Stereophile Review｜Octave MRE 220 SE monoblock power amplifier

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Date: 2024-08-06

Tubes, tubes, tubes—how we love to bask in their glow, roll them, and take their second-harmonic distortion into our hearts as if it were a child or a pet. Some may put out so much heat that we have no choice but to open a window, turn on the air conditioning, or listen in the garb of Adam and Eve before that fatal first bite. As they and you age, you can never be sure who's at their best. Tubes, at least, can be replaced, albeit at significant expense.
The special sound of tubes, which flows like water and sometimes shines like the golden sun you hopefully shielded your eyes from during the recent eclipse, makes it all worth it—if the design and implementation are good and you can't get similar sound from low-maintenance solid state (footnote 1).

I haven't reviewed much tube gear, but when I have—Bruce Moore and VTL (in my pre-Stereophile days), Audio Research, and in our September 2022 issue (footnote 2), the towering Octave Jubilee Mono SE tubed pentode push-pull monoblocks (footnote 3)—I've been enamored of their sound. I waxed ecstatic about the "captivating beauty" and "heavenly" highs of the Jubilee Mono SEs. I can still recall how gorgeous they sounded; every listen was special.

Hence, my enthusiastic "yes" to a solicitation from John Quick, VP of Sales & Marketing for Dynaudio North America, Octave's North American distributor, to review the smaller MRE 220 SE (footnote 4) mono push-pull tube amplifier ($28,500/pair base price, $37,100/pair in the ultimate configuration, which I received). Having discovered that the common output power specifications do not correlate perfectly with an amp's ability to control bass, I reckoned the rated output of 200W average and 220W peak power into 4 ohms would work just fine with my reference Wilson Alexia Vs.

Options, we have options
Each MRE 220 SE monoblock uses four power output tubes—KT88s, KT120s, or KT150s are recommended and available from Octave—and three driver tubes: one 6SN7GTB (a 6SN7 with a maximum plate voltage that's 50% higher) and two ECC802s (footnote 5). A few options are offered with the MRE 220 SE that can affect its sound.

Power Tubes. You can choose among KT88 (supplied with the base unit), KT120 (adds $800/set), or KT150 (adds $1600/set). During an extended Zoom interview with Quick and Octave founder/chief designer Andreas Hofmann, Hofmann said, "To my ears, KT88 sounds quite a little bit soft. Sound and quality-wise, KT150 is best. But it's also a question of the speaker and the quality of the system. If somebody has a less-demanding system than yours, the top is sharp, or the sound is darker, they may prefer KT88."

Power Supply. You can boost the capacity of the MRE 220 SE's internal power supply via an external capacitor bank called the Super Black Box (footnote 6; adds $7000/pair). The Super Black Box connects to the amp via a flexible, screw-on umbilical cord. Adding the SBB to the MRE takes only a few minutes.

The manual says the Super Black Box "significantly increases power supply capacitance to stabilize current delivery and reduce impedance interaction of the load, thus improving dynamic range, separation, depth, soundstage size and articulation. This enables optimizing the MRE 220 in respect to the speaker, Which is a tremendous benefit if the speaker is difficult to drive."

Input Choice. The choice of XLR or RCA will have sonic consequences. Which is better? Octave's Michael Nonnenmann wrote, "We do not prefer any connection method. It depends on what devices you use. Please test for yourself which connection sounds best." Because I was using a dCS Vivaldi Apex DAC as my volume control, I was able to try both options. More on this topic follows.

(In addition to RCA and XLR inputs, the Octave MRE 220 SE has optional RCA 2 input designed to reduce hum and eliminate signal ground/protective ground issues in some countries. RCA 2 is not recommended for US users and was not fitted to the measurement sample.)

Passive Filter. Octave makes a relatively compact but hefty passive filter called the Filter 3-P ($3000), designed to be inserted in the analog interconnect path between a preamp and each SE 220. (It can also be used between preamps and DACs.) The Filter 3-P is designed "to eliminate the problem of propagation of conducted electromagnetic interference," which travels in both directions on ground lines. There are different filters for users of RCA and XLR interconnects; mine was designed for XLR (footnote 7). "It's a special type of transformer with a high capability to block the high frequencies," Hofmann explained. "The so-called common-mode rejection ratio (CMMR) is quite high. The frequency range is flat up to 100kHz, but it blocks high-frequency distortions from the mains that travel in both directions on the ground, from the network, and so on. You can also use it between the DAC and the input section of the preamp to address jitter that can mask details." The Filter 3-P requires an additional set of interconnects.

Ground-Lift Switch. It's on the rear panel. When in the "connected" position, ground resistance is 0 ohms; in the "disconnected" position, it is 18 ohms. In normal use, when no ground-related hum is present, the "connected" position is appropriate. "All manufacturers, including cable manufacturers, treat grounding differently," Hofmann said. "Cable capacitance can create high-frequency distortions on the ground line, and components' internal circuitry can sometimes float above the main earth ground. Some manufacturers reverse pins on XLR, which can cause problems, and some use two-prong power cords that lack a third wire for an independent ground. Even the network may not be properly grounded."

"Issues can develop when you join together the grounds of different components," Quick added. "Ground loops can arise when monoblocks and source components are plugged into different circuits. Lifting the ground allows interconnects to ground everything to one place rather than multiple places that may have different ground potentials." Depending on the precise cause, that may get rid of any ground-loop hum.

Adjustable Damping Factor. The SE 220 has an adjustable high/low damping factor to, in the words of the website, "adapt ... to [and] ideally suit the higher demands of modern loudspeakers." The factory setting is "low."

"With low or reduced feedback, the damping factor is very low," Hofmann said. "In my opinion, a good speaker with good magnetic efficiency like your Wilsons does not require a high damping factor. As much as an amplifier controls the sound, the loudspeaker system must have its own control. Otherwise, it's a poor design. A few exotic or old speakers—an old B&W or Dynaudio, for example—need a high damping factor; the newer Dynaudios do not."

"I've designed amplifiers for 20 or 30 years, and I still feel that we need a certain amount of feedback and a bit of damping, but not too much. Otherwise, the midrange sounds flat or less than good. Adjusting feedback is a very, very sensitive thing. Too many amplifiers have too high a damping factor and too much feedback. Too much feedback kills the sound."

Layout, features, and biasing
The 220 SE arrives with a protective grille over the tubes. The product manual declares, "Operating the amplifier without its protective grille is dangerous and not recommended." Quick gave the okay to remove the grilles.

A small power-rocker switch is located on the left side of the amplifier, close to the front. The front panel holds two large selector knobs. The left knob selects input (RCA 1, RCA 2, XLR, or Mute; you can connect or change cables with the switch in the Mute position, without switching off the amplifier).

The right knob selects one of the four tubes, "V 4, V 3, V 2, or V 1," for biasing. It can also put the amp in either "Eco off " or "Eco on" mode. In "Eco on," the amp remains fully warmed up and ready for use unless no signal is present for approximately 10 minutes. After that, it enters sleep mode, draws 20W idle current, and produces very little heat. When a music signal is detected, the Eco mode circuit powers the unit back on with a 20–30 second warm-up/start-up delay. Eco mode is said to increase tube life and ensure that nothing bad or dangerous happens when you leave the amp switched on. Tube-biasing is only possible with "Eco off."

Pre-measured tubes arrive in a separate box, with amplifier positions labeled for the power tubes. If you use these, you're golden. If you want to roll tubes, manual bias adjustment is necessary. Turn the front-panel bias selector knob to the tube you wish to bias, then use the supplied small screwdriver to adjust the trimmer screw next to the tube. Yellow and green LEDs located between tubes V2 and V3 indicate if bias is too low, correct, or too high. Combinations of yellow and green allow you to tweak bias higher and lower according to tube choice and loudspeaker type. Sayeth the manual, "Certain loudspeakers may benefit from the higher bias current, as it will increase the damping factor and provide slightly better control of the speakers." BIAS Low corresponds to a plate current of 33mA, and BIAS high to a plate current of 40mA.

After biasing the tubes, you can set the right knob to "Eco off " and start listening right away. Alternately, you can remain in "Eco on" and, after a short pause when the start-delay circuit kicks in, signal will begin traveling to your speakers. In either scenario, sound will continually improve over the first 12–15 minutes, until the amps are fully warmed up.

The rear panel contains XLR (pin 2 is positive) and RCA inputs; that ground-lift toggle switch; loudspeaker outputs that accept either spade or 4mm banana connectors; and a place to connect an umbilical cord to the Super Black Box. There's also a "HIGH – 200W/LOW – 140W" power selector. KT120 and KT150 tubes require the high setting while regular KT88s require low; Octave's preferred KT88 S4A Carbon can use either setting. Rounding things out are the serial number and an IEC socket with an integrated fuse holder.

Additional design details
In contrast to most tube amps, it's okay to operate the MRE 220 with no loudspeaker connected because "The MRE 220, like all Octave amplifiers, is fully protected against open circuit operation, ie, the amplifier will come to no harm if it is operated without loudspeakers connected and the input level is in the magnitude of normal listening levels." The amp's soft-start technology is said to greatly extend tube life; tubes "normally sound the same throughout their service life." When an output tube has reached the end of its useful life, you won't be able to adjust the bias correctly.

Octave's website (footnote 8) declares that the MRE 220 SE mono power amplifiers have been "completely redesigned" relative to the non-SE versions, with a new power supply and output transformer section design. The company says the amplifier is stable down to a loudspeaker impedance of 2.5 ohms and that its S/N ratio is >116dB (no ref. level specified). The manual touts the amplifier's double-sided PCB with plated-through holes using 70µm copper, ceramic tube bases with silver contacts, specially selected and burned-in long-life tubes, and professional low-inductance electrolytic capacitors.

I asked Hofmann if he had different design goals for his larger and more costly Jubilee floorstanding monoblocks. "The MRE 220 SE is, in my opinion, the best compromise we can produce in terms of dimensions, price, and power," he answered. "The sonic goals are the same, but with the Ultimate, I want to expand an amplifier's possibilities as much as possible in terms of dynamics, power bandwidth, and so on. That requires a big amplifier with eight tubes, more power from an oversized power supply, and most important, an extremely oversized output transformer. The Jubilee's power bandwidth goes well below 20Hz; this works well when everyone in a big orchestra starts to play at once and this huge shockwave of sound, with frequencies below 20Hz, comes out easily and naturally from the speakers. You cannot hear these frequencies directly, but you can feel them when classical or pop music goes very, very deep.

"Reproducing them correctly also requires less phase shift and less distortion. Everything must be in time and in phase. There is so much information in the lowest frequencies; I think it has more importance than the frequencies above 100kHz. Correct phase response and phase shift at full power contribute greatly to sonic integrity."

What occasioned the redesign of his less expensive monoblocks? "The MRE 220 SE is a refinement of its predecessor, the MRE 220, which was available for close to eight years. After its release, as I worked with different systems and setups, I learned how to improve some aspects of the push-pull design that affected feedback and stability. A good amplifier is a stable amplifier that always has good sound, independent of the speaker. I've made the two signal passes for the plus and the minus push-pull transients as equal as possible. The midrange and even the high frequencies and bass sound clearer and more natural. Body and image size have improved."

XLR or RCA, that is a question
The answer is different than Hamlet's, and the consequences aren't tragic. The Octave MRE 220 SE, whose input stage is single-ended (unbalanced), offers users a choice between XLR (balanced) and RCA (single-ended) inputs. "Andreas feels the best way to design an amplifier's main amplification/power gain stage is to make it single-ended (unbalanced)," Quick said. "As a result, the balanced input includes an additional circuit that converts incoming signals from balanced to unbalanced. To Andreas's way of thinking, using a high-quality converter circuit based on an op-amp, which is the most linear, quiet, and consistent way to convert between single-ended and balanced, is the right solution."

Hofmann affirmed Quick's explanation, offering that "The converter has a minor effect on the sound, but this can still be positive. You cannot say that sound gets worse with the op-amp, because ours is professional grade, not a cheap part, that acts as a professional balanced-to-unbalanced converter.

"Ultimately, the sound depends on the source, on the preamp, and on the DAC. What is the balanced output of the particular brands in your system, and how is the sound affected by the length of the cable, its susceptibility to distortion, and the grounding? Even the power supply of the DAC can have a sonic effect if there is noise on the ground line. There are so many factors at play that it's difficult to predict which will sound better."

In a subsequent email, Quick pointed out an additional advantage of the balanced-to-unbalanced converter circuit. "The MRE 220 SE's XLR input circuit also serves to make levels between XLR and RCA sources more similar without simply shunting half the original signal to ground. This decision maintains more of the original balanced source's signal integrity."

Installation Pt.1
My sample arrived with KT150s and Super Black Boxes. Separately, I received a broken-in Filter 3-P that would accept XLR interconnects. Hofmann does not feel that Octave amplifiers need additional power conditioning. Nonetheless, if someone prefers the sound through their favored power product, he's okay with that. Because work with Edward DeVito of Audio-Ultra has greatly improved my electrical setup, I plugged the amps directly into the wall. I anticipate further work shortly that will upgrade the dedicated wiring and increase the number of circuits.

The Octave MRE 220 SE's footers are, in Quick's words, "not meant to be anything particularly special, just functional." I could say the same about the footers on my reference amps (footnote 9). Because I'm enamored of Wilson Audio Pedestals and use them under virtually every component, I placed them under the MRE 220 SEs. I had a spare set of four that I divided between the two Super Black Boxes. With trial and error, I managed to get them to balance.

I streamed music from Qobuz, Tidal, or my NAS through my optical network and a Nordost QNet switch to the Innuos Statement NG music server/streamer. I also played files stored on the Innuos's internal SSD. From the Statement, the signal proceeded through the three-piece dCS Vivaldi Apex stack to the Octave MRE 220 SEs through top-level Nordost and AudioQuest cabling.

Because the Vivaldi Apex DAC has both RCA and XLR outputs, I tried both. When I output from Vivaldi to monoblocks through RCA, I easily connected my Wilson LōKē subwoofers with XLR-terminated subwoofer cables; when I output to the monoblocks through XLR, I used Cardas XLR-to-RCA adapters to send signal through RCA to the subs.

During the review period, I learned what might happen if I floated the ground or switched the damping factor. I also evaluated sonic differences between XLR and RCA inputs in my system with my components, the effects of the Filter 3-P in my system when XLR interconnects were in use, and the benefits of the Super Black Box power supply. Your mileage may vary.

Installation Pt.2
Quick spent several days in Port Townsend checking out my reference system, installing the amps, and listening. We took our first listen using XLR interconnects. A grand symphony by Mahler sounded anemic, skeletal, and stripped to the bone. Notes came through dimly, shorn of overtones, body, color, and musical essence. I feared I had no choice but to declare the Octave MRE 220 SE monoblocks system-incompatible.

Stereophile's review policy is firm: Do not discuss what you hear with anyone supplying the gear, or anyone else. If you suspect something's wrong, say nothing, consult editor Jim Austin, and go from there. Which doesn't mean that I don't find it difficult to keep a straight face as I witness the desecration of music I love while an intelligent, discerning distributor I know well sits next to me, hearing what I was hearing.

Thankfully, Quick immediately heard that something was way off and proposed potential solutions. First, he explained that he had installed the amps with the ground lifted. "Because your amplifiers were plugged into the wall, the rest of your system was plugged into the Stromtank S 2500 Quantum Mk.II, and some units were connected to Nordost QKore ground units, I thought the amps might sound better with the ground lifted," he said. "Let's see what happens if I engage the ground."

Two switch-flips later, the improvement was major. There was nothing subtle about the transformation. Although intuition suggested that more could be done, I could allow my shoulders to drop most of the way down from beside my ear canals.

"I also set the damping factor to high," Quick said. "Let's switch it to low." I breathed a sigh of relief as I expressed my eagerness to proceed with the review.

Let the music play
First on my musical agenda was a review of the new Takács Quartet recording of Schubert: String Quartet No.15 in G major D887 ù String Quartet No.8 in B flat major D112 (24/96 WAV download, Hyperion, available as CD). In No.15, written two years before the composer's death, the Takács's soft playing was remarkable for its control and sensitivity. Passages of heartbreaking tenderness washed over me, touching the core of my being. As Schubert's joy ceded to pain and came back again, I sat transfixed.

Reviewer mode kicked in, and I couldn't stop myself from changing the interconnects to RCA. With RCA, the sound was more neutral—less overtly warm—than with XLR, and colors seemed more varied. Images weren't as big, nor the presentation as transparent as through my far more costly reference amplifiers, but the music's beauty and emotion came through clearly. I felt totally confident that I was hearing what I needed to hear—and feeling what I needed to feel—to assess the recording.

Shortly after I reviewed (footnote 10) pianist Simon Trpčeski's fabulous performance of the Tchaikovsky Piano Concerto No.1, with the Seattle Symphony under guest conductor Osmo Vänskä, my blessed friend Scott arrived for a listen. I cued up a private recording by Peter McGrath of Trpčeski performing Prokofiev Sonata No.7 on May 2, 2023, in Miami (24/176.4 MQA). The interconnects were XLR, the soundstage was wide, and the sound was warm and inviting. Every note of the piano was solid.

When we inserted Filter 3-P, it softened the piano's leading edge—great for bright systems—truncated harmonics, and lessened air, depth, and the rate of decay of those lovely reverb tails. When I played Mahler's "Revelge" from baritone Samuel Hasselhorn and the Poznan Philharmonic's new recording, Urlicht (24/96 WAV download, Harmonia Mundi), Filter 3-P seemed to muddy the orchestra's lowest notes. On the heavily overdubbed Maya Beiser × Terry Riley: In C (24/48 WAV download, Islandia Music), some lines were smudged, some colors diminished. Layers and textures clarified when I removed the filter.

Nonetheless, with the Filter in, the tribal nature of some of the movements and a multilayered acoustic that invoked a huge gothic church came through strongly. Perhaps in systems burdened with electromagnetic interference, electrically conducted noise, and the like, the Filter 3-P's positive effects will outweigh other considerations. In my system, it seemed best to proceed without it.

Time for a tried and true, Mahler Symphony No.4 performed on period-appropriate instruments by Les Siècles under François-Xavier Roth (24/96 WAV download, Harmonia Mundi). Through XLR, timbres were warm and colorful. All but the lowest bass was tightly focused. I loved what I heard. Yusef Lateef 's continually fascinating Eastern Sounds, remastered in 2023 (24/192 FLAC, Craft Recordings/Qobuz), made both Scott and me very happy.

A few days later, as I sat at the computer at 9:15am, a series of rapid off/on electrical flickers—so quick that my computer didn't shut down or reboot—precipitated a 45-minute internet outage. When I visited the music room, I found the EtherREGEN at the end of my optical network fried and the Nordost QSource linear power supply that fed it so hot I needed gloves to move it.

With streaming temporarily off the table, I played files stored on the Innuos Statement NG's SSD. Thankfully, 4TB is a lot of music. Meanwhile, I took advantage of the disaster to speak with Dennis Bonotto of Nordost and John Giolas of dCS about optimal power cable connections.

On my first new listen, I felt as though I had entered a magical kingdom—as if I had passed through the Looking Glass into Wonderland. It was like listening through a crystal ball. The music sparkled. Voices were unusually clear, open, and lustrous. On DSD64 files of mezzo-soprano Alice Coote and the Netherlands Philharmonic under Marc Albrecht performing two songs from Mahler's Lieder eines fahrenden Gesellen (Songs of a Wayfarer), from Mahler Song Cycles (DSD64 download, Pentatone), the mezzo's voice sounded clearer than I'd ever heard it.

I felt called to embark on an Henri Duparc journey. I played three versions of his great song "Chanson triste" (Song of Sadness). First came mezzo-soprano Marianne Crebassa with Fazil Say on the piano, from Secrets (24/96 FLAC, Warner/Qobuz). Her voice was cool and grounded, beautiful on top, and quite touching. Then came a true French master, baritone Charles Panzéra, on an edgy digital transfer of a 1927 recording (collected on Pearl GEMM CD 9300), making music out of language, treating every word and note with equal importance, savoring every syllable and vowel as though it were a fine wine, and lingering idiomatically in passages modern singers sing straight through.

Finally, from Reflet (24/96 WAV download, Alpha), the wonderful soprano Sandrine Piau, with Orchestre Victor Hugo conducted by Jean-François Verdier, alchemically transformed each high note into a dew-kissed flower opening in the morning sun as it beckoned bees and poets to drink its nectar. Piau may not share Panzéra's gift of rallentando, but her voice matches it for beauty. I left the music room dazed, as if in a dream, to walk the dogs in the dark under a star-filled sky in the still-chilly early spring.

Time for a pause to evaluate the effects of the Super Black Box. Without it, instruments sounded smaller and less impactful. Air diminished, low bass became lighter in weight. It felt as though I was listening through an inferior amplifier. I suppose I was. It's irrefutable: The Super Black Box significantly elevates the Octave MRE 220 SE monoblocks' ability to deliver musical satisfaction.

When Scott and I listened with the RCA interconnects to Ike Quebec and friends playing "Minor Impulse" from Blue & Sentimental (Remastered 2007/Rudy Van Gelder Edition) (24/192 FLAC, Blue Note/Tidal), we felt that low bass could have been a mite tighter. Nonetheless, color, detail, and musicality were so satisfying that I felt the best way to bid the MRE 220 SEs farewell was to play the moving Adagietto from Mahler's Symphony No.5 with the Orchestre symphonique de Montréal under Rafael Payare (24/96 FLAC download, Pentatone), sigh, and bask in the beautiful flow of massed strings. What better way to say goodbye than with heartfelt beauty?

Summing up
With my unique equipment configuration and setup, it felt as though I was reviewing two closely related albeit different monoblocks. Fed by XLR, the Octave MRE 220 SEs sounded warm, round, and full; fed by RCA, they sounded more neutral, with more varied colors but a bit less substantial. In both cases, their beauty of sound and ability to communicate music's transformative essence won out over other considerations.

You'll definitely want to use the Super Black Box to hear all these babies can do. It absolutely lifts the amplifiers to a level that many components aspire to and few achieve: Easy on the ear, satisfying to mind and body, capable of reaching into music's heart and sharing it with all who listen. There's a palpable yet paradoxically intangible truth to the sound of the Octave MRE 220 SE mono tube amplifiers that makes them an easy, solid recommendation to music lovers of all stripes.