December 2021

World Premiere Review!
AGD Tempo di GaN Stereo Amplifier Review
Worthy of the highest recommendation.
Review By Rick Becker

  Alberto Guerra took a dive into high-end audio using his background in the development of GaNFET transistors as his springboard to success. In bold strokes he premiered his Avant-Garde style Vivace monoblock putting out 100 Watts into 8 Ohms, 200 Watts into 4, and the Art Deco-like simulated tube design of The Audion, another monoblock putting out 85 Watts into 8 Ohms and 170 Watts into 4 Ohms. The clever ploy of inserting the power modules into the glass of vacuum tubes and featuring them prominently in his designs, allowed for easy user upgradeability when new power circuits were developed. And indeed, one was developed two years later.

Pull out the old tube; plug in the new one. Suddenly you have a new, improved amplifier at a relatively small cost with a boost of power and an increase in sound quality. In 2020 The Audion was again awarded a Blue Note Award, as was the Andante, an ambitious Preamp with DAC, Streamer, and Phono Stage built-in.

But monoblocks, and especially ones with cutting edge design and technology are for the connoisseur who is willing and able to spend large for two chassis, two power supplies, and eventually, two premium power cords. Just as Porsche saw the wisdom of developing SUVs, a four-door sedan, and now an EV, Alberto knew he had to follow his flagship designs with products having a broader appeal to ensure the financial stability of his company.

About AGD's Tempo di GaN Stereo Amplifier
This brings us to the AGD Tempo di GaN stereo amplifier reviewed here. I was expecting a conventional single chassis stereo amplifier, but I was not expecting it to be as beautiful as the petite polished jewel box I pulled out of the accompanying flight case, complete with a monogrammed polishing cloth, white gloves, and a substantial power cord. I'm old enough to know that presentation isn't everything, but in this case, I was impressed. I hooked it up and let it cook for a couple of days while I admired the AGD logo on top and the Tempo on the face, deeply engraved in the solid billet aluminum chassis. Very rich. The feet were substantial and intentionally engineered to further reduce vibration. The resolution I was hearing suggested I had no need for superior footers, although the amp was placed on a Symposium Acoustics Svelte Shelf that certainly offered a degree of isolation.

The size was impressive, too. I kept thinking "jewel case" as if it were filled with expensive gemstones. The polished aluminum chassis obviously contributes to the allusion. It is way smaller than a typical Class A or Class A/B amplifier. Class D simply does not have to be that large. In opening another manufacturer's Class D amp some years ago, the famous advertising cliché came out of my mouth: "Where's the beef?" The billet chassis of the Tempo contributes to the heft, but I was quite certain the contents are fairly compact to keep the signal paths short.

I was reluctant to open the amp to photograph the internals, but a couple of photos on their website give you the general idea of the complexity and density of their PC boards which are said to be state-of-the-art. Alberto supplied the photo below to show the circuit boards in relation to the size of the chassis. There is not a lot of wasted floor space inside.

AGD's Tempo di GaN stereo amplifier solid 11 pounds combine with the grippy feet to eliminate any tendency to slide around on the shelf or twist from the pull of a heavy power cord. (The Audion monoblocks, for comparison, are just under seven pounds each.) All told, it feels like a very substantial product and the polished finish makes it look far more expensive than it is, despite the $500 upcharge. A word of caution is in order about those white gloves, however. While they may be useful for keeping fingerprints off the chassis, the cotton gloves and the polished finish make for a slippery combination. Personally, I prefer to use my padded fingerless bicycling gloves for a secure grip with audio gear and use the microfiber cloth to wipe off any fingerprints that accrue.

The business side of AGD's Tempo di GaN stereo amplifier is neatly and conveniently laid out with the left and right channel inputs and outputs being mirror images of one another. The power inlet is offset to the right side (when facing it) and thankfully the inlet is a more conventionally designed unit than the original The Audion. (Alberto informed me the new Mk II version of The Audion has a similar simplified AC power inlet module.) This power module has the familiar slide-out fuse holder that will be appreciated by folks who like to upgrade with aftermarket fuses. I've reviewed a hand full of fuses over the years and I'm a big fan of Synergistic Research's Purple fuse, though I haven't yet tried one with the Tempo.

Also on the back are two toggle switches — one to select either RCA or XLR inputs, as they are completely separate. The other switches the amp from stereo operation to bridged mono. The right channel RCA input is labeled "bridge – " in white and the left channel RCA input is labeled "bridge +" in red so there is no confusion about how to convert it. Just reconfigure your interconnects and flip the toggle switch and you've got a monoblock amp that puts out 200 Watts into 8 Ohms and 400 Watts into 4 Ohms. Buy two Tempo amps for a stone and a half more than The Audion monoblocks and you sacrifice the intriguing design of The Audion for twice the power. How cool is that? Unfortunately, I only had one review sample so I couldn't try this out.

Play It Again, Sam
After the Tempo had been sufficiently burned in, I reverted to The Audion to establish my reference level, playing all the usual suspects. Also by this time, the Purple fuses in my Calyx 24/192 hot-rodded DAC and Coincident Speaker Technology Statement Line Stage were in full blossom. The rig was at an all-time high level of performance.

When I switched to the Tempo, the most recognizable difference was in the slight homogeneity of the soundstage. With a stereo amp there is inevitably a small amount of cross-talk between the channels that manifests itself with a slight loss of pinpoint imaging on the soundstage. After a couple of songs, I just didn't give a damn. It was a pretty small difference and I'm not sure I would have picked up on it if I hadn't swapped the amps myself. The soundstage had the same dimensions and the transparency was the same. The speed of the Tempo, the attack, the decay were all virtually identical.

If I were to be nit-picky, I could say The Audion had slightly more bloom and a little better sense of the acoustic volume of the music's venue. But I can also say that was very likely the benefit of the Purple fuses in the monoblocks. Acoustically, they were as similar as identical twins. Did I enjoy the music just as much? Absolutely! I could justifiably send you over to my review of The Audion heree if you want to know more about the sound. Just scroll down to "The Sound Dimension" heading in The Audion review. But the story takes a twist.

The polished Tempo had to go back to Alberto and on to the Capital AudioFest. He was pleased to learn I thought it sounded virtually identical to The Audion and promised to send me another one after the show. At the time, I wasn't sure I would be going down to the show because of Covid. As it turned out, I got a booster shot and went at the last minute, limiting my visit to Friday to reduce my exposure.

Back In The Black
An' here I sit so patiently; Waiting to find out what price;
 You have to pay to get out of; Going through all these things twice.
  —  Bob Dylan, "Stuck Inside of Mobile with the Memphis Blues Again"

To my surprise, and somewhat to my initial dismay, Alberto sent me a new AGD Tempo di GaN stereo amplifier after the show in the black anodized finish. (A third finish, anodized silver, is also available.) This meant another period of burn-in and time was getting short for the December issue. I plugged the Tempo into our video rig in the family room to save a few days on the 101D tubes in the Coincident preamp. We used a custom interconnect cable from Audio Sensibility with a 3.5mm stereo jack on one end and RCAs on the other, coming from the headphone output on the Sony XBR big screen. This enabled us to control the volume with the TV's remote. At night we set the TV to a channel with ambient dreck and let the amp cook 24/7.

When Linda came into the room her first words were "Oh, how much does that cost?" in a tone indicating a very positive WAF. The Tempo is decidedly smaller and less conspicuous than the large Eddie Wong integrated amp that bristles with eight tubes and four transformers. The Tempo is also considerably more powerful than Eddie's amp which puts out six single-ended Watts per channel from two EL34 tubes. With the AGD Tempo di GaN amplifier, the Coincident speakers snapped to attention with higher resolution yielding more intelligible dialog and greater dynamics with tighter bass. The black anodized finish that was less attractive amongst the bling of my big rig was pleasantly unobtrusive in the Adirondack décor of our family room.

The red AGD logo on the power button on the face of the amp drew just enough attention to remind us to turn it off at the end of the evening — though we never did so in the video rig. A switch on the front puts the amp into standby mode and the power switch above the IEC inlet on the back turns the amp completely off. There is also an input and output on the back for trigger cables if you use the amp in the context of a video system. When left in standby mode the Tempo warms up quickly and the difference in sound quality between standby and peak operating temperature is small.

Usually, I futz around for just a minute before starting to listen and after a song or two, it's good to go. It's not at all like warming up my tube amps for twenty minutes before listening to music. And what a smooth quality feeling you have when activating the red power button.

The Form Factor
Once again, to learn more about the sound of the Tempo, go to the review of The Audion and scroll down to "The Sound Dimension" header. While the sound is almost identical, the result is accomplished differently. Missing is the vacuum tube look of their more expensive models. Inside you will find a completely redesigned power section on a very high-grade PCB board. But like the higher-end models with their power stages housed in replaceable glass tubes, this too is replaceable when higher grade GaNFET transistors are developed. So you are protected against evolutionary obsolescence to a large degree.

Another change from The Audion is the speaker cable binding posts which are their design. They protrude from the chassis about an inch and now have a large hole to accept bare wire connection in addition to spade and banana plugs. The offset (from vertical) alignment of these posts makes them easy to attach either spades or bananas and despite the diminutive size of this amp, it was easy to attach speaker cables and interconnects.

The design of the billet chassis with its smooth, rounded edges, beautiful proportions, and superb finish (either polished or black anodized), the tactile red power switch on the front, the quality footers, and the fittings on the back all come together to make this a highly coveted amplifier that could easily become a "forever" component.

Bigger Is Better?
Devialet set the high-end on edge with its slim all-in-one products back in 2007 and that category has taken hold to a modest degree. Manufacturers of individual components like AGD have responded by creating products of a smaller scale and combining multiple components in a single chassis. AGD does this exceptionally well with their Andante Pre-DAC-Phono-Streamer reviewed here), and others strive for this as well. As Boomers age and downsize their living arrangements and younger folks enter the High End with their values of minimalism and simplicity, this trend will likely flourish. This doesn't mean everybody will want smaller gear, but I see this trend expanding. Inflation and the scarcity of resources will likely contribute to its popularity. Couple the Andante with the Tempo and you have a very modest size rig to deliver high sound quality in whatever level of décor you prefer. Choose a speaker, choose a turntable, add a NAS.

We've all been told that Class D is very efficient and AGD boasts of their 94% efficiency. The chassis becomes only warm to the touch, even at loud playback levels. In fact, the chassis is not even ventilated. There are no slots or holes to provide a flow of air. Additionally, the Tempo is capable of driving mean speaker loads, limited only by the maximum output current of 30A (same as the Audion). But a closer look at power consumption is warranted. Most of the music we play requires only the first few Watts of an amplifier.

Class D is a switching topology circuit, and as with all switching circuits, the efficiency is a function of the load, getting up to >95% at higher power and dropping to 75% or less at lower levels. Alberto provided this explanatory diagram to illustrate the concept, in this case comparing Gallium Nitride (GaN) vs. silicon (Si) MOSFETs.

With zero signal input (hence zero current out), the Class-D power stage switches at a constant frequency (>600kHz for the TEMPO, which is commendably high) and with a duty cycle of 50%. In this condition, the only power losses are related to the switching characteristics of the components of the output stage. These power losses are around two to three Watts only. At higher power output the efficiency increases. Let's be grateful for this savings. The real benefit is that it will not overheat your room in the summer and will save the energy required for air-conditioning.

Musicality?
You will notice that I haven't said anything about the musicality of this amp. There is a reason for that. Simply put, the AGD Tempo di GaN stereo amplifier redefines that term in a significant way that begs for a high-quality front end as well as a loudspeaker capable of the speed and resolution this amp can deliver from top to bottom. I should mention deep bass in particular.

One of the listening tests I ran was with "Master Tallis's Testament" from Pipes Rhode Island, an organ piece with some 20Hz notes at the end. My Kharma speakers are rated to 33Hz and being ported, they roll off rather quickly after that. I probably should not expect to hear those notes at all, but there they were, very tight, though low in volume. With my tube amps they sound a bit louder but nowhere near as well focused. If you have speakers designed to go that low, I expect you will be very impressed. The >1000 damping factor of The Audion likely carries over to the Tempo, so this should not be surprising. Toe-tapping? Ease of listening? Emotional involvement?Certainly, but there is something even more.

Tempo di GaN?
In everyday conversation, people will call the amp the Tempo, but Alberto Guerra is a man of depth. He is well connected on Linkedin. So what's with "TEMPO DI GAN"? Being an audiophile, I suspected "TEMPO" was "tempo" as in the speed, pace, or rhythm of the music. "DI", I figured was "of" and "GAN"? Could "GAN" be "gain"?  "Speed of gain", as in the very fast rise time of the GaNFET transistors? Or am I overthinking the whole thing? It's easy to see why I'm not a linguist, I suppose. Alberto graciously explained the double entendre.

Actually, the name is "Tempo di GaN" so the "GaN" is easily identified as the Gallium Nitride FETs that are used in the amp. And yes, "di" is "of". The duality comes into play with "Tempo" which in one sense is the rhythm of music or "rhythm of GaN" which is noticeably faster than with MOSFET transistors. The rise time or attack of the recorded music seems the same as the real-time in which it was performed. The music happens "right now!" And it happens without audible overshoot or irritation. It gives the music an immediacy and presence unlike MOSFETs or tube amplification. And a sense of dynamics that are not "off the charts" but pretty much "define the charts."

All of which is to say that the proper execution of GaNFET technology is a very significant step toward experiencing recorded music as "live." It doesn't give us everything we need (yet?), but it seems to have conquered issues around pace, rhythm, and timing just as computer-regulated speed control has done for analog turntables and digital front ends. Resolution is as sharp as electrostatic speakers give us in the midrange and treble, and stays just as sharp into the low bass.

If you get the sense that I'm telling you GaNFET transistors are a game-changer, you've stumbled onto the second half of Alberto's double entendre. "Tempo" is also "time" and yes, this is the era of GaNFET technology — not only for high-end audio, but other fields as well. There will always be lovers of tube technology, just as there are still lovers of sailboats, but if you are in the solid-state camp, either as a manufacturer or a listener, you need to be listening with GaNFETs.

Summary
There are already several companies putting out GaNFET Class D amplifiers. I've met Merrill Wettasinghe and heard the Merrill amps in a very high-end system that sounded terrific. And Underwood Wally is slowly releasing their new Living Sounds Audio Voyager GaN 350 amp at a lower price point. I expect each will have its own sonic signature but all should have the speed, transparency, and resolution that make this technology stand above traditional MOSFETs. Each of these companies has their own niche in high-end audio and others will no doubt join them in the future.

What makes AGD's GaN amp so special is their superb sound quality, upgradability, and outstanding design and quality of the physical product that endears pride of ownership. That they do all this at what is still a reasonable price makes it an outstanding value. Alberto Guerra is clearly one of the rising stars of high-end audio and as word gets out and people experience what his amplifiers can do, I expect the AGD brand will be up there among the most respected and revered.

AGD's Tempo di GaN stereo amplifier is based on The Audion monoblocks which I own as one of my "forever" components. I look forward to using both of these amplifiers to bi-amp a very unique loudspeaker in the immediate future so expect to hear more about them. Both of these amplifiers are worthy of the highest recommendation.

Specifications
Type: Class D solid-state stereo amplifier
Output Power 100 Watts per channel @ 8 Ohms (200 @ 4 Ohms)
THD+N: 10W @ 1KHz <0.01%
Frequency Response: 5Hz to 80KHz (+/-3dB)
Input Impedance: 40 kOhm (600 Ohm upon request)
Efficiency: >94%
Bridge Mode Output Power: 4 Ohms @ 400W
PowerGaN PWM Frequency: ~600kHz
Dimensions 11" x 3.75" x 7.1" 
Weight: 11 lbs.
Price: $5500 in standard anodized silver or black finish
        $6200 in polished aluminum finish, by special order

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

Website: www.AGDproductions.com