Bryston 4B power amplifier
The Bryston 4B NRB available in stores today incorporates a number of features not found in the original design. While 5 lbs lighter, its power-supply energy storage has been increased by 28% as a consequence of raising the rail voltages from 75V to 85V (footnote 1). Power-supply impedance and IM distortion ratings have been reduced by a factor of four, and twin toroidal transformers have replaced the earlier E-I core transformers.
Having reviewed an earlier version of the 4B for Audio magazine (November 1985, Vol.69 No.11, pp.80–89), I was very interested when designer Chris Russell showed me this version at the 1991 Summer Consumer Electronics Show. Its extraordinary warranty and redesign were reasons enough for me to re-review this classic.
The Bryston 4B NRB amplifier became popular in early 1978, when The Audio Critic declared the then–$1195 amplifier "the best power amplifier we've been able to find so far" (Vol.1 No.4, p.41), praising it as an "unflappable voltage source with an unshakably tight bass response"—but only if warmed up for an hour. Never mind that it was found to be "just a wee bit hard and zippy compared to our current favorites" just two issues later (Vol.1 No.6, p.31); the audiophile community had been alerted. The other amps in that magazine's "search for the perfect audio amplifier"—the Futterman H-33aa, the db systems db-6, the 25Wpc Electrocompaniet, the Quatre DG-250, the RAM 512, and the Threshold 800A—have not been in production for over 15 years.
Bryston has since improved upon the 4B's design. A new output stage was added in 1985. My review that year praised the Bryston 4B's sonics for excellent channel separation and ability to establish a stable soundstage with uncanny directional cues. It had speed, bass control, and lots of "slam." Studio engineers and concert pros liked the Bryston 4B NRB for its ruggedness, reliability, and passing of the "steel toe" test (footnote 2). Although it was a better-sounding product, it joined the Crown DC-300 as a standard tool for recording engineers and touring musicians.
New design features & construction
The Bryston 4B NRB is a class-AB2 solid-state stereo power amplifier. Smaller than some of the monoblock amplifiers reviewed in these pages, its slim profile and 44 pounds belie its power capacity. Bryston rates this amplifier at 250Wpc continuous into 8 ohms, 400Wpc into 4 ohms, and 800W, bridged, into 8 ohms. Its compactness comes from using the whole chassis as a heatsink.
The 1991 Bryston 4B NRB has a totally redesigned front panel. Gone is the flat, 1/8", rolled-aluminum, dull, military black front panel; it's been replaced by a ¼"-thick sculpted rack-sized panel etched with two thin lines. The two front handles have similar grooves. This new panel is made of a single extruded aluminum piece, buff-finished with a fine abrasive like jeweler's rouge. The surface is now so smooth that one cannot leave a mark by rubbing a finger across the surface. The front panel features tri-color LED pilot lights, one per channel. These remain green while the unit is on, but flash first amber when a channel clips momentarily, then red for hard clipping. The square power pushbutton is the only front-panel control.
Rounded heatsinks line the amp's sides, not the back panel as in former Bryston 4Bs. This means that the chassis can be an inch deeper than before, permitting space on the rear panel for a number of new features. These include balanced XLR input sockets, toggle switches for floating or using the ground lead (on the three-prong cable), and a switch for choosing either single-ended or balanced inputs. These switches are greatly appreciated, particularly if you've ever misplaced the tiny U-shaped shorting pins that must be inserted in the XLR input sockets of some other high-end amplifiers (the Mark Levinson No.27, for one) to be able to use the power amp's single-ended RCA input jacks.
Opening the Bryston 4B requires a special screwdriver called a "Robertson." Bryston continues to use excellent threaded steel inserts and Robertson machine screws to attach the top panel. Internal fasteners and screws are used with locking thread-sealer to lower vibration and increase structural stability. As with the earlier models, the new Bryston 4B uses gold-plated board-edge connectors on driver and input boards. Soldered and otherwise gas-tight mechanical connections are used for signal circuits. Circuit boards are very high quality double-sided epoxy-glass, with component-designator screening. After assembly, Bryston amplifiers are bench-tested, then must undergo a rugged burn-in, being driven by a squarewave input signal into a capacitive load, slightly under clipping, for 100 hours. After this reliability check, the 4B is again bench-tested. The resulting printout is shipped with the amplifier. These features, as well as other elements of the amp's fit and finish, are consistent with the best instrument standards.
What's inside? The 4B's chassis houses two completely separate amplifiers, every part duplicated except for the single power cord and the back-panel bridging-circuit board. Also new in this version is a detachable AC cord, a vast improvement over the former heavy, coiled, nondetachable line cord. Two new toroidal transformers are located just behind the front panel, where they receive support when the amp is rack-mounted. Four small 4700µF electrolytic filter caps with higher voltage ratings are used in the 1991 Bryston 4B, instead of the earlier dual 10,000µF cans. Shorter lead lengths bring these filter caps to within an inch of the output circuitry.
The new 4B amplifier's power supply is a relatively "stiff " design, and has been designed to be relatively unaffected by big current draws. The rails are said not to sag from their ±85V, whether the amp is at 0 or 10 amps current draw.
The Bryston 4B's output circuit uses hand-selected devices with matched transistor betas. Chris Russell suggests that the 4B's output stage eliminates any small asymmetry in the zero-crossing region, particularly notch distortion, as well as resulting in a decrease in upper-harmonic distortion. He also claims that the design makes the output stage more tolerant of loading than conventional output-stage topologies.
Footnote 1: Because the energy storage of a power supply varies as the square of the voltage, the new 10V increase in rail voltage makes a significant impact on the Bryston 4B's power-supply capacity.
Footnote 2: The "steel toe" test is a rigorous quality-control procedure. If the audio pro happens to be wearing boots with steel-toe caps, a firm kick checks whether the audio gear can handle the rigors of concert touring: bouncing around in the back of a van, being thrown up on a stage, falling out of a sound tower, etc.