AGD The Audion Monoblock Power Amp Review
Jay Garrett investigates an intriguing pair of amplifiers where all is not as it seems…
The Audion Monoblocks
Monoblock amplifiers are often regarded superior to their stereo siblings, as each amplifier has its own dedicated power supply and specific, one-channel job to do. This means there’s little risk of crosstalk or interference from the other side, and explains why many stereo power amps are actually mono designs, albeit in one case. However, AGD's £7,895 The Audion could confuse those who sit on either side of the tube/transistor fence, as it sports the aesthetics of vacuum tube amplifier designs but is in fact a solid-state Class D device.
Now, before the wailing and gnashing of teeth ensue, this is not something pretending to be a valve amp. Nobody is setting out to deceive here, contrary to what anyone might think. Instead, AGD's Alberto Guerra wanted to offer audiophiles a Class D amplifier using GaN MOSFET transistors that was attractive but also enabled the owner some simple customisation. Modular amplifiers are not a new thing, but these often remain boxy affairs with equally boxy modules that can be swapped out, usually by a service centre - The Audion does things differently.
AGD openly admits that the KT88-sized glass tube pays homage to the original Audion triode vacuum tube, dating back to 1906. But, in the case of The Audion from AGD, it allows you to effectively 'tube roll' the Gallium Nitride MOSFET power amplification unit, which is captive within that glass tube. AGD is on to its second iteration of its so-called GaNTube, and owners of the first edition can buy the new ones to fit in their existing amplifiers, bringing them up-to-date with the latest version featuring an improved layout and design of the GaN MOSFET board. Indeed, Alberto also has plans to offer various interchangeable 'tubes' for the full 'tube rolling' experience. Unfortunately, those supplied with The Audion's larger siblings, the Vivace and Gran Vivace, won't go into The Audion.
Now, where's that other elephant? Firstly, Class D does not stand for Digital. 'D' was simply the letter that followed Classes A to C, and was created by British scientist Alec Reeves in the nineteen fifties. While Class D is often found in budget offerings due to its thermal efficiency – which makes for cool running, obviating the need for expensive heatsinking – it’s starting to be used in high-end designs. For example, Bruno Putzeys' high-end Ncore offerings under the Mola-Mola brand are serious bits of kit.
So, why Gallium Nitride (GaN)? Conventional Class D designs find it challenging to create perfect square waves fast enough, supposedly. I am informed that GaN transistors offer up to one hundred times the switching speed compared to the silicon MOSFET transistors typically used in amplifiers. The result is, apparently, a perfect square wave that contains all of the sound, as well as the harmonics of that sound. Furthermore, the square wave is filtered back to a perfectly preserved, distortion-free sine wave by a simple analogue filter at the amplifier's output.
Moreover, GaN transistors are said to have inherently lower noise thanks to fewer passive effects from the likes of capacitance, resistance, and inductance. Finally, GaN transistors can produce higher power outputs in a smaller package. This power density is the reason for The Audion boasting a greater than 94% efficiency rating. Oh, there's also no warm-up time required - unlike some other designs I could mention, including my own Class A Gryphon Essence, which really does come on song after stretching its legs a bit.
The Audion also features a trick switching power supply that utilises 20,000uF of filter capacitance to deal with the challenges a Class D amplifier section can bring. That's plenty when you consider this is powering something as efficient as the GaN amplifier, which typically renders large amounts of DC filter storage capacitance unnecessary.
Diminutive it might be, measuring 7.5x5.5x7.5-inches (HxWxD), but The Audion dishes out a claimed 85 watts into an 8 ohm loudspeaker, which is said to double to 170 watts into a 4 ohm load. Basically, most speakers will be pushed along merrily by these mini monoblocks, aided by 30 amps apiece of current delivery. “But Class D is notoriously noisy”, you say? The Audion has a noise floor that’s claimed to be better than -130dB. All from something weighing just 2.5kg each, presented in a well-padded Pelican-style flight case.
While not as muscular or full in their presentation as my Gryphon Essence power amplifier, The Audion monoblocks performed remarkably well with the added benefit of not giving me a mischief if and when they needed moving – unlike the 45kg Gryphon! With IsoTek power cables plugged in and XLR and speaker cables from Jorma Design's Trilogy range connecting The Audion to a Gryphon Essence preamp at one end, and a pair of Audiovector R 3 Arreté speakers at the other, the little AGDs looked rather smart in the glow of their virtual tubes.
The Audiovectors aren't particularly tricky to drive, with a claimed 90.5 dB/W/m sensitivity and 8 ohm impedance, but I have grown accustomed to the Gryphon's mastery over the Danish loudspeaker's drivers. Incredibly, this pair of The Audions was able to display a similar level of control, albeit not with the same amount of impact and dynamism. I think it's also fair to state that the Essence power amp costs around £10,000 more than the pair of AGDs.
Initially, there was a little cognitive dissonance from seeing what looked like SET amplifiers yet hearing output without that rounded, warm, and slightly veiled presentation one expects from tube-based designs. However, once past that, The Audion has a lighter disposition than my reference power amp that brings with it a sense of openness with a sprinkling of sweetness.
The opening bars of acoustic guitar in The Verve's Lucky Man was perfect fare for The Audion and was rendered wonderfully. As the band joined in, singer Richard Ashcroft's voice was presented richly with plenty of character. The violins swooped in and out, suspended slightly above the band in a decently high and wide vista buoyed by an illuminated midband.
Piano was also faithfully rendered with speed and depth, as the Bösendorfer of Tori Amos demonstrated as I played Yes, Anastasia. Here, the dynamics of the piece were clearly presented, making the differences in character during the various parts of this nine and a half-minute track easily discernible. Ditto the weight of the hammers striking the strings, and whether the soft, sustain or damping pedal was being employed. At around the five-minute mark, the song erupts with the full string section, enabling The Audion pairing to show its skills in separation and placement, which elicited a nod of approval from yours truly.
During the playback of this favourite of mine from Under The Pink, it was also great to hear that the AGDs were kind to Tori's voice and retained the energy, passion and subtleties in her vocals that can go missing through lesser amps. Additionally, on her higher notes, although there was some audible smoothing to the very top, it was certainly preferable to the amps creating any harshness in the attempt to eke out more detail than they were capable of.
Although The Audion did deliver low frequencies quickly and accurately when needed, it seemed to lack the visceral weight of my reference amp. However, that didn't lessen the enjoyment of 1000 Light Years From Here from Prince's Welcome To America. This was spun on my VPI Prime turntable via a Nagaoka MP500 moving magnet, on the end of a 12-inch Integrity Hifi True-Glider arm, plugged into a YBA Genesis PH1 phono stage. The production of this album no doubt helped here, but the kick drum was impactful and snappy through the Audiovectors. The metalwork was also well represented, with splashes of cymbals seemingly drifting away into a limitless black background. In contrast, the crack of the snare drum was instant and cut effortlessly through the mix.
Once again, The Audions were able to studiously mark the ensemble within a reasonably sizeable audio image. Indeed, the various players, including the three backing vocalists, were ably presented with plenty of space around them, allowing the listener to place them within the soundstage and draw out individual elements without much trouble.
Can The Audion challenge the best that Class A or AB has to offer? Not quite, in my opinion. That said, not everyone wants or needs a power-hungry amplifier that's capable of heating a room when played for any length of time. However, The Audion from AGD offers a great sounding pair of mono amplifiers with an aesthetic that makes many audiophiles go weak at the knees.
Factor in the advantages that GaN transistors offer, along with the option to swap the power stages out for new ones should you wish, and The Audion offers something that few other power amps can. All of this is wrapped up in classic styling in a compact case that delivers plenty of high-quality audio power. I heartily suggest that you audition a pair for yourself.
StereoNET UK’s Editor, bass player, and resident rock star! Jay’s passion for gadgets and Hi-Fi is second only to being a touring musician.
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