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Phillip Holmes discovers a diamond in the rough that is the

StereoKnight M75 mono block tube amplifier

Phillip Holmes March 2009

Manufacturer: StereoKnight HiFi Technology LLC.

7890 Grove CT. E. Germantown, TN, USA

Tel: 1-901-283-0073 Fax: 1-901-248-6866

URL: www.StereoKnight.com

The M75 mono block amplifiers are a little deceiving on the surface. With the champagne colored metal work and the acrylic windows, it suggests that it might be a ¡°hi-fi¡± sounding amp, full of sound and fury. Under the skin though, it proves to be a pragmatic design, and might be the most practical power amps I’ve used. The first clue that it wasn’t a ¡°typical¡± amp design was the specifications.

Specifications:

Tubes: 1 X 6H30, 2 X 6922 4 X KT88 Power output: 2×75W (8¦¸ ULTRALINEAR); 2x 140W (4¦¸

ULTRALINEAR) Input: 1 x RCA, 1 x XLR

Frequency response: 10Hz～60kHz THD: 1%(1kHz)

Input sensitivity: 2 V Input impedance: 50 K¦¸

Output impedance: 8¦¸ , 4¦¸ S/N ratio: > 90 dB

Power consumption: 550W maximum MSRP: $3,850/pair

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You can infer some things from the specifications. First, with a THD of 1%, it probably doesn’t use feedback (it doesn’t). Second, the designer is confident enough of the merit of his design that he didn’t feel obliged to apologize for the distortion value. As a matter of fact, during the majority of my listening, I didn’t hear any audible distortion (only once actually). When I read the M75 information, StereoKnight states that a primary goal is longevity and stability. To that end, the tubes are run more conservatively than many other designs. The M75, with four tubes, puts out what some amps put out with two. I ran the amps for extended periods without checking the bias on the tubes, and never had any incidents. Though the specs don’t specify the -3dB point or power output, 10Hz-60kHz frequency response would be good results from a feedback-free design. It also indicates good output transformers. By the way, the output transformers are ultralinear. Ultralinear operation is a compromise between triode and pentode operation. Actually, the KT88 is a power beam tetrode, and doesn’t behave like a pentode. It’s efficient like a pentode, but has much lower 3rd harmonic distortion than a pentode. When used in an ultralinear circuit, it comes darn close to mimicking the sound of a triode, but with better efficiency. A good ultralinear transformer and circuit design can give you sound that is sweet and low distortion like a well-built triode amp, while giving you power reserves that most triode amps can only dream about. The design is a true balanced push-pull amp from input to output, but uses a transformer phase splitter if you don’t have a balanced preamp. The amp has better bass extension when using the balanced inputs, and seems to have better resolution. With the transformer input, the sound is more luxurious, with more midrange presence. This gives the amp a split personality; no pun intended. Depending on input choice, the amp will sound different. Not only that, but because it uses no negative feedback, tube rolling is the order of the day. Since the output tubes are run conservatively, you could try using some old stock Tung-Sol 6550 or Genelex KT88 (I did not). And you have hundreds of choices to try in place of the 6922; I tried Bugle Boy ecc88 and US Amperex 7308 JAN tubes. The 6H30 doesn’t lend itself to rolling, but that’s not a big deal. If you take into account cable choices, tube choices, bias tweaking, transformer phase splitter or XLR balanced input, you can get radically different sounds from this amp. After living with low-powered tube amps for a while, it was fun to listen to a powerful one. I’ve noticed that even when using efficient speakers, I missed the extra current reserves of a big amp. It put restrictions on my musical choices. Large classical works, heavy metal and big band are some of my favorite genres, but I didn’t enjoy them as much with low-powered amps. VERSATILITY

I used the M75 amps with three different speaker systems, all with radically different designs and sounds. The MaxxHorn Immersion, my modified Maggie 2.6r, and the Jaton REAL A&V-803. All three are radically different, with radically different impedances and sensitivities. Starting with the MaxxHorn Immersion, the M75 proved to be delicate enough to go head-to-head with single-ended designs. It was interesting to compare the radically different Audio 300B single-ended amp with the M75. Though the single-ended amp, with its simpler circuit and built-in passive preamp, had the edge in micro details, the M75 had a similar sonic fabric to the small, single-ended amp. To keep the comparison as similar as possible, I used a passive single-ended stepped attenuator,

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eschewing a fully active preamp. I might hypothesize that there might be some losses in the phase splitter input transformer used in the input of the M75. Head-to-head with an extremely efficient design, the M75 held its own in the detail and tone department. However, when it comes to slam, the M75 quickly put the hammer down on the competition. Wanting to see just how much abuse the Immersions could handle (and the driver is a ¡°pro¡± driver), I pulled out some heavy metal, some Beastie Boys, United States Marines Drum And Bugle Corps, etc… The Immersions could handle much more than my ears could. It was some of the loudest, cleanest, most dynamic sound I’ve heard anywhere. It did get a touch forward, but I blame that on the driver used in the Immersion, which seems to have a slight resonance in the presence region, like the coil former or tweeter phase cap is resonating. 7 watts versus 75 watts sounds like it would be ten times as much sound, but it’s not. It’s closer to 3 ½ times as loud. Even while it was huffing and puffing and blowing my walls down, it managed to maintain a realistic image. It’s hard to figure if it was the critical damping of Johan van Zyl’s cabinet design, or the amp, but the system never lost control, even when the reviewer did. Though I enjoyed the combination, I couldn’t wait to connect my Maggies. I paired the M75 with StereoKnight’s Silverstone-Balance (a fully balanced transformer volume control), an Art Audio Vinyl Reference phono stage, and my usual turntable setup of Denon DP80, SME V arm and Lyra Argo. It was a breath of fresh air to hear the big speakers come to life. Maggies, regardless of their rated efficiency or the relative advantages of a line source speaker over a point source, need lots of power. It might just be tube lover hyperbole, but the M75 sounded like a 300-watt transistor amp sans the sand sound. That’s not too much of a stretch though. The M75 is rated at 140 watts into 4 ohms and the Maggies dip to 3.4 ohms. I would guess that it was putting out more than 160 watts on more than a few occasions. Since the M75 doesn’t have feedback or the gross clipping distortion that comes with feedback, it compresses gently and loses steam at the frequency extremes, much like a SE-DHT amp with no feedback. Listening to the LP box set The Complete Pacific Jazz and Capitol Recordings of the Original Gerry Mulligan Quartet (Mosaic MR5-203), I was struck at how dimensional sounding these recordings were with the M75. For mono recordings from 1953, they could almost fool you into thinking they were in stereo. This was markedly better than the last tube amp I used with the Maggies. These recordings are pretty dynamic too. With the M75, both large dynamic swings and small nuances were handled with ease. The start and stop of notes were fast and clean. The net result of the good dynamics and speed was a propulsive sound that was ¡°jazzier¡± than most. Bass had speed and weight. Drums were ultra clean with very fast attacks.

make this record an audiophile favorite can take away from the music’s message. With the StereoKnights, the audiophile stuff was handled with an ease that didn’t draw attention to the ¡°sound¡± of the recording. Listening to one of the new and improved Classic Records pressings of Duke Ellington and Louis Armstrong Recording Together for the First Time (CLASSIC/ROULETTE SR52074-45QPC, 200g, 45RPM, single sided, clear vinyl—whew!!!), the clarinet of Jimmy Hamilton was as reedy and palpable sounding as I’ve heard in my system. The clarinet is a difficult instrument to reproduce, with a hollow sound that most systems get wrong. The clarinet’s structure suppresses even-order harmonic overtones, allowing primarily odd-order overtones through. This gives the instrument the woody hollow tone. When played back on a tube system that has too much even-order harmonic distortion, the clarinet loses some of its hollow sound. With bad sounding transistors, the clarinet sounds shrieking and aggressive. How much of this was the amp is hard to say, but the M75 has the

The M75s made the Maggies sound as if they had another octave of bass resolution, especially when playing at high volumes. With other amps, there are times that these speakers will sound bright when things get loud. These maintained the correct tonal balance regardless of how challenging the material. StereoKnight makes a big deal about their power supply, and it sounds like it has the current reserves to handle difficult speakers and recordings. Moving on to ¡°serious audiophile music¡±, I played Breaking Silence (Analogue Productions APP-027) which seemed to be more human and less of a demonstration disc. There were more tonal inflections and nuance in Ian’s voice. The qualities that

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transparency to allow the appreciation of really good 45rpm vinyl (the closest we will get to the sound of the master tape). I could also pick out multiple distinct images. Cream’s ¡°Strange Brew¡± was a riot. There was an ideal amount of bite in the sound. The guitar sound was a perfect combination of body, string and distortion. It didn’t get too forward or aggressive, even when pushed to very loud levels. Clapton’s voice was dulcet, and contrasted with the sound of the ensemble. On Dark End of the Street by Cat Power (Matador Records, on two 10¡± LPs), Chan’s voice was as sweet and sensuous as it usually sounds, but now there was more drive. The bass line on the title track was intense. The amps kicked out some serious volume while leaving Chan’s voice unscathed. The cover of Fortunate Son is also excellent. You should pick up these discs as soon as possible. As of this writing, they are still available for $16—that’s chicken feed. The amp’s ability to image outside the speakers was emphasized on A Night In Tunisia, Art Blakey BST-84049 (45 RPM Music Matters pressing). This album has a massive acoustic space, some of it created by van Gelder’s studio work. Besides the space, there is the issue of handling the tremendous scale of Art Blakey’s drum sound. Between the dynamic contrasts and realistic imaging, it sounded like I was hooked up to a microphone feed on the other side of the recording booth glass. When going to the Jaton REAL A&V-803 speakers, a relatively efficient 3-way bass reflex design, it sounded like I could achieve rock concert levels with impunity. These speakers aren’t as ¡°picky¡± as the high efficiency MaxxHorns or the transparent Magnepans. What the speakers do though, especially in combination with the StereoKnight M75, is combine some of the virtues of the big horn and the planar into a package that plays difficult pop and rock music with grace and clarity. I could follow the vocal lines in overly processed recordings, some for the first time. The pairing was very successful with music that can sound threadbare and jumbled together with other combinations. THE LITTLE PARTNER THAT CAN!

Silverstone-Balance, I much preferred the sound of the Silverstone-Balance. There was no noise, better bass and dynamics, and no added midrange grain that I heard with the active tube preamp. I heard no limitations in the highs or the lows with the Silverstone-Balance. The sound of the Art Audio Vinyl Reference driving the Silverstone-Balance, into the M75 monoblocks, all wired up with Purist Audio cabling, was some of the quietest and cleanest sound I’ve had. An interesting point to make about the Silverstone-Balance is that you can hook either a single-ended RCA or an XLR to the input of the preamp, and then drive the input of the power amp with an XLR, bypassing the built-in phase splitter transformer. Regardless of whether I used the Silverstone-Balance with single-ended or balanced inputs, the sound was superior to driving the

I would be remiss if I didn’t talk about what I feel is the natural partner to the M75 amps, the Silverstone-Balance passive transformer volume control. The build quality is first rate. The body is CNC machined from a solid billet, the Teflon coated copper wire used is meticulously soldered, and the Elma switch is about as good as money can buy. The unit is configured for either zero or 6dB of gain. I preferred the sound of the unit in the 0dB gain setting, where the bass seemed a little firmer and deeper. You can choose between balanced inputs and RCA inputs. It has one set of balanced outputs and RCA outputs (again, you must choose one or the other).. When I switched between my tube preamp, my stepped passive resistor attenuator and the

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M75 with a single input driven by the passive ladder volume control or active preamp. I’m sure some of this is due to the nice cables on loan from Purist Audio Design, but the majority was the excellent transformer volume control. Time didn’t permit me to audition the Silverstone-Balance with other amps, but I wouldn’t be surprised if it were an affordable option that stands toe to toe with the best preamps in the world. THE FIRST TRUE LOVE

There was a reason that I became an audiophile. It wasn’t that I like pretty equipment, or spending large sums of money. Music is my first love. Back when I thought girls were nasty, before my hormones kicked in, there was music. I was lucky to grow up in an environment that exposed me to stimulating sounds. My parents were both musicians and my father had the hi-fi bug. Mother was a band director, and who knows how those sounds might have affected me before I was even born. I grew up with good music and sound. But sometimes it becomes an effort to listen. Aren’t there times when you ask: Do I need to turn off the system and give it a break? Am I trying to ¡°one-up¡± my audio buddies? Am I trying to play ¡°sophisticated¡± music and thereby gain approval from a musical snob? These are questions I ask myself when my mind starts to wander while listening. Thankfully, some components and recordings revive the excitement that got me into this hobby. Just as I was getting into another listening funk (funk meaning blah, not like George Clinton or James Brown), the StereoKnight amps rekindled the flame. I believe these amps combine virtues that you would find in totally different design topologies, such as the grace of a 300B amp with the bass grunt of a solid-state design. They feature ¡°name brand¡± parts and expensive machine work, but don’t cost what you would expect to pay for a name-brand amp of similar sonic qualities. The only time I got them to sound ugly was when I tried to clip them on purpose (with Dark Side of the Moon). It would’ve been more shocking had the amps not ran out of steam. There were minor quibbles. Occasionally, when driven with the RCA inputs, the amps had a slightly wooden bass quality. Then, there is the issue of the aesthetics. I prefer black. However, these are picayune critiques. Both the amp and transformer volume control are highly recommended and I will be sad to see them go. Not all products that come through tempt me to purchase them, but these did. Honestly speaking, if I had the extra cash, they’d be mine. Maybe later this year? Here’s hoping the stimulus bill works!

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