my opinion, the experience of enjoying a recorded musical moment should abandon all but a singular objective where it pertains to the electronic instruments generating the moment – The degree to which the veil has been removed between reality and artificiality in the re-creation of the musical moment. All other predispositions as they apply to the listening experience have, in my opinion, no place in the listening room.

If we are to consider the future of audiophile pleasures, the musical output must remain paramount. In the context of critical listening, AGD Productions offers this opportunity at the highest levels. The future of high-end recorded musical reproduction through the eyes and ears of AGD Productions offers an exciting lens into fine music. The journey is afoot and in good hands. Let curiosity continue to be our guide!

Reference System
Preamplifier: VAC Master (w/ phono)
Amplifier: VAC Statement 450S iQ
Digital Source: Esoteric K-03
Analog Source: Kronos Sparta / Kronoscope RS arm (with Dynavector XV-1s cart)
Speakers: 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, Gingko Isolation Platform, Furutech NCF Cable Boosters, Nitty-Gritty 2.5Fi Vinyl Cleaner

Specifications
SOLO Limited Edition Monoblock Amplifier
Frequency Response: 0Hz to 80kHz (+/-3dB)
Nominal Output Power at 0.1%
THD+N, 20Hz to 20kHz, 4 Ohm 600 Watt
THD+N 10W/1KHz <0.005%
Maximum Output Power at 0.1%
THD+N, 20Hz to 20KHz, 8 Ohm  300 Watt
Input Impedance 40kOhm (600Ohm upon request)
Gain 23db
GaN Power Module PWM Frequency ~800kHz
Dynamic Range >120dB
Dimensions 14.5" x 11.25" x 6.5"
Weight 45 lbs
Price: $23,500 per pair

Andante MKII PRE / DAC / Phonostage / Music Streamer
Phono Input Moving Magnet and Moving Coil selectable via Menu MM Input Impedance: 47kOhm, S/R ≥ 85db
MC Input Impedance: 420Ohm, S/R ≥75db
RIAA Accuracy: <0.1dB Frequency Response 20Hz-20kHz ±0.1db THD%: ≤0.05% 
Line Analog Inputs 2 Single ended (RCA2, RCA3), 1 XLR inputs (XLR) Frequency Response: 1Hz-100kHz ±0.2db (Passive front-end direct coupling) 
Analog Outputs 1 XLR Output, 2 RCA Outputs (Output #1&2 full range, Output #3 Sub-Out or Hi-Pass filter, 120Hz -3dB) Output Impedance: <50ohm
Frequency Response: 1Hz-100kHz ±0.2db THD+N (Balanced): ≤0.005% THD+N (Single Ended): ≤0.009% S/R ≥130db
Direct output coupling. 
Digital Inputs: USB, AES/EBU (XLR), Coax, TosLink (max sample rate 192 kHz) LAN (Ethernet Port) 
File Formats MP3, WMA, MPEG-4 AAC, WAV, FLAC, AIFF, ALAC, DSD 
DNLA 1.5
USB XMOS XU208 Process based Chipset
Frequency: 44.1kHz, 48kHz, 88.2kHz, 96kHz, 176.4kHz, 192kHz, 352.8kHz, 384kHz, 705.6kHz, 768kHz Resolution: 16, 24 & 32-bit.
DSD64 (2.8224MHz and 3.0720MHz), DSD128 (5.6448MHz and 6.1440MHz) and DSD256 (11.2896MHz and 12.2880MHz), DSD512 (22.5792MHz and 24.576MHz). 
Streaming Functionality Via LAN Port DNLA, UPnP, AUdirvana, ROON (via Airplay only), iTunes AirPlay.
Preferred apps: AUDIRVANA, Bubble UPNP, Mconnect HD. 
Bluetooth aptX HD (24-bit/48kHz.) 
Net Dimension 11"x11"x3.5" (280x280x90mm)
Remote Apple Infrared 
Trigger Output 12V, 0.1A DC (Custom Request)
Flight Case Dimension 22"x18"x10" (560x457x254mm)
Net Weight 22lb/10kg
Gross Weight 29lb/13kg
Input voltage Universal (110-240V, 50-60Hz)
Price: $12,999 in satin ($13,999 fully polished)

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

Voice: (888) 652-0104
Email: agdsupport@agdproduction.com 
Website: AGDProduction.com