Atma-Sphere's Ralph Karsten has been manufacturing direct-coupled, differential-circuit tube amplifiers for about 13 years using a special version of OTL (Output TransformerLess) circuit called the Circlotron. To drastically simplify this circuit, just imagine two parallel-output, Class-A SET amps on the same chassis, one working the plus side of the audio waveform, and the other the negative, resulting in a "pure" balanced amplifier. A preamplifier can be made to work in a similar way.
In the past, Karsten's MA-1 Mk II.2 and the MA-2 Mk II.2 amps have received attention in our pages, but his bread-and-butter model, the monoblock M-60 Mk II.2, has not. Weighing only 30 pounds each, using eight 6AS7G triode output tubes, four 6SN7 input/driver tubes, and no output transformers, these amps are easy to move and easier to like. Fast, bold, and of remarkable bandwidth, the M-60s have a wide-open, seamless, grain-free sound that straddles the line between a classic tube signature and that of a fine solid-state design. With the right speakers (although OTL designs need to be more carefully matched to loudspeakers than do conventional push-pull tube amps, Karsten's designs are more flexible in this regard than most of their ilk), the M-60 Mk II.2s are amazingly neutral, with tight, subterranean bass response that is at the same time airy, weighty, and nimble; a spectacularly luminous midband with the dimensionality and liquidity of tubes and the speed and articulation of solid state; and a treble at once soft, sweet, and remarkably extended. One might think that the highs were rolled, but playing something with true upper-octave response proves this a false notion, doubtless brought on by the amps remarkable lack of both HF distortion and glassy signature.
Ostensibly intended to be driven in balanced mode, M-60 Mk II.2s perform quite well using single-ended inputs; however, I have found that, as Karsten suggests, they sound best driven via balanced inputs, as I did with several preamps including the fantastic (aesthetically and sonically) MBL 6010D and Karsten's own stunning MP-1 Mk II (both preamplifiers capable of correctly handling MC cartridges).
Several years ago, I used, for an extended period, the Jadis JP80 MC preamplifier as primary reference in HP's Music Room 1, especially with LPs. Though it was not perfect, this single-ended preamp had a special way with music; the essence of compositions always made their magic known without fanfare or emphasis. Each record or CD had its own sonic signature; the preamp pretty much stayed out of the way of the sound (except for a slight softening and sweetening of the high treble), bringing about magnificent dynamics and an effortless, live quality to the music that was equal parts elegant circuitry (correctly designed and implemented) and electronic alchemy. The separate, massive tubed power supply did not hurt matters, either.
The MP-1 Mk II is such a two-piece preamp, but vastly less expensive and more specialized in that it is designed primarily to accept balanced sources, including phono (though it does allow a single-ended source). It also handily surpasses the Jadis in frequency extension at both extremes and has even greater dynamic wallop, with the best 3-D soundstaging I have heard. To me, the MP-1 Mk II is a glorified phono section with the bonus of line-input capability.
Both of these components are excellent on their own terms, but used together, the M-60 Mk II.2s and the MP-1 Mk II make music in a way and on a scale that is compelling and seductive in the extreme.
|Atma-Sphere MP-1 Mk II Preamplifier|