Solid State Power Amp Reviews

When Editor Jim Austin asked me to review the new Rogue Audio DragoN monoblock amplifier, my first thought was, "Isn't that Herb's beat?"

A quick scan shows that Rogue Audio has been prominent in Stereophile. The magazine has reviewed quite a few Rogue Audio products, including the RP1 preamplifier, the several versions of the Sphinx integrated, the Pharaoh II integrated, and the Stereo 100 power amplifier. Indeed, Rogue has had a near-continuous presence over the past decade on Stereophile's coveted "Recommended Components" list. That's an impressive track record, and accessibly priced tube gear has been Herb Reichert's forté.

As a new Stereophile reviewer, that led me to a moment of panic: What if I get it wrong? Then I thought of something I read in this very magazine: Don't think about the sound. Focus instead on how the review component impacts the music. And so I shall.

About the DragoN
The DragoN monoblock is the latest amplifier from Rogue Audio's Mark O'Brien. The DragoN is a hybrid of tubes and class-D, using Hypex's class-D NCore module in the output stage. Hence the big N in the name of the product.

Rogue Audio calls this its "proprietary tubeD circuit." Rogue says the DragoN combines power, musicality, and finesse for "dynamics and transparency without any of the edgy, etched, or grainy sound that often accompanies solid state designs." That you can read on the web, on the product description page (footnote 1). I asked Rogue General Manager Nick Fitzsimmons to expand on the concepts and design choices that make the DragoN monoblock different from Rogue's other amplifiers.

"The tubeD circuit topology is our in-house method of combining tubes and class-D and encapsulating them in a global feedback network, which allows the tube to saturate the audio circuit," Fitzsimmons wrote in an email. "The tubes are actually included in the class-D circuit vis-à-vis a small global feedback circuit that forces the MOSFET class-D output to behave (and sound) like a tube circuit. ... We feel this provides the end user with an amplifier that has solid state punch combined with the midrange magic that only tubes can achieve.

"Our hybrid, tubeD amplifiers offer a higher damping factor for tighter bass control, and they also produce higher power into 4-ohm loads; this allows for a wider range of speaker compatibility, and it also helps improve low-end response with difficult-to-drive speakers."

Rogue has used class-D modules from Hypex since 2012, when the company released the Medusa and Hydra power amplifiers. I asked Fitzsimmons why Rogue chose Hypex NCore modules over those from other companies.

"We experimented with various modules before choosing Hypex for our designs," he replied. "We found the Hypex modules offered a very neutral sonic presentation, which allowed us to inject our own tube flavor. They also allowed us to bypass some of the built-in circuitry so that we could use our own input and buffer stages, as well as our own linear power supplies."

Unboxing
I had roofers stomping on and spray-foaming my home's flat roof on the day of delivery, a process that continued through the following weekend. I used the noisy time to reorganize the layout of my Massif Audio Design "Dogma" rack to accommodate the DragoNs on the bottom shelf, then streamed the local classical/jazz station (footnote 2) during my workdays until I began critical listening.

The boxes arrived well-packed, and the amps snug within. Each 40lb package was manageable; I was able to move it from the doorstep to my living room without enlisting the help of the perpetually annoyed longhaired teenager down the hall.

Size-wise, the DragoN resembles an extra-thick briefcase, with an understated aesthetic I dig. Each measures 12.5" W × 5.5" H × 19" D and weighs 33lb. Also contained in the boxes were a printed user manual, a branded sticker, and a standard power cord of good quality.

I had opted for black faceplates to match the rest of my gear; a silver/aluminum finish is also available. Centered on the milled-aluminum faceplate is a "Standby" pushbutton within a circular recess; the button is surrounded by three LEDs that alert the listener to the amp's status: blue above the button indicates the amplifier is fully operational; yellow at bottom left indicates standby mode; red at bottom right indicates a problem. The brand's name is engraved in italicized block letters beneath the recess and was not observable from my listening chair. A small, rectangular mesh grille/vent is centered at the top panel near the front panel. Through it, you can view part of the PC board, an assortment of capacitors, and a vacuum tube.

Around back are the usual bits: a power switch at bottom left, with the Rogue Audio logo and "DragoN MONOBLOCK AMPLIFIER" screen-printed above it; high-quality speaker output terminals sit top center; and an IEC receptacle is mounted beneath, with a fuse to its right. A toggle switch at right allows you to select input options, XLR/Balanced or RCA. The actual connections are far right, to the right of the toggle switch, XLR above, RCA below.

Each DragoN monoblock's class-D output stage is capable of putting out 325W and 525W into 8 ohm and 4 ohm loads, respectively. And then there's that single vacuum tube: an ECC802S, which is a long-plated 12AU7 dual-triode preamplifier tube, used here in the input stage. Fitzsimmons said that the tubes Rogue Audio selects for the DragoNs are new-production JJs, manufactured in the Slovak Republic, internally balanced and matched.

"Tube rolling is part of the fun of owning tube equipment, and the DragoN amps are very responsive to tube rolling," he said, adding that the use of aftermarket tubes will not impact the warranty as long as they are the correct type. MSRP is $5995 per pair.

Setup
I used the stock power cords connected to the high-current zones of my AudioQuest Niagara 1200 power conditioners, each of which is plugged into a dedicated 20A circuit via a hospital-grade outlet.

The preamplifier was my reference Linear Tube Audio (LTA) microZOTL, which, in my reference system, is matched to an ZOTL40 Reference+ stereo amplifier (footnote 3)—a push/pull, class A/B output-transformerless amplifier specified to produce a mere fraction of the Rogue monoblock's output power: above 50Wpc via four EL34 tubes, two for each channel. The microZOTL preamp does not have balanced outputs, so, for all of my listening, I used AudioQuest Black Mamba II single-ended interconnects and the DragoNs' RCA inputs. Speaker cables were AudioQuest Rocket 44.

Fitzsimmons told me that the DragoN monoblocks are friendly to a diverse group of speaker types; in-house, Rogue Audio tests its amplifiers with speakers from Joseph Audio, Magnepan, MartinLogan, Klipsch, EgglestonWorks, Zu Audio, and GoldenEar. But I doubt the DragoN had ever encountered speakers like mine.

The ESS Transtatic I is a hybrid-electrostatic loudspeaker (footnote 4). The fabric-wrapped 120lb cabinets are three-walled, open on the inside face. Low frequencies are transmission-line loaded and radiate from bass ports above each laminate-veneered loudspeaker pedestal's cardinal directions.

Each speaker uses a 9" × 12" foam, flat-piston KEF B139 oval "racetrack" low-frequency driver and a rear-ported KEF B110 midfrequency driver, coordinated with an array of three RTR electrostatic panels. These drivers were fairly common in the 1970s, used by various manufacturers including Infinity. According to the specifications, the Transtatic I's lows extend below 15Hz, the highs up to 32kHz, with a ±3dB range of 25Hz–20kHz. Admirably, ESS specified a minimum—not nominal—impedance, of 4 ohms. Sensitivity wasn't specified, but I estimate it as in the vicinity of 90dB/W/m. The user manual suggests a minimum amplifier power of 60W and says it can take "greater than 650-watt musical peaks without distortion."

Uncolored and unboxy, these are the best-sounding loudspeakers I've ever heard at low volume. For this review, I used them exclusively.

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Footnote 1: See rogueaudio.com/Products_DragoN-mb.htm.

Footnote 2: See capradio.org.

Footnote 3: Mine is evolutions newer than the amp HR reviewed.

Footnote 4: See hifi-wiki.com/index.php/ESS_Transtatic.