Boulder 850 Monoblock Amplifier

Perfecting the technique of “deep focus”

The ability of these amps to provide the listener with more, and more realistic, musical information extended to the low frequencies. Bass passages were consistently clean, taut, and weighty. In addition, the 850s’ low-level resolution meant that they did not cut off the tails of decaying notes. Bass drum notes, for example, just hung in the air until they drifted away. Treble was airy and accurate without glare. And transients were fast and clean. Overall, the 850’s were simply very musical. I really enjoyed the Boulder 850 amps. They brought a new level of realism to my system. And while they are not cheap at $10,000, they are certainly worth considering if you’re shopping in this price range. TAS

The Technical Scoop

Boulder’s 850 amplifier is relatively small, just 8.5" wide, and weighs only about 30 pounds. Though the design is simpler and hence less costly than Boulder’s other efforts, the front plate, top cover, and rear plate are all made of machined aluminum. The appearance of the 850 is rather utilitarian, resembling an elongated shoebox with a perforated side. However, Boulder used the same feet on the 850 that it uses on all of its 1000 series products (the feet are made of constrained-layer dampening materials), and, according to Boulder, no additional isolation should be necessary.

The 850 has a three-stage, balanced, instrumentation-style input section. This reputedly lowers noise and reduces distortion while at the same time providing a benign load to any preamp or source driving the amp. The amp is entirely linear and uses no switching circuits, either in the power supply or the output section. Power output is rated at 200 watts into 8 ohms. The 850 has a maximum output power of 800W. AC power is filtered internally and power-supply common-mode rejection is extremely high to keep power clean, quiet, and optimized. The amplifier is fully protected from overvoltages, current and voltage clipping, DC, under-voltages, thermal overload, and shorted outputs.

The output section is Class AB, using an active bias system: The amplifier detects the load and current draw at the outputs and ramps up bias to match the outputs section’s needs accordingly. The bias then slowly ramps down over a period of 30 seconds unless another peak is detected and the bias must be kicked up again. Biasing is not based on the input signal, as it is more important to know what the output section of the amp is doing than what the preamp is passing along. This keeps the amp running as efficiently as possible and holds generated and radiated heat to a minimum. Multiple smaller filter capacitors are used (instead of two large ones) to lower power-supply impedance. Multiple microcircuit gain stages are also used, with the majority of the gain being handled by the first stage to maximize bandwidth.

As much of the amplifier’s design as possible was completed with surface-mount parts. This eliminates lead inductance and reduces the overall size of the circuitry, thus reducing capacitance, increasing board-layout efficiency, lowering noise, and allowing for four-layer circuit-board construction. Full external power control (standby/on) by means of Boulderlink or 12V trigger in custom installation applications is possible. MS