Country of Origin
Reviewer: Srajan Ebaen
Financial interests: click here
Sources: Retina 5K 27" iMac (4GHz quad-core with Turbo boost, 32GB RAM, 3TB FusionDrive, OSX Yosemite. iTunes 14.4), PureMusic 3.02, Audirvana 3, Qobuz Hifi, Tidal Hifi, COS Engineering D1, Denafrips Terminator, Soundaware D300Ref as USB bridge/SD transport
Preamplifier: Vinnie Rossi L2 Signature with WE 300B/VT52 or Elrog ER50/300B; Nagra Classic, Wyred4Sound STP-SE Stage II, COS Engineering D1, Vinnie Rossi LIO (AVT module)
Power & integrated amplifiers: Pass Labs XA30.8; FirstWatt SIT1 monos, SIT3. F5, F6, F7; Goldmund/Job 225; Wyred4Sound mINT; Nord Acoustics NC500 monos; LinnenberG Audio Liszt monos, Bakoon AMP-13R, SAG AIO
Loudspeakers: Audio Physic Codex; Cube Audio Nenuphar; Kroma Audio Mimí; EnigmAcoustics M1; Albedo Audio Aptica; EnigmAcoustics Mythology 1; Boenicke Audio W5se; Zu Audio Druid V, VI & Submission; German Physiks HRS-120; Eversound Essence; Fram Midi 150 & 120; Acelec Audio Model One; Node Audio Hylixa [on review]; Rethm Aarka [on review]; sound|kaos Vox 3f [on loan] Cables: Complete loom of Allnic Audio ZL 3000; Zu Event; KingRex uArt double-header USB cables; Tombo Trøn S/PDIF; van den Hul AES/EBU; AudioQuest Diamond glass-fibre Toslink; Black Cat Cable redlevel Lupo; Ocellia OCC Silver
Power delivery: Vibex Granada/Alhambra on all source components, Vibex One 11R on amps/sub
Equipment rack: Artesania Audio Exoteryc double-wide 3-tier with optional glass shelves, Exoteryc Krion and glass amp stands
Sundry accessories: Acoustic System resonators
Room: 4 x 6m with high gabled beam ceiling opening into 4 x 8m kitchen and 5 x 8m living room, hence no wall behind the listening chairs
Review component retail: $15'000/pr for Vivace, $7'500/pr for Audion
Alien Gimmicky Design? It's absolutely not what AGD Productions stands for. But with only a brief glance at micro transistor PCB embedded back to back inside a KT88 glass envelope replete with eight working pins… well, you'd be forgiven for such a faux pas. Yet read designer Alberto Guerra's CV—add 'Design' to his initials and the company name is clear—and you quickly appreciate just how serious his background in hard science and hi-tech is. In it you'll come across International Rectifier now Infineon where he worked with new semiconductor materials and emerging power conversion technologies. You'll see work with integrated circuits at Vishay Siliconix. You'll read about 10 patents, two of which are for novel power module topologies, two for GaN driver ICs and GaN power Mosfets. Being Italian, Alberto also inherited his nation's famous flair for design. Clearly his class D Vivace monaural amplifiers with gallium-nitride transistors couldn't help but look like modern art in very unique fashion. And if it hadn't occurred to you yet, mounting an output stage inside a removable glass envelope makes it replaceable should upgrades develop. Styling it like a tube simply adds visual pizzazz to a modular feature. Try that with transistors inside a hermetically sealed case like Merrill Wettasinghe's just reviewed Element 114. Not!
But like the Merrill stereo amp, the same-priced more compact but twinned 100/200W into 8/4Ω Vivace mono amps exploit new power devices not in silicon. Those can turn on/off far more rapidly than traditional Mosfets do in class D switching output stages, here at up to 768kHz PWM. About their own GaN parts [example at right, not what's in Vivace], Infineon say that they "offer fundamental advantages over silicon. In particular the higher critical electrical field makes it very attractive for power semiconductor devices with outstanding specific dynamic on-state resistance and smaller capacitances compared to silicon switches. That makes GaN great for high-speed switching." A PDF by Efficient Power Conversion Corp. explains the science behind these lateral parts which are promoted as replacements for the ubiquitous silicon-based Mosfet. "The lateral structure of the GaN transistor makes it a very low-charge device. It can switch hundreds of volts in nanoseconds, giving it multiple megahertz capability. This will lead to smaller power converters and higher-fidelity class D amplifiers." This link goes to an AES paper on the subject, this link to Texas Instruments.
By late 2019, Merrill's Element amplifiers, Alberto's Vivace & Audion, Technics amps and Java Hifi's integrated all had boarded the GaN train. NuPrime and/or NuForce were said to be imminent, the LSA Electronics Voyager and Leonid Ayzenshtat's Orchard Audio Bosc amps were about to bow. As US dealer Mike Powell of Verrastarr told me who first suggested this assignment, "I'd not heard anything like the Vivace when I first auditioned them. I see big promise for their GaN technology. One thing to note? AGD use a patented GaN which no competitor has access to. In fact, Alberto was involved in its design which is optimized for audio not radar. Also, this circuit is his very own, not someone else's OEM module." Given AG's background, that didn't surprise. No MeToo entry here.
For his GaN power stage, Alberto claims virtually zero stray inductance and 15ns fully symmetrical rise and fall times. "Even with a 3'700V/μs slew rate, the output waveform at the switching node of our power stage ahead of the LC filter is completely free of oscillation." That's wicked fast and squeaky clean. His published graphs show a noise floor of ~120dB for proper 20-bit resolution in a high-level circuit where thermal noise remains the limiter. Capacitance is 48'000µF. Other specs are less than 0.005% THD+N at 10W/1kHz, 5Hz-100kHz bandwidth -3dB, better than 94% power efficiency and A-weighted output noise of 45μV.
All of it fits into a milled billet case of 11×11" footprint with just 5" of height without the GaNTube™ and a smoky hand-polished acrylic truncated pyramid whose lopped-off top seats the 8-pin output stage lit back in dark orange from below. So weight is a solid 10kg. Global voltage selects between 115/230V by changing the fuse cards of the IEC power inlet. That adaptability hints at the ZVS-oscillator high-voltage SMPS under the hood. RCA or XLR select via toggle between the winged binding posts.
Vivace carries an official CE and FCC marking from the G.S.D. test lab in Pisa/Italy, thus conforms with all the regulatory emission controls of Europe. Alberto's portfolio also has his tiny $7'500/pr Audion monos which slightly scale power down to 85W, then seriously shrink and simplify the Vivace enclosure whilst using the same GanTube™ at 400kHz.
In short, a more than cursory tour of the Vivace quickly dispels any potential first 'gimmick' reactions tied to the faux valve that's proudly emblazoned with AGD's oval logo. One soon understands that this is a very serious attempt at a modern statement amplifier which just so happens to include novel industrial design and a fresh solution to making a solid-state output stage modular. To be complete, here one must mention the LISST device. That packs transistor outputs into metal canisters with tube pins to become plug'n'play alternatives for octal tubes in Schiit's Lyr, Mjolnir, Saga and Freya models.
"There are more things in heaven and earth, Horatio, than are dreamt of in our philosophy."
At RMAF 2019 with Sigma Acoustics Maat Vector XAC speakers, dual 15" woofers per side. On lower shelf, new fully balanced AGD Andante preamp with MM/MC phono, 4 analog inputs, 3 analog outputs,
256-step passive resistor volume, 32/384 R2R DAC with DSD, HD AptX Bluetooth and LAN streamer; Audion monos on outside, Vivace between.
To learn more not just dream, I contacted Alberto. "The first question might be why you decided on class D in particular. Wouldn't GaN parts lend themselves equally to class A or AB? Or are they particularly suited only to faster-switching applications? The second question could be how a GaN transistor optimized for audio differs from one designed for industrial apps like radar and lidar. It begs the question what device parameters were changed/tweaked from that of 'non-audio' variants which your competitors would be stuck with. Next we could dive a bit deeper into your circuit. Most shoppers are familiar with Hypex/Ncore class D and equivalents from ICEpower and Pascal. In that field, Bruno Putzeys is famous for saying 'the more negative feedback the better (if one knows how to do it correctly)'. Merrill now take the opposite approach. They claim that the use of GaN's higher switching speeds allows a stable circuit without any negative feedback yet zero dead time. Where between those two design polarities do you come down? You publish a switching frequency for Vivace that's higher than what other firms seem to use. Could you explain the benefits and challenges of switching that fast and how circuit bandwidth and slew rates factor into audible performance? At 94% claimed power efficiency and 100-watt/8Ω output, your GaN devices must need very little to no cooling that mounting them on tiny PCB inside a glass tube is sufficient thermal conduction? It's a clever way for modular power devices and unique for transistors. Do you plan on alternate 'tubes' to, like with vacuum tubes, eventually offer listeners different sonic flavors and introduce the concept of GaN rolling? Finally, I think people would like to know how, aside from slightly lower power, a simpler smaller 3.5kg enclosure, a lower switching frequency, 20'000μF capacitance and different input opamps, the Audion monos differ from the Vivace."
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