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Reviewer: Srajan Ebaen
Source: Audio Aero Prima SE; Accustic Arts Drive-1
Preamp/Integrated: AUDIOPAX Model 5
Amp: AUDIOPAX Model 88
Speakers: Avantgarde Acoustics Duo; Gallo Acoustics Reference 3
Cables: Crystal Cable Reference complete wire set of analog and digital interconnects, speaker cables and power cords; Z-Cable Reference Cyclone power cords on both powerline conditioner; 2 x Stealth Audio Cables Indra analogue & Sextet S/PDIF cable
Stands: 2 x Grand Prix Audio Monaco four-tier
Powerline conditioning: BPT BP-3.5 Signature Plus; Velocitor; Velocitor S [on review]; Quantum Symphony Pro [on review]
Sundry accessories: GPA Formula Carbon/Kevlar shelf for tube amps; GPA Apex footers underneath stand and speakers; Walker Audio SST on all connections; Walker Audio Vivid CD cleaner; Furutech RD-2 CD demagnetizer; WorldPower cryo'd Hubbell and IsoClean wall sockets; Musse Audio resonance dampers on DUO subs
Room size: 30' w x 18' d x 10' h [sloping ceiling] in long-wall setup in one half, with open adjoining living room for a total of ca.1000 squ.ft floor plan and significant 'active' cubic air volume of essentially the entire (small) house
Review component retail: $1,800


It's a solid 58 lbs. There's remote-switchable pentode/ultralinear operation for 24/18wpc of Class A push/pull power. There's adjustable NFB, a switched preamp input, four line level inputs, one tape out and remote volume and input switching. So you'd reluctantly think that Alex Yeung's first full-size heavyweight must surely be aimed at a different audience than his justly popular MiniMax Series of petite-yet-potent and very affordable tube components. Alas, our man from Hong Kong, local celebrity with the Tube Audio Society, continues to practice Confucian wisdom. "Do unto others before they do you." This piece sells for all of $1,800 directly from US importer and partner Bill O'Connell of MorningStar Audio. What for other makers is the 1-meter diving board from whence to get an introductory wet here becomes the 10-meter board. The M520 is a big splash, full contact statement-level effort within deliberate reach of real-world budgets.


Tube-rectified with one 5AR4 per channel, preamplified with an EF86 followed by a 12AU7 phase splitter for the duet of EL34s, the nomenclature M520 refers to a famous historic Mullard circuit that was implemented by such brands as Leak, Eico and Audio Master. Planted like a miniature fire hydrant in front of each tube alley atop the stainless steel cover is a 3-position toggle for min/center/max negative feedback. The motor-driven attenuator sports a thoughtful green LED to show position. The same LEDs blinks during mute when activated from the all-metal custom remote and also during the initial 30-second thermal stabilization cycle.


The business end with its aluminum plate sports the quartet of relay-switched single-ended inputs, the tape-out and switched pre-in sockets, the central power inlet and unusual 16/8-ohm transformer taps instead of the more customary 8/4-ohm connections. Solid dress plates which surround the longitudinal arrays of valves carry clearly silk-screened tube identifers and two grab rails come in handy when it's time to position this hunking integrated with the substantial transformer iron into place.


Things are just as tidy under the hood and the following close-ups will give credit to the masterful handiwork in evidence.


Known as a golden ear in Hong Kong, designer Alex Yeung voices all of his valved components to do justice to music's innate beauty. Music first, test tones ninth. He clearly doesn't subscribe to elitist snobbery either. No $500-a-pop 300Bs, not even the magic three letters S.E.T. Conversely, he doesn't seem to subscribe to the mondo power religion either. The choice of the EL34 for his current magnum opus is telling as well. It and the 6V6 are the quintessential musician's or Blues tubes, beloved by guitar players for their tone. Where others might upgrade their dearer models to go exotic by reserving the provincial EL34 for commoner's fare, Alex Yeung specifically selected it and the rarely seen EF86. That and the Mullard circuit reference aroused my curiosity. Time to admit ignorance and learn from our tribal elders. According to our own Steve Marsh, it was one of the circuits that Mullard had published in the public domain. The 510 preceded it, which meant five tubes to produce 10 watts. 520 was for 20 watts of course. The Williamson circuit was the other dominant design of the time (still mono) and later adapted by Heath for the 807 tube in its W-1 amp using Peerless iron. Our man in the UK Edward Barker owns a slight variant of the original 520, the BBC AM8/4A monoblocks which were used for most BBC recordings and playback during the 50s. According to Edward, "they are among the finest amps I've heard (admittedly these are very tricked out). The Jadis JA 30 is another variant. Here in the UK you can get them from £250 to £50,000 depending on spec. All in all, it's my favorite 'normal' amp." Bill O'Connell then kindly arranged for Alex to ring me from Hong Kong. The big sky scraper city from afar would connect with my nearly 3rd world remote outpost. Viva la technologia. Here's what I learned:


Alex's M520 differs from the original circuit by replacing the 12AX phase splitter with the 12AU7 and adding true pentode operation. As Alex explained, so-called pentode amps on the market nowadays nearly exclusively operate in ultralinear which runs the screen grid off a transformer tap at a value typically 40-43% between plate and B+ voltages. Scoping a tube connected in such a fashion on a tube tracer would reveal specs about half-way between triode and pentode operation. His amplifier uses a separate power supply rather than transformer tap for the screen grid. It allows the EL34 output valves to operate in default pentode mode which Alex prefers.


In keeping with his love of British vintage valve amplifiers, Yeung's loudspeaker of choice is the BBC LS3/5A but he also owns a ProAc Signature and has Dali Royals and a vintage Tannoy corner horn with Tannoy Silver woofers in his shop. His partner owns a pair of original Siemens Klangfilm monos and according to Alex, his new M520 offers nearly identical performance. He's quick to add that the EF86 preamp tube with the mesh-plate shielding was deliberately designed for audio rather than military or communications applications and as such, offers certain unique qualities. When used in pre-in mode, the gain of this valve is reduced significantly to turn into a de-facto driver stage. Input sensitivity then drops to about 1V and direct-drive from attenuator-fitted sources is not recommended. The Eastern Electric branded EL34s are of Chinese origin and offer what Alex believes to be excellent performance and reliability. Tube rolling with rare NOS valves is encouraged for those who like to experiment but not considered mandatory at all for full enjoyment.


Incidentally, the center feedback setting isn't exactly zero but Alex' default position for his favored amount. On the subject of push/pull vs. single-ended, Alex Yeung takes no sides nor favorites. In fact, his first one-up amplifier built for himself was a single-ended paralleled EL34 unit. Once could conclude that Yeung holds this tube in special regard. Asked about his output transformers, Alex divulged that the cores are ultra-premium Japanese issue while the windings are done in China by a team of expert old-timers with more than 20 years of experience. Because the M520 employs a single power transformer, it isn't strictly dual-mono and hence properly marketed as a stereo integrated. Asked whether adding an outboard preamplifier via the pre-in feature would add anything worthwhile, Alex merely offered that he wanted to allow owners of statement-level preamps the opportunity to season the final sonics if so desired. However, my distinct impression is that his primary vision for the M520 was to design a perfectly voiced single-box solution for ambitious music lovers on a budget. So there you have it, a new product introduction from a designer who has already proven to be blessed with exceptional ears and a real knack for voicing even affordable components to achieve more than their price tags might suggest. How would the M520 sound?