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.
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.
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, 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.
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