Bryston 7B NRB-THX, ST, & SST monoblock power amplifier
Bryston may be the most senior of the bunch. Founded in the early '60s, it has been manufacturing continuously ever since, exporting a good proportion of its wares to the US. Unlike most high-end manufacturers, however, it targets the professional market at least as heavily as the consumer market. This pro orientation lends a certain distinctiveness to its designs. Its amplifiers, in particular, can be easily moved around by one person. Its cosmetics are attractive but in no way ostentatious. There are no half-inch-thick front panels or brushed gold anodizing—just solid but simple black casework.
The Bryston 7B NRB-THX amplifier, has, in addition to the above characteristics, a complete set of inputs and switches occupying its back panel, some of them familiar, some unusual. There are the now-expected unbalanced and balanced inputs (with an unexpected two of the latter, one for a three-conductor phone plug and one for the more common XLR), two pairs of loudspeaker terminals (why there are two pairs on a monoblock will be explained shortly), and four toggle switches. One of the latter selects between the balanced and unbalanced inputs, another inverts the polarity of the output signal, and a third disconnects the chassis ground (the power cord's third or round prong) from the signal ground. The latter eliminates the ground loop which is sometimes formed when the preamp also has a three-pronged power cord, a situation that most often occurs when preamp and power amp are plugged into different outlets—not unheard of these days with power amplifiers located near the loudspeakers. But, unlike the situation when using a "cheater" plug to accomplish the same end, the Bryston's ground lift switch retains the ground connection between the wall socket and the amplifier's chassis.
The fourth toggle switch—mechanically shielded to make it difficult to engage because it should not be thrown with the amplifier turned on—selects between two operating modes, "series" or "parallel." This feature is unique, in my experience, among high-end amplifiers. Each 7B monoblock is composed of two internal modules, each having its own independent power supply with ±55V rails, and each driven from the main amplifier input. The series/parallel toggle switch, used in combination with the two pairs of output terminals (one pair connected to the output of each module), enables the modules to be connected in either a series or parallel mode.
The toggle switch configures the module inputs; the user takes care of the module outputs by the manner in which he or she hooks up the loudspeaker to the four output terminals—one pair driven from each internal module. The parallel mode of operation optimizes the 7B for loads as low as 1 ohm, connecting both modules in parallel at their inputs and outputs. In the series mode—the type of connection commonly used when amplifiers are bridged—the mode switch reverses the phase of the input to one module and the loudspeaker is connected to the two positive output terminals. With this arrangement, the amplifier is optimized for loads of 3 ohms and above. Hookup in either configuration is actually a very simple matter, harder to describe than to perform.
This two-module, bridged design simplifies the amplifier designer's task in comparison with what is faced in designing most all-things-to-all-loudspeakers amplifiers, but somewhat complicates the user's task. Since most loudspeakers' impedances vary significantly across the frequency range, no one mode will be absolutely optimal at all times. But the situation is manageable; the most difficult task for any amplifier is driving low-impedance loads—those demanding the greatest current capability. If your loudspeakers measure below 3 ohms for a substantial portion of the frequency range (a good example is the Thiel CS3.6), you should use the parallel mode. If they dip below 3 ohms here and there, you should probably use the parallel mode, but can likely experiment safely with the series connection. Stereophile prints impedance measurements in its loudspeaker reviews; if your loudspeaker has not been reviewed, you should be able to get appropriate guidance from the loudspeaker manufacturer.
The internal circuitry of the 7B is completely symmetrical. The output section of each module is composed of multiple pairs of PNP and NPN bipolar output transistors, each pair controlled by a single driver transistor. Each of these sets—output pair and driver—is treated as a composite output device with a very linear transfer function; Bryston refers to this as its "Quad-Complementary" configuration, and although other fully complementary designs are similar, Bryston argues that its particular implementation is unique and proprietary, yielding high overall linearity and mainly low-order distortion products. Feedback is primarily local in nature, and the open-loop distortion is said to be on the order of 0.05%.
Utilizing dual toroidal power transformers—one for each internal amplifier module—the 7B's power supply is said to incorporate circuitry which rejects radio-frequency interference and minimizes the effects of line spikes. The use of external line conditioners is discouraged by Bryston. The front-panel, power-on LED, while normally glowing green, will turn yellow as clipping is approached and red when the amplifier is driven into overload (I experienced nothing but green during my time with the 7Bs). The Bryston uses internal protection circuitry, but it's designed to protect the amplifier while still allowing it to drive even highly complex loads without complaint. One potentially useful feature of the Bryston's power supply is its "soft start" circuit, which prevents an immediate drain on the power line when the unit is turned on. It's potentially useful because this is not likely to be of great importance to a user with a conventional stereo rig. But for the owner having racks of equipment and multiple pairs of 7Bs in, say, a professional or Home Theater installation, it might just keep start-up from shutting down the whole neighborhood.
The Bryston 7B NRB-THX is, as its name indicates, a THX-certified product. For those who may have recently returned from Alpha Centauri, the folks from Lucasfilm who bring you THX movie sound have established a set of standards for various audio components which these products must meet to wear the THX label and be used officially in THX systems—home or theater (footnote 1) Bryston also manufactures a non-THX version of the same amplifier (the 7B NRB) which, the company claims, differs from the THX version only in that it has 1dB more gain and lacks the THX front-panel logo. It also costs $300 less per pair.
Either amplifier, of course, can be used in either an audio-only system or an audio/video setup. I would like to be able to relate what sort of special characteristics an amplifier must have to qualify for the THX seal of approval, but that information is kept confidential by Lucasfilm, and, at Lucasfilm's insistence, by its THX licensees. One characteristic which, deduced from circumstantial evidence, seems to be part of the THX amplifier specifications is a high dynamic headroom—the ratio of instantaneous to continuous power output.
Auditioning began in a system consisting of a C.E.C. TL 1 transport, Mark Levinson No.35 D/A converter, Rowland Consummate preamp, and Immedia RPM1 turntable with Wheaton Triplanar III tonearm and Lyra Parnassus cartridge feeding an Audio Research PH2 phono stage. The loudspeakers were Wilson Audio WATTs/Puppies. Interconnects were AudioQuest Lapis (balanced) from phono preamp stage to Consummate, TARA Labs Master RSC from D/A converter to Consummate, and Cardas Hexlink from Consummate to power amps (the latter two interconnects were unbalanced). TARA Labs RSC loudspeaker cables connected the power amps to the loudspeakers. The transport-converter link was Ensemble Digiflux coax.
My first exposure to the sound of the Bryston 7Bs was during my recent auditioning of the Mirage M-1si loudspeakers for the June issue (Vol.16 No.6, p.192). I had been very impressed by the combination's ease and lack of irritating qualities, its smooth, palpable midrange, and its sweet, silky highs. I had also found the bass and midbass to be deep and rich, leaning more to fullness than to tightness.
The pair of Bryston 7Bs used with the no longer in-house Mirages were a different set from those which I now had hooked up to the WATTs/Puppies in my listening room. The earlier pair were non-THX, while the newer pair had the THX logo tastefully emblazoned on the front panel. "A sign of the times," I thought, though Bryston had assured me that the two pairs of amplifiers were otherwise essentially identical. With the Mirages I had driven the Brystons in their serial mode. The WATTs/Puppies, however, have a sharp but brief flirtation with 1.75 ohms at about 2200Hz, and retain a relatively low (but not that low) impedance at other points above about 150Hz.
I therefore followed the guidance in the 7B's owner's manual and drove the Wilsons with the Brystons in their parallel mode. Later auditioning in the series mode revealed no particular change—positive or negative—in performance, but, with no lack of power in the parallel connection, there was no reason not to adhere to Bryston's recommendations for the bulk of my listening.
I was feeling in need of a bit of Christmas in July—Santa Fe having a hotter than usual summer—so I began by playing Christmas Time with The Judds (RCA 6422-2-R), a recording most notable for its clean, open detail and well-chosen perspectives. The result was striking, with a tight image, well-defined soundstage, and plenty of natural, unexaggerated detail. There seemed to be just the right degree of richness, especially in the three-dimensional vocals. The breath sounds from the children's chorus on "Away in a Manger" sounded like real breathing, not pink noise, and the guitars combined warmth and detail in just the right amounts.
Footnote 1: Use of a non-official amplifier in a home THX system will, as we all know, bring down the wrath of the THX Police. JGH, you're in big trouble.