Rogue Audio Hydra Amplifier

The Absolute Sound

Garrett Hongo

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.

 With the Hydra sitting on a stand in front of my main audio rack rather than on its bottom shelf, I needed interconnects a little longer than my reference 1m Siltech 330i ICs in order to make the distance to the preamp easily. I swapped in a 1.5m pair of Cardas Clear ICs at first, going from preamp to amp, keeping all my other Siltech ICs in, plus one pair of Shunyata Python ICs in the DAC-to-preamp run. That sound turned out to be a bit harsh—tipped up and edgy, sibilant on female vocals, chesty on some males. So I switched to a new pair of Audience Au24SE ICs. Immediately, there were big gains in naturalness and ease. The new Audience cables sounded so good, in fact, I ended up going all Audience Au24SE in the system. The result was a wonderfully pleasing sound, speedy but somehow relaxed with a touch of warmth. I think the new ultra-low-mass, high-purity, cryo-treated tellurium-copper headshells on the Au24SEs are a serious upgrade over the older “e” style connectors, increasing speed and resolution to a startling degree. These new cables were a perfect complement to the Rogue Hydra amp. For speaker wires, I stuck with my reference Siltech 330L cables and jumpers. For a power cord, I used a Siltech Ruby Hill II rather than the stock cord.

I also experimented with a couple of different linestages, searching for a good complement to the Hydra. With my reference deHavilland Mercury 3, itself a tubed unit that works perfectly with my tube monoblocks, I thought the system sounded thin and wiry. I quickly switched to the Lamm LL2.1, another tubed linestage, which, with its inherently more full-bodied sound, was a perfect match with the Hydra.


O’Brien mentioned rolling a different pair of ECC82 (12AU7) into the amp’s input section. Stock are Tungsram shortplates or JJ longplates (mine came with Tungsram) and though I’ve lots of 12AU7s around, including Mullard longplates, which are the current darlings, I thought the amp sounded very good stock. In the end, however, I did roll in a pair of new issue Shuguang Psvane 12AU7-Ts, just for giggles.

For most of the review period, I used Von Schweikert VR- 44 Aktive speakers (90dB/8ohm), my new reference. But these speakers have powered woofers, with a 300W plate amp in each woofer cabinet that takes the audio signal from the system amplifier and boosts it. So, to test the Hydra’s bass and slam, I also used my previous reference, a pair of Von Schweikert VR-5 HSE passive speakers (93dB/6-ohm).

Listening with Powered Speakers

I played mostly digital music while I had the Hydra in my system—CDs, rips, and downloads. I let it run in for about 100 hours before I took any notes, but, to tell the truth, I didn’t hear much change in sound (once I switched over to the Audience ICs). It also seemed to accomplish its basic sound on first powering on—no warm up! On rock, jazz, and world music, the Hydra’s sound was immediate and lively. It did pretty well (with some caveats) on orchestral, choral, and even operatic music too. And, besides its transient speed and dynamic power, I’d say its main characteristics are a pure and satisfying midrange, a fine tonal saturation on piano and woodwinds, and great slam and sparkle on electric blues and rock.

The Hydra excelled at the presentation of the female voice. Renée Fleming’s “Quando me’n vo” from her eponymous Decca CD had a shimmering, liquid quality to her high notes and the inner tone of her chest voice was rich and full of body. “Under the Boardwalk” by Ricky Lee Jones from Girl at her Volcano (iTunes LossLess) demonstrated a rich complexity of rhythmic ensemble work—marimbas, vibraphones, congas, and percussion—but it was Jones’s sinuous, wailing voice that took over the tune, harmonizing in duet with a soul tenor’s in the choruses, bouncy and full of sass and musical personality in the verses. I also liked how the amp was able to render Chrissy Hynde’s chameleon-like transformations in her vocal—going from a dry, husky chest- voice into nasal, then wiry, finally to strident wails on “Chill Factor” and “Back on the Chain Gang” from the Pretenders live The Isle of View CD [Warner Brothers]. But, for my money, most impressive was the rich, dark, sinuous-as-a-violin vocal of Sarah Vaughan on “In a Sentimental Mood” from her Duke Ellington Songbook, Vol. I [Pablo]. There was a superb integration of her vocal with the tapestry of the band (mostly veterans from Count Basie), and I felt chills on the refrain as she slid into melismas during her incomparable jazz roulades. And her ornaments—a hesitation staccato of phrasing, her swooping flats, and skittering accelerandos—oh my! The Hydra tracked her every filigree.

The Hydra performed equally well on male vocals and rhythmically complex music like that on “Saludo Compay” from Eliades Ochoa y El Quarteto Patria’s Sublime Illusion (iTunes LossLess). The track features a slinky guitar intro with a brass chorus backing it. There are multiple percussion instruments including a shaker, cowbell, and congas. And underneath them all is a bass quatro that’s percussively plucked. The Hydra rendered all these as well as Ochoa’s soaring vaquero vocals with ease and perfect timing, distinguishing each instrument’s natural timbre from the rest, giving air and space to the choral voices in call and response with Ochoa’s gritty lead.

 

My strongest impression, though, had to do with the amp’s ability to render a lifelike presence to the music and maintain control over all its elements. This Cuban music had verve and drive, and the system’s bass and timing were awfully tight. Yet, I knew that the VR-44 Aktive speakers could have had a lot to do with that.

By far the most difficult music for the Hydra to render well was orchestral, particularly the sound of violins or “massed strings” as they are called in audio. I eschew solid-state amps and preamps for the most part (aside from the exceptional Herron Audio M1 monos), because, even if they don’t turn violins glassy or edgy, these devices tend to obscure vibrato and other performance details having to do with the micro-dynamics of phrasing. They also lose the main reasons we crave violin sound—for their sweet highs, their bountiful harmonics. In this, the Hydra was not completely exempt. I can’t say the sound was bad, per se, just not glorious—a bit pedestrian. On Vivaldi: Eleven Concertos [Sony], performed by the baroque ensemble Tafelmusik, violins were more incisive than sweet on Concerto for Strings in G Minor. And, although there was no glassiness or glare, there was also no richness of harmonics and, to my ears, string sound thereby seemed stripped. I wondered whether Hydra’s sound might be improved by swapping out its stock 12AU7 tubes.

Yet, even without changing tubes, the hybrid amp still had its grand moments. For example, in the second movement of Brahms’ First Symphony, performed by the Budapest Festival Orchestra conducted by Ivàn Fischer (iTunes, Apple LossLess), the Hydra produced a string sound that captured the natural and distinct timbres of the instruments. Cellos swelled with the violas and doublebasses, creating a marvelous foundation for the violins. From the opening bars of the movement, I heard great air and sweetness to the orchestra as a whole, a brilliant and clearly rendered oboe, and a pleasing lightness to the violins, which never turned glassy. Sure, compared to a Class A tube amp, the Hydra’s string sound was a little over-polished and under-detailed—but it was never “electronic.” The Rogue amp properly reproduced the warmth of the main theme as it traversed the orchestra and ended up being sweetly sung in the brief solos of the concertmaster’s violin.

Listening with Passive Speakers

All of the above was noted with the Hydra and the VR-44 Aktive speakers. What would it be like with a pair of unpowered, passive speakers? To find out, I had to pull the VR-44s out of my system and swap in the older VR-5 HSE speakers.

What did I hear? Well, first, a warmer, perhaps richer, but also a more veiled sound. The VR-5 HSEs aren’t the transparency and resolution meisters that the new VR-44s are. But this assisted the Hydra in sounding more bounteous in the mids, less penetrating and less forward (though that forwardness was thrilling). In a way, the Hydra sounded more balanced throughout the frequency range on the VR-5 HSEs. There was less sparkle, less snap, but Benny Goodman’s clarinet on “Mean to Me” from Stealin’ Apples sounded woodier, a touch warmer, and much less piquant (a quality I liked, by the way). Piano notes were less defined and startling in attack and decay, sounding more rounded.

But I changed speakers better to listen for the amp’s natural bass, slam, and foundational contributions without the assist of the VR-44s booster amps. And I was not disappointed. My big test for this sort of thing isn’t head-banger rock, but Igor Stravinsky’s The Rite of Spring as performed by the Kirov Orchestra under Valery Gergiev [Philips]. After the Introduction with sweet oboes, mellow bassoons, and airy flute parts, “Dances of the Young Girls” begins as the strings play repeated forte eighth notes with strong irregular accents, and I heard both power and speed in the amp. Then, when the passage culminates in fanfares, a drum roll, and a powerful bass drum strike that slams and then reverberates into a slow decay, I felt the Hydra did quite wonderfully. It rendered each preliminary strike as an individual sonic event, and then delivered the final drum whack with thundering authority. Later in the piece, there are even more thrilling drum strikes in combination with full orchestral crescendos. In these passages, the Hydra gave the brass and horns finely expressive tonal colors, the strings and woodwinds a sweet warmth, with speed of impact and great slam to the drums. The Rogue passed my test with flying colors on bass, current demand, and easeful orchestral scaling.

What about swapping out the stock 12AU7 tubes? Well, just before the end of the review period, I broke down and replaced the stock Tungsram tubes with a pair of Shuguang Psvane 12AU7-T tubes I’d gotten from Grant Fidelity. Though these aren’t the NOS tubes preferred by so many, I’ve come to value them for their availability, reliability, and very good sound. And, at $83/pair for “better” and $99/pair for “best,” their prices are relatively reasonable. With the Psvane tubes in, there were obvious and immediate gains in depth, openness, dimensionality, and sweetness. String sound was more real and open, with greater body. But violins, on the same Vivaldi: Eleven Concertos CD I’d played before, still lacked a certain tonal richness, the startling bounty of harmonics I am accustomed to hearing with my reference deHavilland tube electronics. On other recordings, particularly small-ensemble acoustic jazz and vocal music, there were noticeable upticks in depth, image definition, and smoothness. On balance, I’d say the Shuguang Psvane tubes are a definite upgrade over the stock tubes, affording the Rogue amp more of a “triode-type” sound overall, giving it more spectral balance through the frequency range and less top-end emphasis.

Yet, I heard no loss of jump factor with the Psvane tubes in. The Hydra absolutely cranked with slam, sparkle, and superb bass definition on electric blues, studio hip-hop, and rock. On “Milkcow’s Calf Blues” by Eric Clapton from his Sessions for Robert J CD [Reprise], Clapton’s electric slide screamed along as the rhythm section locked in to the beat with real weight and momentum. Electric bass can sound one-note on some systems, but the Hydra and VR-5 HSE combo let me hear the taut and articulate travel of Nathan East’s precision playing as he laid down the funk. In “Let’s Get Retarded,” that infamously politically incorrectly entitled track from elephunk [A&M] by the Black-Eyed Peas, the bass line was even tauter—so much so, I thought for a moment it was synth bass I was hearing. Listening more carefully, I recognized a familiar Family Stone figure worked into the otherwise stylistically monotonous bump on the bottom. And “Right Down the Line,” from Bonnie Raitt’s new Slipstream CD [Redwing Records], had a tuneful reggae-like bass with a pleasing, chest-thumping impact amidst the sweet snarl of Raitt’s bottleneck sliding across the strings of her Stratocaster. It’s also worth noting that the Hydra’s powers of resolution easily sorted out instrumental lines and produced a consistently wide and deep soundstage with instruments and voices placed vividly within it. The Hydra’s soundfield is sturdy and detailed.


At first, I didn’t dare play choral voices with the Hydra in the system, but as I grew more and more pleased with its nimbleness and ability to produce fine tonal colors and render the differences in instrumental textures and vocal timbres, I broke down and gave it a go. I played a real “system-crusher” of a piece, Thomas Tallis’ Spem in alium from the Utiopia Triumphans CD [Sony] of the Huelgas Ensemble directed by Paul Van Nevel. Tallis’ composition is a forty-part motet but Van Nevel employed forty-three voices—eleven sopranos, six altos, fifteen tenors, and eleven basses. On a system with poor amplification, the voices cave in on themselves in a weltering mosh pit of audio hash without any differentiation among the choral parts. The sound is not only hard and edgy, it’s glassy, full of time smear and distortion. But not so with the Rogue Hydra driving the VSA VR-5 HSE speakers. Voices were clear, sections distinct, the polyphonic vocal lines pulsing as called for by Tallis’ magnificent music. Though I did hear an occasional touch of hardness here and there, these were rare and minor and the presentation was mainly clean and airy, the full choir sometimes cloudlike amidst a wide soundstage. It was an impressive feat for any amp, let alone one with a Class D output.

Conclusion

Rogue Audio’s Mark O’Brien has really got something with his Hydra hybrid tubeD stereo amp. It delivers the sonic goods, is easy to use, and is seemingly bulletproof. Best of all, it produces sound of clarity, strength, and fine resolution in a consistently broad and vivid soundstage. Though I found some flaws in its presentation, mainly in orchestral string sound and limitations in clarity and smoothness to top-end extension, you can still tailor its presentation a bit by rolling input tubes. I think it a very worthy piece of electronics much superior to any Class D amp I’ve yet heard. In fact, I think it holds its own alongside most any amp at its price point, tube or solid-state. The slight upper- midrange forwardness I heard (with stock tubes) mainly added to its appeal, lending to piano, woodwinds, brass, and vocals a pleasing saturation of tonal colors and a kind of sweet authority I’ve not found the like of with any other piece of electronics. Add to all this its “green” efficiency and affordable price, and I think it’s definitely a product that fills the needs of a lot of audio buyers out there. For anyone looking for a moderately priced, seriously good-sounding stereo amp, the Rogue Hydra could be just the answer.