Solid State Power Amp Reviews

There are peripheral issues. To get the most from the Supercharger, you need more expensive jumpers and AC cords than the ones supplied, but these are minor costs. I used the supplied jumpers, along with my reference Shunyata Research Helix Anacondas.

Let's play!
First up was the 100Wpc Music Reference RM-200, my reference tube amplifier and a reasonably priced, rugged, ultra-reliable component. JA's measurements, which accompanied my review in the April 2002 issue, showed that the RM-200 delivers 95W via its 4 ohm tap at 1% distortion. It can drive the 92dB-sensitive Wilson Audio MAXX 2s, but with only 19.8dBW available, that's only about 105dB, which puts it in the range of "Average Quality" on Musical Fidelity's sliding scale. While the RM-200's sound when driving the MAXX 2s is velvet-smooth, rich in the midband and airy on top, the dynamics aren't exactly realistic; the combo is best used for background and easy listening.

Keith Jarrett's stunning The Carnegie Hall Concert, a solo-piano recording (2 CDs, ECM 1989/90), demonstrated that. Jarrett's fierce attacks—especially in the heavily rhythmic, tuneful, gospel-like "Part VII"—were unnaturally muted and softened as the body of the instrument blended into the ambience. The best I can say about the RM-200's sound on its own is that it places me in the upper balcony of Carnegie—which is not the perspective of the recording.

When I played Classic Records' reissue, on 200gm vinyl, of Led Zeppelin II (and if you think this record is bright, blame the messenger and do something about your system), the sound was just plain flaccid. And Cisco's superb new reissue of Leonard Bernstein and the New York Philharmonic's recording of Shostakovich's Symphony 5 (Columbia MS 6115) merely simmered when I know that it boils.

Then I added the 550Ks to the system, driving them from the RM-200's 16-ohm taps. Dynamically, the combination was literally off the "High Quality" end of Musical Fidelity's "System Diagnostic" calculator. But could the 550K increase dynamic range without destroying the essence of the RM-200's sound?

The answer was an unequivocal "Yes." Adding the 550K delivered the RM-200 intact, but on steroids. The Jarrett CDs sprang to life dynamically in every sense. The woody richness of the piano remained intact, as did the airiness of the overall presentation and the sense of musical flow that tube lovers crave. But Jarrett's attacks, from the softest to the most insistent keyboard strokes, produced not just more loudness at the top end of the scale, but a world of dynamic gradations in between. That, more than the accurate production of dynamic peaks, was what helped sell the sound. Adding to the sense of reality was the far clearer delineation of the piano relative to the real reverberant field of Carnegie Hall, as opposed to the cloudy one that I conjecture was produced by the tube amplifier clipping.

The experience with the RM-200 alone seemed to prove Musical Fidelity's point: aside from the generous improvement in dynamics, the 550K itself seemed to be MIA. There was no solid-state etch, or dryness, or whatever else it is about transistors that tube lovers don't like. Nor was there any improvement—or, if you prefer, change—in bass extension or punch. The bass remained, for better or worse, "tubey"; that is, it lacked the punch, drive, and control that, in my experience, only big, solid-state amps produce.

This review could have ended right here. However, a newly restored Scott 299D integrated amplifier from 1964, sitting on a shelf in another room, proved too tempting. There was no point in trying to drive the Wilson MAXX 2s with the 18Wpc Scott, but adding the 550K made it possible. If the 550K was really adding nothing but a boost in dynamic range then the sound produced by the 299D should have been totally different from that of the RM-200.

The sound of the Scott 229D plus MF 550K was not only totally different from that of the Music Reference RM-200 plus 550K, it was in many ways better: a massive attack that was more vivid and immediate, bigger without losing focus, more three-dimensional, and faster, yet still very definitely tube-like, with excellent bass extension and control.

How to explain the satisfaction of listening to a supposedly washed-up, low-powered, veteran amp—one that, refurbished or not, is supposedly ready for the audio glue factory—instead performing like a thoroughbred as, with the assistance of the 550K Supercharger, it drove a pair of Wilson MAXX 2s with total control? This is the sort of thing that makes this hobby fun. The 299D now sounded both loud and dynamically complete—and its volume control never got past 11 o'clock.

The Jarrett album's explosiveness and the piano's crisp attack led me to Live at Merkin Hall (CD, Stereophile STPH018-2), the recent album by Stereophile writer Bob Reina's band, Attention Screen, recorded by John Atkinson. Wow! Perhaps the music's magical, river-like flow and ease (aside from the excellent recording job) can be attributed to the 43-year-old amp's tube rectification, or to its output transformers, or to everything together. Whatever, it was almost another week before I could let go of the Scott-plus-MF's fast, detailed transient attacks and overall liquidity.

Later, I tried a Hafler DH-200 power amp and a refurbished Scott LK-72 integrated (love those garage sales). The 550K worked its dynamic magic on both. Then I tried Musical Fidelity's X-T100, a moderately priced integrated that puts out 50Wpc into 8 ohms (80W into 4 ohms), with MF's X-Ray^V8 CD player, both powered by MF's outboard Triple-X power supply. On their own, as expected, they weren't really up to driving the MAXX 2s. Boosted by the 550Ks, they were.

Compared to the Scott 299D, the MF X-T100 exerted better control at the bottom end and was cleaner and faster at the very top, with better overall resolution of inner details. But the magic of tubes—the colors, the vibrancy, the flow—was gone. In fairness, the X-T100 was auditioned cold; but then, the Scott 299D had commenced its magic immediately on power-up, after spending months in the garage.

The bottom line: The 550K Supercharger was like a chameleon, adding power without making its presence known by any telltale sonic signature.

Conclusion
There's nothing particularly new about the technology behind Musical Fidelity's 550K Supercharger. Anyone old enough to remember first-generation car-stereo separates can recall that these were strictly "speaker out" devices; adding more power required "speaker-level," input-based amplifiers. The 550K's input circuitry is similar, as is the input circuitry of powered subwoofers that can be driven by amplifier outputs and claim to take on their tonal character. But despite these obvious antecedents, until now no one, to my knowledge, has thought of using the technology in a full-frequency power booster for a high-performance audio system.

Using a variety of very different-sounding amplifiers of various power outputs overwhelmingly demonstrated to me that the 550K Supercharger will retain the sonic attributes of your favorite low- or medium-powered amp (50–200Wpc), whether tubed or solid-state, while increasing its output by 10dB or more. The result will be dynamic realism and, in most cases, better overall performance. You can have your cake and make it rock, too.

If you attend a lot of concerts—whether of classical, rock, jazz, or all three—and are thus attuned to the true dynamic range of live music, listening at home to a limp, dynamically restricted stereo is disappointing. But once you're hooked on real-world dynamics at home, it's hard to give them up. That's why I can't part with my 1000W Musical Fidelity kW monoblocks, and that's why the 440W mbl 9007 monoblocks I reviewed in September 2006 proved so satisfying.

Dynamics aren't everything, of course. But if you feel that your system has everything but full dynamic range, or if you're curious to hear if it's possible to improve your system in that area, there's now a way of getting realistic dynamic performance without sacrificing any aspect of what you already like about your system.

The 550K Supercharger is an innovative but no-brainer, "Why didn't I think of it?" product that offers massive reserves of power at a reasonable price. I have no doubt it will measure well—most Musical Fidelity products do. Dealers should let shoppers take 550K Superchargers home and try them for themselves before buying. Few will be returned.

Footnote 1: Musical Fidelity offers a "System Diagnostic" calculator on their website, where you match your speaker's voltage 1m sensitivity in dB with your amplifier's maximum power rating to get the maximum sound pressure level in your room. (It appears to subtract around 7dB to allow for the fact that you sit further away from your speakers than 1m.) It then rates that spl as "Average Quality" of dynamic realism for 102–105dB, while one capable of only 97–101dB is of "Low Quality." Higher than 106dB and your system is rated as "High Quality."—John Atkinson