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 September 1, 2008 Audio Zone AMP-D2 Mono Amplifiers
Of course, class-A isn’t the only game in town. My reference Simaudio Moon Classic i5.3 integrated amplifier, for example, employs a pure class-A output for only the first 10W of output power; after that, the i5.3 operates in the more efficient class-AB mode. Whereas class-A designs conduct current through the full plus and minus phases of each wave, class-AB devices push current for just a little bit more than half the wave cycle. This allows a class-AB output device to operate at a low current state through a portion of its operating cycle, which therefore reduces heat buildup and increases electrical efficiency. Unlike the always-on class-A type, however, class-AB amplifiers can be subject to crossover distortion as each output transistor (or tube) ramps its output up or down. Such distortion can affect the sound, so care must be taken when designing a class-AB amplifier to avoid any sonic penalty. The newest kids on the block are class-D amps, sometimes erroneously referred to as "digital" amplifiers. Through their use of extremely fast switching technology, class-D amps can achieve very high efficiency -- better than 90% of input current is converted to output power. A class-D amplifier switches its output devices on and off tens or even hundreds of thousands of times per second. This high-speed switching is the source of the "digital" appellation, in the sense that the class-D output device is either fully on (binary 1) or fully off (binary 0). However, the signal is not converted to digital, but is amplified in the analog domain. As with the class-AB topology, class-D takes advantage of its off state to run at cooler temperatures. When switched so rapidly, output devices are never on long enough to become very hot, and what heat they do build up can be dissipated during the off cycle. This allows a class-D amplifier to output significant amounts of power -- even several hundred watts -- while needing only very small heatsinks. And because these amplifiers operate at low temperatures, less robust components can be used, and can be placed closer together on smaller circuit boards. So not only is the class-D topology energy-efficient, it’s quite frugal when it comes to chassis real estate. Without the need for parts rated to operate dependably at high temperatures, a large chassis, or massive, expensive heatsinks, class-D amps can be manufactured at much lower cost than equivalent class-A or -AB designs, and thus put more amplifier power into audiophiles’ hands for less money. Unfortunately, class-D’s thermal efficiency can exact a sonic toll. Because they switch on and off so quickly, class-D amplifiers can emit enough radio-frequency interference (RFI) to be easily audible via the average radio tuner. Combine a class-D power amp with a high-speed switching power supply, and the RFI can spill over into the power line, contaminating other audio components with unwanted noise. These are worst-case scenarios, however; in practice, RFI is a problem only in poorly designed class-D components.
Indeed, the contest to extract audiophile-pleasing performance from class-D amplifiers has led to something of an arms race in the amplifier world. Three different takes on class-D have emerged as market and technology leaders: Bang & Olufsen and their ICEpower module, Tripath’s "class-T" variant, and Hypex. The last, an offshoot of electronics behemoth Philips, was created to exploit the advanced class-D designs of the topology’s leading proponent and best-known designer, Bruno Putzeys. Hypex and B&O seem to have divided the high end of the audio world between them, with class-D designs from both companies finding their ways into products built by many other audio firms. Toronto-based Audio Zone saw the more modular Hypex designs, in this case the UcD400, as the ideal candidate for their new monoblock amplifier. Audio Zone is best known for its well-received AMP-1 Gainclone chip power amp and the matching PRE-TI preamplifier, which is as much a metal sculpture as it is an audio component. Gainclone amplifiers gained a huge following in the DIY world when it was discovered that the spectacular sonic performance of the 47 Labs Gaincard owed a debt to the very inexpensive National Semiconductor LM1875 amp-on-a-chip. The subsequent swell of support for class-D technology led to the creation of the AMP-1 for audiophiles who don’t own a soldering iron. And the success of the AMP-1, in turn, inspired the AMP-D2 monoblock ($2395 USD per pair). Description Audio Zone is a joint venture of well-known Toronto audio retailer George Tordai and designers Michael Kerstner and Michael Thomson. Kerstner made a name for himself several years back by designing digital circuits for Audio Note. His Audio Zone responsibilities have centered on the AMP-D2’s power supply, a clean-sheet linear (nonswitching) design. Audio Zone chose to go the traditional linear route to avoid the pitfalls of switching power supplies, most notably their potential to introduce unwanted noise. By designing their own power supply, Audio Zone’s creative team could also control parts selection, and have a power transformer built to match their amplifier’s precise requirements. While Kerstner worked on the power supply, Michael Thomson was responsible for tweaking the basic Hypex module to provide what Tordai says is cleaner, faster performance. Thomson’s rethinking of the Hypex module is also claimed to produce deeper, tighter bass than the stock factory version. The modifications include complete replacement of all capacitors with audio-grade types, and an upgrade of the UcD400’s buffer op-amp to a higher specification.
Setup Monoblock amplifiers offer some placement flexibility: it’s possible to use long interconnects to facilitate placing each amp close to the speaker it powers. The benefit is that you can then use shorter speaker cables -- the amplifier’s binding posts might be only 3’ or 4’ away from the speaker’s connectors. As interconnects are generally less expensive per foot than speaker cable, this can save you some serious money. But I have no short lengths of speaker cable; I placed the amps on my listening-room rack. I did have one setup problem with the AMP-D2s that bears mentioning. When I tried to remove a PS Audio Power Plus cable from the AMP-D2’s female IEC connector, the cable didn’t want to let go; as a result, the amp’s power connector came free of the chassis. Once I was able to get the PSA cable out of the IEC socket, the combo IEC/switch housing snapped back into the D2’s chassis -- no harm done. I suspect that the PSA cable achieves its terrific grip in part because its male IEC connector is slightly larger than specifications call for -- which could explain why the cable was hard to insert in the first place. That said, I was surprised that the IEC connectors came out of both AMP-D2s. System My Simaudio Moon Classic i5.3 integrated amplifier (85Wpc into 8 ohms, 130Wpc into 4 ohms) took its usual spot in Reference Central, this time serving as only a preamp. To reassure the purists out there, the i5.3’s preamp section was more than up to the task -- it can easily hold its own with dedicated line stages in the $1500-$2000 range. My digital source is a laptop computer feeding losslessly compressed audio files to a Blue Circle USB Thingee, which in turn sends signals to an Audio Note Kits DAC 2.1 D/A converter. Sound The Audio Zone AMP-D2s delivered bass and slam like nobody’s business. With 200W on tap and a massive power transformer feeding them heaps of current, the AMP-D2s produced an abundance of clean, tight sound while never even hinting that they were being worked very hard. Even the Mark & Daniel Audio Labs Maximus Monitor speakers didn’t in the least faze the Audio Zones as they coaxed those difficult-to-drive speakers into pounding out deep, rich bass. Lately I’ve been using a solo drum track from Ray Kimber’s IsoMike Tests 2005B test CD (Kimber Kable IsoMike Recordings 2005B) as my reference for assessing a component’s ability to deal with explosive dynamic peaks. This CD actually comes with a label warning listeners to keep the volume low or risk damaging their speakers. That caution should be heeded by AMP-D2 owners -- the amps had so much dynamic headroom that a less robust speaker might not stand up to the onslaught the D2s could deliver. In addition to conveying the impact of the drum kit, the tone of each tom-tom and the shimmer of the cymbals were delivered in quite satisfying manner, the latter with good decay. The AMP-D2s weren’t quite as convincing when delivering the spatial cues of the auditorium in which this track was recorded. It was always clear that the drums had been recorded in a hall, but I’ve heard this track sound bigger through other amplifiers. The AMP-D2’s treble performance was even through most of the upper range, if perhaps a bit soft at the very top. I found that this quality didn’t detract from the sound when the D2s were driving the Guru Pro Audio QM10 speakers, which tend toward a little forwardness in the treble region. But with the Mark & Daniel speakers, which could use some extra sparkle up top, the sound was a trifle flat. This had more to do with the speaker than the amplifier, though, because when providing motive force for the superbly balanced Exodus Audio Kepler speakers, the AMP-D2s were as luscious as freshly picked strawberries. Luscious was the adjective that came to mind as Nicholas Ward and the Northern Chamber Orchestra’s performance of Mozart’s Symphony No.15 (CD, Naxos 8.550874) effused from the Keplers. Although I’m generally not a big classical listener, with the AMP-D2 and Kepler combination at work I found myself wielding more air baton than I have in a long while. Chalk that up at least partially to the AMP-D2’s ability to exert a high degree of control over the Kepler’s drivers. This quality did much to light a fire under high-energy ovations, but also to slam on the brakes when, metaphorically speaking, the thunder suddenly gave way to a gentle rain. Imaging and detail, too, were AMP-D2 strengths. The Zenph Studios "re-performance" of Glenn Gould’s 1955 recording of J.S. Bach’s Goldberg Variations (SACD/CD, Sony Classical 88697), in which a sophisticated computer takes the place of the late virtuoso, was wonderfully, spookily real. You know you’ve got a special combination of stereo equipment when piano notes float freely, forming a concert grand right there in front of you. The combination of AMP-D2s and Keplers wasn’t holographic or completely transparent, and there was some warmth in the middle notes that could be ascribed to the amplifiers, but the pairing was so enjoyable to listen to that I made my way through the entire Zenph CD for the first time. As I said, I’m not a big classical listener -- though I was while the AMP-D2s were here. Of course, the AMP-D2s could rock, too -- or, perhaps more accurately, they could dance. My nostalgia for the 1980s has me buying all manner of greatest-hits collections lately, and the AMP-D2s were on station for Bang! . . . The Greatest Hits of Frankie Goes to Hollywood (CD, Atlantic 99647). It was certainly hard to "Relax" while listening to Frankie wail about "Two Tribes" going to "War," what with the AMP-D2s and the Guru speakers conspiring to deliver sound levels worthy of a dance club. Even with the volume higher than I or my neighbors usually like, the Audio Zone amps maintained their composure while delivering a pleasurable listening experience. In simpler terms: These amps had good boogie factor. Comparison It seemed only natural to pit the Simaudio Moon Classic i5.3’s power-amp section against the Audio Zone AMP-D2s. That might seem unfair, considering that the i5.3 is an integrated amp and that its power and preamp sections were designed to work together. But when you recall that the Audio Zones sounded great when paired with the Simaudio’s pre-outs, the playing field was actually quite level.
The i5.3 is my reference because it’s neutral, clean, and transparent without being dry or lifeless. It’s the kind of amplifier that reveals the true nature of ancillary gear because it is remarkably adept at adding no signature of its own. But I’d settle for a little bit of color if I could have my i5.3 and the effortless power delivered by the AMP-D2s. Driving less efficient speakers, such as the Mark & Daniel and Guru designs mentioned above, the i5.3 doesn’t sound strained, but then it doesn’t have much power in reserve, either. After listening to the i5.3 in its native, integrated mode, I was able to confirm that the Audio Zone amps sounded, in comparison, a bit soft in the upper registers. The AMP-D2s were also a couple of veils shy of the i5.3’s level of clarity and detail. The AMP-D2s produced deeper, more satisfying bass, but it wasn’t as tight or as punchy as through the i5.3. Overall, the Simaudio Moon Classic i5.3 is the more refined amplifier -- but at $3200, it costs almost $1000 more than a pair of Audio Zones. Conclusion The Audio Zone AMP-D2 is a fine iteration of class-D design that has been tweaked to take advantage of class-D’s benefits while avoiding the topology’s pitfalls. The AMP-D2’s outstanding punch and terrific drive deliver an engrossing sonic experience, and yet, at its asking price of $2395 per pair, it’s actually something of a bargain. If high-grade amplification in a package that won’t heat up the listening room is on your list, be sure to give the AMP-D2s a listen. . . . Colin Smith Audio Zone AMP-D2 Mono Amplifiers Audio Zone/Audio Oasis E-mail: info@audiooasis.com
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Class-A amplifiers have
long ruled the roost as the ultimate expression of audiophile amplifier technology.
However, this topology’s performance comes at a price. Class-A amps consume many more
watts of electricity than they produce at their outputs, with the excess energy -- more
than 80% -- being dissipated as heat. This is why big class-A amps are fitted with chunky
heatsinks, and why many can do a reasonable impression of a fireplace.
The AMP-D2 delivers a
claimed 200W into an 8 ohm load, and twice that into 4 ohms. Instantaneous peak power
output is said to be 500W. Audio Zone states a frequency response of 10Hz-50kHz, +0/-3dB,
and typical total harmonic distortion (THD) of less than 0.005% at an unspecified power
output level. Each AMP-D2 monoblock is housed in a steel chassis measuring 8.25"W x
4"H x 14"D and weighing a hefty 22 pounds. Each amplifier is fitted with a front
panel of thick, machined aluminum with a red power-on LED in the center. On the rear panel
are one RCA and one balanced input (switch selectable), a set of five-way binding posts,
and an integrated assembly of power inlet, fuse holder, and power switch. The AMP-D2 has a
delayed soft-start circuit and comes with a three-year factory warranty.
