Rogue Audio Magnum M-120 monoblock power amplifier
Handsome Is as Handsome Does
With its steel chassis and 3/8"-thick aluminum faceplate, the Magnum M-120 tubed monoblock amplifier bears witness to the unadorned functionality and high-value performance that have become Rogue Audio trademarks. Set off against a black circular recess in the center of the machined faceplate, a heavy aluminum button triggers the amp's slow start/turn-on mechanism. The simple, elegant retro look echoes the straightforward, refined topology within—because handsome is as handsome does, and Rogue Audio is committed to channeling its resources into performance features you can actually hear.
That's because, when chief designer Mark O'Brien and his partners, Phil Koch and Mark Walker, came to the High End from the Lucent division of Bell Labs back in 1994, they did so with the enthusiasm of dedicated audio fans—three top engineers determined to have some fun. O'Brien is Rogue's chief designer, while Walker and Cook are responsible for producing and manufacturing those concepts. O'Brien has some very definite ideas about audio design, reflected in his use of an output stage configurable for either triode or ultralinear operation, and, in the Magnum line, in his insistence on using premium parts throughout. The standard M-120 costs $2995/pair. The Magnum models include larger, higher-performance output transformers; heftier, specially tweaked power supplies; high-quality interstage coupling capacitors; Dale-Vishay resistors; better binding posts; gold tube sockets and silver wiring; and premium input and output tubes—all of which bump the price to $3495/pair.
A defining aspect of all Rogue circuit designs is their use of mu-followers. O'Brien explains: "It's a way of taking two tubes—or two halves of a dual-triode, in our case—and using one half as a constant current source. Because the current can't change, the voltage is amplified. We employ two 12AU7s as driver tubes in the Magnum M-120, whereas most driver circuits employ only one. This allows the tube to function in a very linear portion of its operating range, which means that in bumping up a fairly small signal to 10V, 20V, or more, you're driving the output stage without adding any significant distortions. The other benefit is that mu-followers really excel at rejecting any power-supply noises."
Another distinctive aspect of the Magnum M-120's tube circuits is the use of cathode biasing. "There've been cathode-biased amps since the '50s," says O'Brien. "Because it's such a reliable design, with our amps it's just plug and play: you can replace tubes without having to do any tweaking or rebiasing. Which means that you can employ different output tubes—such as 6550s, EL34s, or KT88s—in the same sockets. And that's a really popular feature. It's also a very musical tube sound, with excellent transparency. The tradeoff is that cathode biasing typically produces around 70% the output power of a fixed-bias amp, so the amps run a little hotter."
The reference system I used with the Magnum M-120 (see Sidebar, "Associated Equipment") was the same as that used for last month's review of the VTL 5.5 line stage, with one exception: Toward the end of my time with the M-120, the Sony SCD-777ES SACD/CD player began skipping, to the point where I had to put it on ice until I could get it fixed. I returned to my California Audio Labs Delta transport and the 24-bit/96kHz version of CAL's Alpha tubed D/A converter, and was impressed anew by the amount of bass energy the Delta conveyed, as well as its firm grip on dynamics. While its depth of resolution and transparency weren't on a par with those of the Sony, the way the Delta accentuated the Alpha's hypnotic liquidity and holographic qualities was very satisfying.