Solid State Power Amp Reviews

For several decades—from well before I toured the Boulder factory in 2016—I've wanted to get a handle on the best sound the now–41-year-old company can offer. Multiple listening sessions at shows had more than hinted at excellence. But neither my time in Boulder's large, dedicated music room, which was intentionally dry, nor exposure at various hi-fi shows left me convinced that I'd heard Boulder's full potential.

My first opportunity for an in-home audition came in 2021, when I reviewed the 866 stereo integrated amplifier ($17,500 with DAC, $16,000 without). But that entry-level (by Boulder standards) product, which Senior Engineer Jameson Ludlam said was released to build brand awareness and expand the company's reach by offering "a more accessible product that provides the features we think many people are looking for with the performance they have come to expect from Boulder," only provided a peek at the excellence I expected Boulder to achieve.

So when a last-minute review cancellation opened space to review the just-released Boulder 1151 mono power amplifier ($47,000/pair), I thought, "At last!" The 1151 is currently topped in Boulder's mono line by only the 2150 ($130,000/pair) and the 3050 ($306,000/pair), which are too massive for my space. But then, when a press release for Boulder's new models arrived, I realized that what I was about to hear represented a major departure for the company.

As Boulder's entry-point mono amplifier, the fully balanced 1151 delivers a more-than-respectable 250W of continuous power into 8, 4, or 2 ohms, peaking at 350W into 8 ohms, 600W into 4 ohms, and 750W into 2 ohms. It is also the first monoblock in the 1100 series said to offer full "class-A" sound and the first to use Boulder's just-introduced proprietary "Smart Current" technology (footnote 1), which is intended to address the notorious inefficiency of traditional hot-running, power-sucking class-A.

"Boulder's Smart Current output stage continuously monitors the current drawn by the speaker and self-adjusts as needed," the press release declared. Accomplished "exclusively with the use of analog parts," Smart Current Bias is said to deliver "higher performance, more details, better reliability, and a more natural musical listening experience."

For now, Smart Current technology is only available in the 1100 series; the higher-level 2100 and 3000 series amplifiers use Boulder's class-A Sliding Bias. Was I about to hear a new variation on the older core sound of a company that only changes its product line when it has something significantly new and different to offer (footnote 2), or is Smart Current bias the first glimpse of a whole new Boulder sound?

These new monoblocks have less metal in them; that much is clear. The 1151 monoblocks weigh just 54lb each. For once, I was able to unpack, position, and repack a high-end monoblock amplifier without assistance. It was hard to sit on my couch and gaze upon the Boulders without feeling a sense of liberation.

What we have here
The 1151's front-panel design derives from a topographical map of Flagstaff Mountain, which is near Boulder's Colorado facility. The modestly sized heatsinks are machined from a solid aluminum billet, and the feet have "special constrained layer damping material for vibration."

The output section of this two-stage design, which was designed to deliver "distortion-free high current to the speaker," uses "multiple smaller, faster filter capacitors instead of fewer larger caps to deliver dynamic impact." That's 40 output transistors and 12 supply capacitors, arrayed to ensure stable power "into any load while maintaining detail and clarity."

Boulder Founder/Owner/Chief Engineer Jeff Nelson's white paper on Smart Current Biasing claims that "regardless of the audio frequency or level," this new technology "eliminates the damaging effects of parasitic capacitance. The occurrence of crossover distortion is so reduced to near zero as to be totally inaudible."

To learn more, I zoomed with Logan Rosencrans, Boulder's international sales manager. "Sliding Bias," which is used in the 2100 series, "doesn't really run hot because it jumps into class-A as soon as it senses signal," he said. "But Smart Current Bias looks at the output; it looks at what the speakers are doing and automatically adjusts. As the music signal changes, speaker cones need changing amounts of current to meet demand. Smart Current Bias 'sees' how much current the speaker demands and adjusts accordingly.

"We don't think anyone else is doing what we are doing in terms of bias. It doesn't fit into any traditional definitions of class-A, AB, or B. The closest thing we can call it is class-A, because the current flows through both transistors when music plays. Ours is a more advanced version of class-A. Most class-A amplifiers use trim pots to set the bias; ours do not. We set bias automatically through analog circuitry that monitors and adjusts according to the speaker demand. This gets rid of notch distortion," which occurs in class-B and also class-AB depending on the bias current and the operating conditions. "It also keeps the amplifier from running hot"—as amplifiers do in class-A—"which is damaging over time, and prevents energy waste when you aren't playing music."

I asked if Nelson and his team of five engineers work to achieve a certain "Boulder sound"? Rosencrans said no. "Jeff is very much an engineer who strives for accuracy. That's been his design philosophy for over 50 years. Each product that we've designed gets closer and closer and closer to that goal."

In his original capacity as an equipment builder for broadcast studios in California and throughout the US, Nelson strove for sound as accurate to the original recording as was possible. Rosencrans told me that all Boulder engineering, including chassis machining, is done in its 9-year-old facility. The only things the company doesn't do are wind their own transformers, make raw PC boards, and anodize the casework, which requires the use of nasty chemicals. "Boulder County wouldn't have us doing that in this building," he said.

What I had in my building
Thanks to the 1151's manageable weight and modest size (compared to the behemoths that have crossed my threshold), it was a snap to put them on a pair of Grand Prix Monza amp stands. Because more weight is concentrated in the front half, I moved them far back on the stands to help even the load. I placed them atop the same Wilson Audio Pedestals I use under my reference amplifiers.

Boulder's speaker terminals are extremely easy to tighten by hand. I chose the set that best separated the speaker cabling from the 15A power cord. Once everything was hooked up, I flipped the main power switches on the 1151's rear panels. In short order, the amps settled into standby mode, the white LEDs on the front panel blinking on and off. It took just a push of the front panels' only button to turn the 1151s on fully. I allotted a full hour for warm-up. Rosencrans thought that 15–20 minutes would be sufficient.

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Footnote 1: A white paper on Smart Current technology can be found at boulderamp.com/wp-content/uploads/Smart-Current-White-Paper-121824.pdf.

Footnote 2: Boulder's original 2000 series lasted over 15 years before it was updated to the current 2100 series.