NHT Evolution T6 loudspeaker system
The heart of the T6 is the M6 monitor, a three-way, sealed-box design using a pair of plastic-cone woofers, one positioned either side of a midrange cone and a metal-dome tweeter. A solid-feeling speaker, the M6's rigidly braced cabinet is fabricated from ¾" MDF, with a separate chamber with nonparallel walls for the midrange unit. The rear of the tweeter's neodymium magnet is fastened to a solid aluminum rod that both acts as a heatsink for the tweeter and holds the enclosure's front and rear panels together. A two-way switch allows the M6's output between 80 and 500Hz to be optimized for free-space placement or close to a wall.
The M6 can be used vertically, with the midrange to the side of the tweeter, for L/R HT or music reproduction; or horizontally, as a center-channel speaker, in which case the tweeter sits below the midrange to aim the sound down from above the TV. The M6, available separately for $600 each, can be used with the matching P6 pedestal ($200 each) or sitting atop the B6 bass module, in which case the M6 is bolted to the subwoofer.
Like its 3.3 grandparent, the B6 is much deeper than it is wide, its drive-units—in this case a pair of 12" aluminum-cone units—mounted on its side. A pair of B6es is "handed"; ie, the woofers of one of the pair are mounted on the opposite side to those on the other. Each bass module sits on a pair of solid aluminum stabilizer bars fitted with carpet-piercing spikes.
The T6 system includes a line-level crossover, the X1, which can be operated in both balanced and single-ended modes. While it doesn't appear to be manufactured to the highest of audiophile aspirations—it runs off a wall-wart supply and uses a mix of TL072 and LM833 dual–op-amp chips—the X1 offers a versatile selection of high- and low-pass filter options, and I didn't find it to be a limiting factor in my auditioning. The X1 also applies the EQ required by the B6.
The B6 modules can be driven by a summed-mono output from the X1 or by stereo bass signals. An amplifier is required, of course, and NHT supplies two A1 monoblocks for the T6 system. This solid-state design is specified as delivering 250W into the B6's 6 ohm impedance; unusually, it features a class-G output stage rather than the ubiquitous switching amplifiers used for subwoofers these days. (A class-G amplifier uses relatively low voltage rails with a high current capability for most of the time, switching in high-voltage rails with limited current capability to handle musical transients.)
The electronics are made in Taiwan, the speakers in China. There is a great deal more technology and features on offer than I have room to describe here. You can find white papers on the T6 at the NHT website.
Setup was tricky, with the vertically oriented M6 capable of being used with its tweeter either to the speaker's outside or inside edges and the B6 with its woofers facing toward or away from the other B6. Then there was the question of the X1 crossover settings to be resolved. After a long period of experimentation, I ended up with tweeters on the inside edges of the M6es, midrange units on the outside edges, with the speakers firing straight ahead—the latter is NHT's recommendation—and the B6 woofers facing each other. The M6 boundary switches were set to "0," their free-space position. With the X1, I began with the settings recommended in the manual, then adjusted high- and low-pass controls until I got what sounded to be the best integration between satellites and subwoofers on the widest range of recordings. (The manual, by the way, offers excellent advice on achieving this goal, with examples given of what changes in control settings sound like.)
My final settings were: High-Pass feed to the M6es set to 50Hz, Low-Pass feed to the B6es set to 65Hz, subwoofer Phase 180 degrees, Boundary Gain +2, and Master Gain set to 11:00 (though I kept fiddling with this).
Once I'd gotten the Evolution T6 system working optimally in my room, I reached for my usual list of diagnostic discs, including Editor's Choice, a compilation of my own recordings (CD, Stereophile STPH016-2). The half-step spaced tonebursts revealed a smooth integration between the satellites and subs, with full weight available at the lowest frequency of 32Hz. The 1/3-octave warble tones indicated that the speaker rolled off sharply below 32Hz, with very much reduced output apparent at 20Hz. Pink noise sounded smooth, though I could hear a slight accentuation in the presence region. NHT warns in their manual that auditioning the speakers without the grilles would result in a slightly bright balance. However, on balance—ha!—I preferred the speakers au naturel.