Solid State Power Amp Reviews

The 9B-THX is derived from Bryston's 3B-ST, reviewed positively in these pages in October 1996 (Vol.19 No.10). The modules' design was further developed, in Bryston's PowerPac series, as a 120W ($795) amplifier mounted on a flat plate that could be attached to the back of a speaker to make it a powered monitor. For the 9B-THX, this single-channel amp was transformed into a thin, 17"-long plug-in card. The card's length keeps the front power supply a good distance from the rear input stages, to minimize hum.

But Bryston's Chris Russell decided that the 9B-THX required more than a reshaped circuit board. For example, the grounding is far more complex than the schemes found in Bryston's two-channel amps. All five amplifiers are first grounded separately, then connected through a large bridge rectifier. Dual 25-amp diodes keep differences between channels to less than 1V.

Besides electrical hum, five toroidal power transformers squeezed into a single chassis can generate a bit of physical noise. To combat this, a special grade of steel was selected for each toroid's core. Furthermore, the filter capacitors are mounted to the PCBs using 5-pin rather than 3-pin connectors, so that no torsion of the amplifier will easily twist the filter cap off the board. The filter capacitors, selected for longest life, are guaranteed to run at 105 degrees for 5000 hours. The 9B-THX is designed to shut down if it reaches a temperature that would make the filter caps exceed 70 degrees F. This extends their mean time between failures to 400,000 hours. Such overdesigning is necessary to justify Bryston's 20-year warranty.

The 9B-THX's five 60V rails could really pull down the AC voltage on startup when large amounts of current are needed to initially charge the electrolytics. Bryston uses a sophisticated soft-start circuit to prevent the 9B-THX from tripping your home's circuit breakers. The startup components and circuit are placed on the sixth board, at the far right of the chassis.

The 9B-THX resembles other Bryston amplifiers in sporting pairs of handles fore and aft. The front panel is of ?"-thick sculpted aluminum with two narrow horizontal grooves; the two front handles are similarly grooved. Because the faceplate is buff-finished with a fine abrasive like jeweler's rouge, its surface is very smooth to the touch. Besides the company name, the only lettering on my review sample's face were two logos: "THX" and "ST." The former is self-evident; the latter are the initials of Bryston's circuit designer, Stuart Taylor.

Amplifier functions are addressed by a square pushbutton power switch and five bicolor power-indicator LED. These glow green while the unit is powered, turning red only when the amplifier is clipping or suffering an internal fault. The clip-sensing circuit uses a comparator to detect signal distortion, clipping, short circuits in cabling, excessive DC, or ultrasonic signals.

The rear panel is laid out in simple fashion. At extreme left sits the detachable AC connector and the remote turn-on input. The remainder of the rear panel is divided into five narrow, identical panels, each with clearly printed instructions in white lacquer that make it possible to set up the 9B-THX without having to consult the written instruction sheet. There is a switch to set the channel to an unbalanced RCA input, or to balanced or balanced +6dB inputs. The balanced input uses a dual-function connector by Neutrik that takes a balanced XLR plug (pin 2 positive) or a TRS ("tip-ring-sleeve") balanced ?" phone plug (tip positive).

Each amplifier's output uses a new speaker binding post (rated at 60 amps) that allows you to insert one banana plug in the back while screwing another into the same post from the top. These binding posts have been designed to meet the rigorous CE standard, which changed the spacing of speaker binding posts from 19mm to 25mm or more. The posts themselves are plastic-shrouded so fingers can't get to the contacts when the amplifier is playing. The first 9B-THXs came with slots in the plastic shrouds to allow for admit ?"-thick spade-lug connectors. Speaker-cable manufacturers quietly exceeded that thickness—as I found out (see sidebar, "Welcome to the Real World"). Current-production 9B-THXs have 5/16"-wide slots for their speaker binding terminals.

The ST logo signifies that the 9B-THX has the latest Bryston circuit innovations. These include lower power-supply impedance, a distortion-lowering input buffer amplifier, 28% more energy storage, improved power-transformer layout, shorter lead lengths, and improved signal isolation. Compensation for capacitive loads is provided by a single loop of wire in the output stage. The new toroidals are precision-wound, each with exactly the same length of double wire.

After bench-testing, the 9B-THX gets a grueling 100-hour factory burn-in consisting of a squarewave input signal clipping the amp into a capacitive load. In theory, this load draws an infinite amount of current on the rising leg of the squarewave, stressing the power supply to the max. Unlike a resistive load, which dissipates all the energy as heat, a capacitive load feeds back the entire signal into the amplifier, which puts maximal stress on the output stages. The driving signal is gated one hour on and one hour off, this cycle repeated many times. This heats up the amplifier, cools it, and heats it again, the resulting expansion and contraction exposing any loose connectors and shaking out any devices subject to early failure. After burn-in, each 9B-THX is again bench-tested, and the results are shipped with the amplifier.