AGD Vivace power amps


I thought I saw it all in Audio Hi-Fi design, but honestly, a complete solid-state audio amplifier power stage, realized in an ultra-compact PCB size, and inserted in a 6550/KT88 tube... well, that one I never happened to come across.

Alberto Guerra of AGD Production must be a big “simpatico” because, even though he can claim a training and professional background of first class in the electronic field, I think he demonstrates in his products something that is unconventionally extra. Almost all of the best designers in the audio field have been in their own way non-conventional, almost as if they belonged to an artistic environment, or at least to the world of creativity and not to the barely technical one. You can find Alberto Guerra’s CV here: compare it to that of any manufacturer or designer in this field and you will immediately realize what we are trying to prove.

One could have suspected it to be a "made in China” questionable quality item, but we are instead in the presence of a true original product, in some respects a disruptively new one, from the perspective of design, technology and performance. A 100% “Made in USA” product with a unique Italian style touch. We are referring to the AGD Vivace monoblock amplifiers, whose retail price is $15,000 USD for the pair.

Its creator not only has the designing imagination mentioned above but can also claim a certain number of international patents in the semiconductors field, and furthermore countless publications, and first-class professional achievements in Silicon Valley, with companies like International Rectifier, now InfineonVishay-Siliconix and Semtech. In short terms, he is a guy that “knows a thing or two” also in the field of hi-end audio reproduction, which is unfortunately overcrowded by way too many mediocre, if not incompetent, designers.

Enough about the man, now let’s talk about his creation.

The two monoblocks, as you can see in the image, present the absolute unique feature of having the complete Class-D power stage designed around a Gallium Nitride – not Silicon! – 70mohm/200V power MOSFET, integrated in a 6550/KT88 like glass tube, complete with a canonical octal socket, as if it were a real thermionic tube.

For the fans of hi-end Audio and Glass Tubes, the whole thing might seem a little blasphemous, and I must admit that at first I myself found the thing a little weird, but at the end I must recognize that the final product, other than being extremely well constructed, is also elegant in revealing, when turned-on, a mesmerizing amber-like light which comes from the chassis’ interior, through a thick multi-angle faced fumè acrylic material, that shows a see through of the internal power electronic and analog section boards.

The dimensions are anyway quite small, and this is mainly due to the adoption of a high efficiency low noise switching power supply – audio specific variable PWM frequency SMPS with ZVS topology, and the complete lack of bulky heatsinks, not needed because of the type of topology used for the power stage. Its weight is also quite acceptable, 10kg/22lbs, essentially because of the massive polished aluminum chassis, milled from a single billet, with high precisions multi-axis CNC machines.

All the electronic components, apart the impressive bulk capacitors bank of 48,000µF and few other power resistors, are entirely SMD based and thus all the PCBs are super-compact. Their layouts have been conceived taking into account specific high frequency signal routing design rules, thus minimizing the parasitic components, which are the ones generating EMI. The AGD Vivace monoblocks have been already certified CE and FCC – the certificate is available for downloading from the AGD productions website.


Since this is not an academic technical review, I will not describe how a switching amplifier, or Class-D, works, but something must be clarified about a few of Vivace’s key innovative solutions, based on what we gathered during a call with the designer.

The critical points of such an amplification are essentially three.

First off, the power stage switching speed: that is the power MOSFET’s on-off frequency, which, in the case of the AGDs, it can reach up to the 768 kHz. That in theory simplifies the realization of the delicate output filter – second issue, which has to filter out those spurious emissions of switching that are the farthest from the audio range, other than delivering the continuum analog amplified signal.

The first original Class-D power amplifiers were characterized by a poor linear response in the higher spectrum of the audio frequency band, precisely because of the filter effect, as similarly happened in the first CD systems, while the Vivace is linear even above 100 kHz.

In addition, the extreme speed of the new MOSFET, realized with Gallium Nitride instead of Silicon, will generate a lower distortion rate, which is usually higher in switching amplifier topologies, making a considerable reduction of negative feedback possible in that way – third issue.

All of what I mentioned above would also represent a tangible progress in performance, since I would not expect these amplifiers to be too sensitive to the variation of the load impedance. That is a typical Achille’s heel of traditional Class-D amplifiers, which makes them extremely variable in the sound performances in relation to the different speakers used.

So, technically speaking, the effort is clear, and the solutions employed in this design seem to represent a further progress in the design of switching topologies. This is maybe the only field of power electronics that took a relevant evolutionary path in the last twenty years.

From these technical perspectives, I believe that these types of amplifiers are the most complicated objects to be exhaustively reviewed under a normal listening test, which was also confirmed in this case.

For instance, the Vivace amps demonstrated some sensitivity to the cables, mostly the power cables, and also, obviously, to the speakers.

Anyway, as far as the power cables are concerned, the Vivace amps under review, come with two massive power cables with Furutech plugs, having certain high commercial value, and are also demonstrated to be of excellent sonic quality.

However, similarly to all other class D amplifiers, the Vivace amps are extremely power efficient and operate practically without heat dissipation. Moreover, they proved to be absolutely silent and stable.


Context of the test

The declared output power is 100W / 8 ohm, but we sensed it to be more and yet at very low acceptable distortion level. I believe that the Vivace amps are able to drive almost all type of speakers on the market because I had the impression of an extremely generous output stage. However, I must say that only a few watts were necessary, when testing them with the Klipsch Heresy and the Cornwall, both in-production speaker types.

On the contrary, with a pair of Focal Micro Utopia BE all the power was needed, but in my opinion, I pushed the speakers to their limits way before doing so to the amps. Giuseppe also tested the Vivace amps with at least half a dozen other speakers, among which the Mantra Sound Daiko, Infinity 9K A, Cerwin-Vega V-152, and Vandersteen 2ce, reaching the same conclusions of a “wide range coverage”. Giuseppe’s listening goals were also focused on the interfacing compatibilities of these amps, having paired them with a few solid-state preamps, direct high-end level input coupling – CD player/DAC, and a redundant number of speakers’ cables.

The signal source was digital only for logistical purposes, consisting of an amazing CD player CEC TL53Z, utilized even exclusively as a mechanical component with the DAC of the Japanese company DA1N. I apologize, but I don’t use streaming sources, since it triggers in me a fastidious form of allergy.

The preamp was a NuForce P9, created by Demian Martin’s pen – are you perhaps familiar with the name Spectral? – and conceived to drive at their best the power amps of the American company that, like the AGD Vivace, work in Class-D.

It is a 2 chassis preamplifier that made transparency and speed its very own key statement, with a true neutral timbrical characteristic, let’s say a miniaturized Spectral, but not really.

As far as the speaker’s cables are concerned, I preferred not to push too much into the detail and the timbrical dryness, opting for an evergreen MIT 350 CVT, detailed but also very plastic and “harmonic”.


Let’s get to the point

All my previous listening of switching-mode amps topologies, given each individual qualitative distinction, had always left me unsatisfied, even in those cases in which I began listening with a certain enthusiasm a sound that can at first be impressive.

All Class-D systems reveal right at the beginning a great sense of detail and transparency, but a detectable sensation of harmonic poverty. The harmonic content of a reproduction is what most helps us to better distinguish voices and instruments, increasing the sense of reality. It is precisely this harmonic limitation that creates the illusion of transparency in previous Class-D testing, since the transparency sensation was the result of a simplification of the musical content, of a subtraction of details. And when from a context, either visual or acoustic, we subtract information, what’s left seems to us clearer and more easily perceivable. Disconnect the woofers from your speakers, and you will be convinced that you are listening to the medium-high spectrum with greater detail and transparency, while on the contrary, you are listening to fewer things. That is what made me believe, before today, that the Class-D amplifiers’ sound was similar to watching modern cartoons on a high-resolution screen, with a great richness of details, but an undisputable sense of fiction. I see really clearly, but what I see is not real: now go ahead and substitute the verb “see” with the verb “listen”.

With the AGD Vivace, I would conclude, the step ahead in the solution to such a problem seemed to me well solved, since the harmonic content is fully present in a quantity totally comparable, if not better, than the best traditional solid-state Class-A or Class-AB amplification systems.

Probably, the harmonic richness of my Triode Mastersound 845 Reference amplifier is possibly a step ahead, but the result is totally aligned to the class of this product, and of similar products, which go from being very expensive to astronomically expensive. The sound reproduced by the AGD Vivace is rich, but extremely balanced, linear but not sterile, and most of all, equipped with a very impressive level of detail. All the elements of the audio spectrum are clearly described, and fortunately with a superlative linearity which never brings up unpleasant timbrical nuances.

The Vivace amps never screech, not even with the worst recordings of the 90s: I was even able, with the Klipsch Cornwall, to listen, at a sound pressure of a stadium, to the first U2 album Boy, which seems recorded with my grandma’s open-reel Italian tape recorder Geloso from inside a hallway.

The middle range always seems extremely well contained, never overwhelming, not even with the horns of the Klipsch, always very balanced and very clear.

The transparency also reaches impressive thresholds, allowing me to easily listen to all the single lines of instruments, but never pushing to that sense of structural disconnectedness coming from musical instruments each playing in separate rooms.

This, mostly in the classical music, helps to immediately recognize each section of the orchestra, scanning with apparent easiness the different textures of the musical content – it is easy to recognize how a choir is positioned in front of the orchestra, but also “how much” in front of it – in a homogeneous reconstruction, with the right blended mix that we attribute to a live event.

And here the “old” Class-D would miserably fail, mostly under the effect of a certain tendency to flatten the musical scene, never so structured like with the Vivace, whose clarity and introspective ability extends also to the objects located far back. The image appears therefore very deep and definitively broad, but without any loss of information and focus, which is generally very high. The sense of very distinctive lightness never constricts or deems the mid frequency range, in the same way in which the high-end extreme of the audio spectrum seems really extended and detailed.

The high frequencies are very subtle and distortion free, while in the low-end the adoption of a switching power supply made me fear a certain lightness. Those fears were instead totally and absolutely dismissed by an excellent extension and solidity of the lows: even with the Heresy I was able to hear well down the 40 Hz, which is quite unusual.


So, control and extension, micro-contrast and detail, broad and precise musical stage, very fine texture: thus, all parameters that characterize an excellent amplification are entirely present. It is a distinct solid-state sound, physical and yet fascinating, for the ability to reproduce without omissions everything that is fed-in, and to drive the speakers to an unthinkable level of performance. The finesse achieved by the “humble” Klipsch, thanks to those AGD Vivace amps, was almost emotional, when listening to such speakers that, with just a few watts, are capable of making the walls vibrate, and playing string quartets or baroque music with a micro-contrast, detail, and mind-blowing timbrical fidelity, worth a world Guinness record.

The only comment worth mentioning is that, with the Focal, I sensed a speed not really at the maximum level, as if the great control, key peculiarity of the Vivace, sometimes trespassed into a containment of the perceived dynamic, and not really because of a lack of driving capabilities, but indeed because such extreme balance that characterizes the Vivace, turns at times into a sort of holding-back effect. Editor’s note | This might be probably due to the low impedance of analog input of the Vivace units under test. The input impedance of these units was set at 600ohm, for the PRO applications, instead of the standard 40kohm of the hi-end version. With solid-state preamps and/or high output level sources, this effect noted by the reviewer was totally absent.

The advantage of that, anyway, stands in the absolute – and I underline “absolute” – lack of listening fatigue, since everything flows smooth and fluid, definite and clear, but also rich of all boundary elements that often make a difference between a sterile and a complete full-bodied sound. There is no musical genre that resulted not congenial to the Vivace, from Nirvana’s rock, with Cobain’s voice just a little younger than the original, to the classic Bruckner’s music, in whose symphonies the musical paroxysm of some passages was untangled with an outstanding easiness, without ever trespassing into confusion or lack of details.

It is indeed with the classical music that one can benefit of the great introspective qualities of those amps, that stand uniquely capable of describing everything with richness of details, and never artificially doing so.

When finally, one gets into the Jazz realm, it’s even too easy to follow the melodic lines of the various instruments, in particular the piano, never exaggerating in the middle-low and always well distinguishable.

Perhaps never in my life had I listened to the accompanying part of Red Garland’s blues in Soultrane so clearly: defined, and perfectly extracted from the music background.

When it comes to the voices, then one can experience an exceptional sense of realism, with a clear and supremely defined vocal dimension, specifically in the opera, where it is very easy to perceive the cantato’s lyrics.

I will spare you the plethora of listened recordings, some of which, I must admit, revealed to me so much new sound with the AGD Vivace, as if the bar of what is physically perceivable had been raised a little higher.


About the “change”

you might skip this part right away

While evaluating the AGD Vivace Monoblocks, I was reflecting days ago on the concept of change in the present global society. Nowadays any change is considered like a positive value, independently from its content. In the billboards recently posted in some cities for the imminent European elections, many of the slogans focus on the change, but without telling us what they want to change and how (…a little marginal detail). In this case, the change is seen as a sign of progress “per se”, without evaluating if the “change” is synonymous of regression, like unfortunately sometimes it happens.

We change anyway, no matter what, because changing is “per se” a good thing.

If after that my bank account goes from positive to negative, it doesn’t matter.

If moreover I change job earning less and working more hours, it doesn’t matter.

If I move out and go to live from a downtown attic to a suburban garret, it doesn’t matter.

One has changed; therefore, one did what was expected to be done.

Let me call it for what it is, a big cheat.

I’m afraid that, with this philosophy of changing at any cost, we may risk worsening the status of our society, even the one of singular individuals, badly impacting values achieved perhaps with a huge effort. If you don’t change, you are frozen. But if making a move means proceeding towards a precipice…well, thank you very much, I was better off just staying put.

Changing is only worth an improvement, in the literal and broad sense of the word. If I am doomed to get worse, I maintain, if I can improve, I change, so I truly believe.

Back to the point, within the current empty culture of changing, testing these amplifiers made me indeed reflect on the fact that they really represent a fundamental improvement of a relatively young technology, which in summary was in need of improving and therefore of changing.

Alberto Guerra is an authentic professional, not an amateur, and he gave his contribution to a true authentic change, he improved a technology.

So, I am also thinking of how many people are convinced to improve their audio system, while in reality they are downgrading it.

Starting from today, I consider the Class-D amplifier topology a mature technology, in the sense that it has proven to be capable of delivering a level of sound quality which stands at the very top.

This is the type of change that I like.


I think I said, if not all, a lot.

The Vivace do cost a lot, but give a lot, they have many competitors among excellent brands in the field of amplification, and therefore they must compete with the best, but they are up for the match, offering also the advantages of Class-D, that, let’s not forget, can claim a disarming user-friendliness, considering the almost zero heat produced – and I, using a class A with triodes, know a little bit about that – and insignificant energy consumption.

Their performance in our testing room has achieved the highest level in respect to some parameters and absolutely at top position in respect to others, and always giving the impression of a super performing product.

If I had to evaluate an amplifier based on its ability to make people want to listen to music, I’d have to say that with the Vivace I always regretted when the time came that I had to turn them off, especially when listening at night, when the great clearness and resolution of those amps matched the great revealing ability of the horns-based speakers. With this type of speakers, the Vivace unexpectedly revealed to be very synergetic, and that for me is enough to consider them universal amplifiers, even outlining potentially some variability of results, depending on the speaker utilized, which always happens.

In my opinion, a stellar amplification.



Evaluation grade from * to *****


Tonal richness ****1/2 | Neutral without being aseptic. Great extensions and linearity with a mid-range moderately in the background and equipped with great clarity. Absent of artifacts. Full harmonics range in line with the top line solid-state amplifiers – conventional topologies.


Dynamics **** | Of excellent level, even though slightly variable with the change of speakers load impedance, better anyway the micro-contrast.


Details ***** | At the maximum levels achievable with any advanced technology today. Details that remain hidden with other amplifications are here easily perceived.


Transparency *****| Same as above for details: we are at the maximum obtainable today. All Instruments well outlined and yet connected in an excellent musical blend.


Image ***** | Extremely deep and broad, and yet very precise, thanks to an excellent scansion of different musical layers.


Speed **** | High speed, even though not the absolute best. This is the element that resents more of the context in which they are inserted, see Editor’s note.


Design ****1/2 | Accurately designed and constructed, as to be for an object of precious value. Outstanding and unconventional appearance, therefore absolutely not ordinary.


Proportion quality/price **** | Definitely not low-price, but here we are dealing with technical value and performance that justifies it. Of course, there are many alternatives, but if you opt for a Class-D, none sounds so well.



Technical Specs declared

Class-D Monoblock Power Amplifier with GaN, Gallium Nitride, MOSFET 200V-70mohm power Stage GaNTubetm technology

Output Power: 100W at 0,01% THD+N, 20Hz÷20KHz, 8ohm

THD+N: <0,005% at 1W/1KHz

Maximum Output Power: 200W at 0,1% THD+N, 1KHz, 4ohm

Output Current limit : 31A

Bandwidth: 5Hz÷100kHz ±3dB

Input impedance: 40kOhm standard XLR and RCA, 600ohm PRO version as tested

Gain: 23/29dB RCA/XLR

Efficiency: >94%

Noise level: ≤ 45μV Weighted A

PWM Frequency: 400kHz to 768kHz

Signal to noise ratio: >120dB

Dimension: 279x127x279mm LxHxD

Weight: 10kg/22lbs

Input Voltage: 110-240V manually selectable

Equipped with Furukawa Electric super pure copper multi conductor power cable Made in Japan

Packaged and delivered in Pelican Professional Case and individually packaged

For additional Info: AGD Vivace Manual available to download here


Distributors: Direct sales, Looking for European Distis, AGD Production web site

Retail price at the time of the review: 15.000,00 USD pair

Test Setup: by Paolo Di Marcoberardino

by Paolo
Di Marcoberardino
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