Audio Research 610T Monoblock Power Amplifier

Masterpiece!

It is a little sobering to look back to 1973 and the first pieces of Audio Research gear I heard—the SP-3 preamp and D75 amp powering Magneplanar I-Us in a stereo demo that, for me, has never been bettered or forgotten— and then to consider how consistently and naturally all the subsequent ARC gear I’ve heard, and I’ve heard a lot, has been voiced. From the start, ARC components had a sound that was uniquely, indelibly, addictively “right.” ARC’s designer and founder William Zane Johnson called this sound “high definition,” a trademark that still appears on the faceplate of each and every ARC component. And even in 1973 his creations were astonishingly high in definition; indeed, their standard-setting resolution, lifelike size, bloom, and airy brightness, and exceptionally low levels of tube-like coloration were a large part of what set them apart from the darker, thicker, blatantly euphonic sound of the tube preamps and amps that preceded them.

Not that everyone preferred high definition tube sound. There were those, then, and are those, now, who thought and think that tubes should invariably make music sweet, round, and rosy, that prettifying sources is the vacuum tube’s job in life—the very thing that sets it apart from the crisp, clean, “neutral” presentation of the transistor. Johnson never bought into this model. Indeed, it was the superior accuracy and neutrality of transistor electronics—which in the late 60s and early 70s were even more in the ascendant than they are today—that inspired him to outdo solid-state at its own game. His tube products were expressly designed not to artificially prettify recordings; instead, they were intended to be as faithful and transparent to sources as electronics can be made to be. When it comes to 12AX7- based preamps and 6550-based power amps, the very concept of transparency was really born with Audio Research.

The inspiration for these ruminations is the 610T monoblock power amplifier—the latest in ARC’s long, storied line of highpowered tube amplifiers and the subject of this review. Replacing the celebrated Ref 600 MkIII (much beloved by our EIC, Robert Harley) and capable of better than 600 watts into any load this side of a short circuit, it is the culmination of William Zane Johnson’s almost forty years of experience in amplifier design and constitutes both a continuation of and a significant advance beyond the virtues of previous ARC superamps.

The changes in the 610T begin with the way it looks. Designed and built on a vertical rather than a horizontal chassis, it is a tall, rectangular, 170-pound brushed-aluminum tower, which mounts its input and output tubes in the open on its top plate, without a surrounding cage. (In the dark all those lit-up 6550s glow like rows of candles on a tubelover’s birthday cake.) Concealed within the massive tower is a ground floor that houses output and power transformers, and a second story for power-storage-andregulation components. The tower design allows the 610T to rely on convection rather than built-in fans (with built-in fan noise) to cool its complement of twentythree tubes. That said, don’t count on convection to cool the 610T overly much. On a hot afternoon, the forty-six tubes in a pair of these amps can raise the ambient temperature of your listening room by a good 15–20 degrees in next to no time, making warm-weather listening necessarily a challenge (and necessarily brief).

Although the 610T, like virtually every Johnson power amp, uses 6550C pentode output tubes—sixteen of them in sets of eight matched pairs—in a fully balanced, push-pull, Class AB circuit, it also uses (for the first time) two 6550Cs as driver tubes—each controlling one bank of eight output tubes operated in unison. Twin 6N1P triodes and a 6H30 follower make up the input gain stage. The combination of the 6550 drivers and 6N1Ps/6H30 inputs has allowed for a welcome simplification of power-tube biasing; instead of having to adjust the 16 output tubes individually (as you did with the Ref 600 and previous ARC amps), in the 610T you need only bias the first two output tubes via set screws in the front of the chassis and the amp’s nifty vacuum-fluorescent display, which reads out bias measurements for each tube (as well as A.C. line-voltage levels, power output, and total hours of tube usage).

ARC says that power-supply energystorage has been increased substantially in the 610T to an astonishing 1000 joules, with substantial impact on sound quality (for which, see below). In a hybridizing move that Johnson has made successfully before in other products, ouput stage power-supply rectification and regulation is solid-state, while input gain stage regulation is both tube and solid-state. As usual with ARC, the 610T’s ultrawide- bandwidth output transformers are custom-made, with separate 4, 8, and 16- ohm output taps.2 Be aware that the 610T’s input is balanced only, so you’re going to need a preamp with balanced outputs. You’re also going to need dedicated 20- or 30-amp lines to feed a pair of 610Ts, each of which can draw as much as 2300 watts from your wall outlets (1700 at rated output).