Bryston 14B-SST power amplifier
How did bridged output stages acquire this reputation? Bridged circuits double the voltage of the amplifier's single-ended output, boosting the amplifier's output power and headroom. However, while the available voltage swing is doubled, the maximum current remains that of one of the single-ended outputs. Low-impedance speakers can thus result in the output stage's safe operating areas for voltage, current, and heat being more readily exceeded.
So it came as a surprise when Bryston's Chris Russell told me that the company's new flagship solid-state amplifier, the 14B-SST, is a bridged-output design. Why would a large, heavy amp with separate power supplies for each channel, computer-designed heatsinks, switchable input gain, rugged bipolar output transistors, and mechanically quiet power transformers need a bridged output circuit?
Why bridged amplifiers can sing
Bridged circuits offer a designer the ability to cancel out distortion. If this can be accomplished—and it must be part of the amplifier's design from the beginning—the bridged circuit can then deliver the highest possible current to the speakers at the lowest possible distortion.
Examples include the 600Wpc Electrocompaniet Nemo (reviewed in Vol.23 No.3) and Bryston's 500W 7B-ST monoblock (Vol.19 No.10, Vol.22 No.3). On the top, the Nemo had a "crystalline, translucent quality," and its bottom-end extension had "ample focus and detail without being overly pronounced or tightfisted." The 7B-ST "excelled in speed, drive, slam, bass control, and effortless dynamics, and made dynamic contrasts more seamless."
Like those amplifiers, the 14B-SST was designed only for bridged operation. Each of its channels contains two amplifiers, wired in series and operated out of phase with each other. The 14B-SST's two ±65V rails allow each channel to swing almost ±130V. If the owner wants to drive very-low-impedance loudspeakers—say you're a diehard fan of the 1 ohm Apogee Scintilla—then you must return the amp to the factory to have it reconfigured. This differs from Bryston's 7B-ST, which allowed the owner to switch the arrangement of the bridged circuits between series and parallel. Why? Bryston engineers found that the 7B-ST's circuit-configuration switch was the source of a tiny but measurable amount of distortion. For that reason, 7B-STs produced after May 2002 will be sold, without the switch, as 7B-SSTs (for "Super Stuart Taylor"—see Sidebar).
The 14B-SST is a dual-mono design; ie, one power supply per bridged channel, both mounted in a single chassis. The two amplifier channels are arranged on either side, with common circuit boards for power-up, fault detection, logic, and front-panel LED control. Short lead lengths bring the eight 27,000µF electrolytic filter caps to within 1" of the output circuitry. The protection circuitry is designed to handle most fault conditions, including shorts and DC offset. The 14B-SST's discrete input circuitry features the ST buffer circuit, now standard in all ST amplifiers for both their single-ended and balanced inputs. The input circuits are completely symmetrical.
The 14B-SST's output stage uses 32 (eight pairs per channel) of a new type of bipolar output transistor, each PNP/NPN pair controlled by a single driver transistor. The power transistors in the 14B-SST (and now in the SST line) are larger, faster, and can handle more power than those in the 7B-ST, and have been proven to be more reliable, says Bryston. Output devices are hand-selected to ensure that the transistor gains are matched.
Chris Russell noted that the 14B-SST gained from the extensive redesign done for Bryston's five-channel 9B-THX. This involved a computer simulation to implement a more sophisticated grounding scheme to combat electrical hum and physical noise from large toroidal transformers. A special grade of steel was selected for each toroid's core. Furthermore, filter capacitors are mounted to the PCB using five-pin rather than three-pin connectors, so that any torsion of amplifier movement will not twist the filter caps off the board. The power supply's eight 27,000µF filter capacitors are guaranteed to run at 105 degrees F for 5000 hours, but the 14B-SST is designed to shut down when it reaches a temperature that would make the temperature of the filter caps exceed 70 degrees F (footnote 1).
Footnote 1: Each time a filter capacitor's temperature is lowered by 5 degrees C, its operating life is doubled.