Rogue Audio Hydra Amplifier

Class D With a Difference

Equipment report
Solid-state power amplifiers
Rogue Audio Hyrda
Rogue Audio Hydra Amplifier

When TAS editor Robert Harley proposed I review the new $2995 Rogue Hydra, a hybrid stereo amp with Class D output stage and tube input, I must admit, being mainly a tube amp guy, I felt a twinge of trepidation. Some six years ago I auditioned a pair of highly vaunted Class D monoblocks at the urging of a dealer friend who himself owned them. The sound I got on my favorite music—a lot of Renaissance choral, opera, and classical orchestral—was like a hard rain of microscopic nails. The amps went back within a couple of days. Then, a few years later, I house-sat a friend’s place in SoCal. He had an audio system with a tube preamp and a different, newer pair of Class D monos, also vaunted, but that system also felt edgy on my music. I resorted to playing mostly smooth jazz and soft rock the week or so I was at his place.

But Class D has since come a long way. That this particular Class D amp was designed by Mark O’Brien of Rogue Audio also certainly grabbed my attention. O’Brien is an engineer with great ears and a lot of experience manufacturing very fine tube gear at reasonable prices. “If he’s doing it...,” I thought, and accepted the assignment with what’s fair to say was a “cautious optimism.”

The Hydra is a “completely different kind of Class D amp,” in the words of O’Brien, who’s studied Class D topologies and come up with a circuit unlike anything previously used in consumer stereo (see sidebar interview). It’s not simply a tube circuit placed in front of a Class D output section, but an integration of a double-triode ECC82 (12AU7) tube section into the amplifier that, he says, combines the best of both solid-state and tube technologies and results in a smooth and natural sound that normally only tubes can provide. In his circuit, which he calls “tubeD,” O’Brien claims there is none of the dreaded edginess, grain, or etched sound of Class D amplification in the past. And I gotta say that, much to my delight, I pretty much agree with him.

No, the Hydra doesn’t sound like a classic tube amp or even like modern ones made by the likes of Audio Research or VAC, but it also isn’t a creature that spits and hisses and bites your ears like those first Class D amps I heard back when. Fast, powerful, and resolving, the Hydra reproduces instrumental tones and timbres with accuracy and texture, has good spectral balance, and gave me lots of real and revelatory listening pleasure. It doesn’t get hot, it never faltered or caused any problems during the review period, and it was easy to use. Besides all that, it’s energy efficient and relatively “green” in terms of power consumption.

Setup and Operation

The Hydra is rated at 100Wpc at 8 ohms and 200Wpc at 4 ohms. My review unit came in black (silver is also available) and was a snap to set up. The casework is powder-coated steel with rounded edges. I’d say the overall look is utilitarian with a touch of class. The Hydra came fully tested, burned in, and auditioned with a three-year limited warranty (6 months on tubes). The seven-page owner’s manual explained all operations clearly and succinctly.

The layout at the back has a power switch, an EIC power inlet, knurled gold-plated-brass binding posts (solid and easy to use), and both balanced and single-ended (RCA) inputs. I used only the RCAs. On the front, the Hydra has a faceplate of machined aluminum and a brushed aluminum Standby/Power-On button that’s recessed into a small circular area with a trio of LED lights around it that indicate Standby (left), On (top), and Error (right). The Error LED comes on if the outputs are shorted or there is an over-current situation such as a faulty crossover in the speakers. (That never happened during the review period.) Once the amp is powered on, indicated by a backlit blue LED, it soft- starts the tube circuit and sends current to the power supplies. It’s fully operational in seconds. Little or no warm-up is required.