Krell S-150m Monoblock Power Amplifier (TAS 205)

Small, Lightweight, and Affordable—But Still A Krell

When I began writing for TAS in 1983, if you wanted a state-of-the-art solid-state amplifier that could drive any speaker you purchased either a Krell or a Levinson. Period. Flash forward to 2010. Both Levinson and Krell are still around, although under new proprietorships. But unlike 1983, audiophiles now have far more choices in amplification, including some options that make both Krell and Levinson seem modestly priced in comparison.

Krell’s least expensive amplifier nowadays is the S-150m, priced at just $5000 per pair. It’s in a compact (by Krell standards) cabinet that remains cool enough for a cat to sleep on. Back in 1983 no one would have dreamed that Krell would offer an energy-efficient amplifier, or one that didn’t require a Fabio-physique to carry, not drag, around solo. But Krell’s new under-30-pounds S-150m monoblock claims to be exactly that—a Krell for those of us who don’t live in the gym. So how Krell-like can the S-150m possibly be? Let’s find out.

When I asked Krell’s director of engineering, Todd Eichenbaum, about the S-150m’s overall design he said, “The S-150m is basically a scaled down Evolution Series amplifier. It has fewer output transistors, a simpler power supply, and the circuit itself is simpler. But the design goal was to retain as much of the performance of a full-blown E Series as possible.”

When I asked Todd Eichenbaum why the S-150m ran so cool he replied, “It’s the heat-sink system. It was originally developed for the Showcase multichannel amplifier, where it had to cool seven channels. It is extremely efficient. Couple that with only having to do one channel and you have a cool-running amplifier.”

Throughout my phone interview with Todd he stressed that the S-150 isn’t about new technology as much as it is about refining and simplifying existing Krell designs. Technology that Krell first developed and used in 1996 can still be found in the S-150m. Although this contemporary incarnation is simpler, it still embraces the same design philosophy as, and circuit topologies similar to, other “classic” Krell designs, in addition to drawing on Krell’s newer technologies.

I asked Krell to send me three S-150m amps so I could power all the front channels in both of my room-based systems. Installation was simple, and except for making sure the balanced/single-ended toggle switch on the back of the amplifier was in the correct position, unremarkable. Once the power switch on the back of the amplifier is turned on the circular light which surrounds the front-panel on/off switch glows bright red. Turning on the amplifier changes the bright red circle to bright blue. And I do mean bright blue. If you’re not into vibrantly colored lights, you can turn this one off via an internal jumper switch. A second internal jumper can disable the front on/off switch completely. You turn on the amplifier by plugging it in. This feature is designed for integration into 12V-triggered AV systems.

During the review period all three of the S-150m monoblocks performed without a glitch or problem of any kind. It’s now thunderstorm season in the Rockies, so the S-150m’s were subjected to several power outages, thunderstorms, and other power vagaries. Again, no signs that these sorts of real-world AC phenomena had any effects on the amplifiers. The amplifiers were all connected to AC through PS Audio Quintet and Quartet AC conditioners most of the time, especially when I was away from home. For some listening sessions I plugged the amplifiers directly into the wall without additional power devices.

I typically run stereo subwoofers with the crossover point anywhere between 55Hz and 80Hz. The S-150m proved itself practically “plug-compatible” with the last basic power amplifier I had in my system, the Bel Canto Ref 1000 Mk II, which also produces 26dB gain. During both manual and automatic system set-up procedures the prior settings I used with the Bel Cantos were also perfect for the Krells.

From the first notes several unmistakably Krell-like characteristics emanated from the S-150m amplifiers. Whether they were connected to Dunlavy SC-VI, Genesis 6.1, or AV 123 Skiing Ninja-modified X-Statik speakers, the S-150m amps exhibited a level of dynamic control and ease that made me sit up and take notice. From Abra Moore to Yoel Levi and the Atlanta Symphony Orchestra (the first and last artists in my iTunes library) the S-150m displayed a dynamic liveliness and dexterity that at times verged on the uncanny. On the Punch Brothers’ Punch! the background singers’ subtle micro-dynamic shadings came through with mesmerizing clarity.

Krell amplifiers have always had a reputation for tight yet musically convincing bass. The S-150m maintains Krell’s rep. The bass lines on Was Not Was’ cover of “Papa Was a Rollin’ Stone” pulse with enough mirror-ball disco thump to make all your stuff go bump. Acoustic basses, such as on Angela Easterling’s “One Microphone” from her Blacktop Road CD, should pop in their initial attack. The S-150m preserved the opening pop as well as the subsequent rumble.