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Equipment Review

September 2004

Audio Research 150.2 Stereo Amplifier

by John Crossett


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Review Summary
Sound "Neutrality" -- "faithfully passes along the signal sent through it with little or no editorializing"; "doesn’t have classic tube warmth and roundness, but it always portrays a sense of three-dimensional people in real space"; "re-created a wonderful sense of tactile realness, which in turn allowed me to suspend disbelief far more easily."
Features "A class-T digital power amplifier that delivers 150Wpc into 8 ohms, and 300Wpc into 4 ohms at 0.1% THD"; single-ended and balanced inputs; "this is one cool-running amp, a quality that will assist in keeping your power bill low and be especially attractive if you are one who leaves your amp powered up 24/7."
Use "I encountered one small glitch while using the 150.2. I use rabbit ears to grab the local CBS affiliate off the airwaves, and the 150.2’s digital circuitry interfered with signal reception on every TV in the house.... Audio Research says that the problem may have been eliminated due to CE testing for European sales, however."
Value "The 150.2 offers plenty of power and wonderful sound in a cool-running, compact, reasonably priced package."

"Step right up folks and prepare to be amazed. Come and peer into the future."

I felt a bit like some carny huckster as I prepared my notes and began writing this review. The fact is that the Audio Research 150.2 really does provide a glimpse into the future of two-channel music reproduction. Whether we want to admit it or not, digital amplification is a technology we audiophiles will have to consider as we continue into the new century. Interestingly, Audio Research, that most venerable of high-end companies, the company that helped keep the vacuum tube from becoming a footnote in audio history, has recognized this trend. Moreover, the engineers at ARC have used their creative energy to produce a digital amp that fits in nicely amongst the company's many tube products.

The 150.2 ($2495 USD) is a class-T digital power amplifier that delivers 150Wpc into 8 ohms and 300Wpc into 4 ohms, both at 0.1% THD. It is a balanced design with a single-ended push-pull output that's fully balanced when the amp is used as a monoblock. A class-T amplifier is a high-speed, extremely efficient digital switching amplifier. The benefit of modulating the switching-frequency amplitude (anywhere from 1.2MHz to 200kHz) and dithering the switching frequency to adjust dead time is that it allows for far more of the incoming electricity to be converted into output power and not be wasted as heat. The 150.2's analog input stage feeds the Tripath module, which creates a 1-bit pulse train from the incoming analog signal. That pulse train drives MOSFETs, which then drive a passive low-pass filter. The output of the filter is the amplified version of the input signal, and that is what is sent to the speaker. Another benefit of class-T amps in particular is Tripath’s proprietary "Digital Power Processing" (DPP) -- as opposed to the Pulse Width Modulation (PWM) method used for class-D digital amplification. DPP, as implemented by Tripath, allows for both high signal fidelity and high power efficiency. In terms of the real-world implementation of many class-D amps, these have ended up being an either/or proposition.

Surprisingly, given its wealth of output power, the 150.2 is relatively svelte: 19"W x 5 1/4"H x 14 1/4"D and 30 pounds. Outside the 150.2 looks every inch an Audio Research product, with its silver faceplate (black is also available) adorned with the Audio Research badge top center and the power switch below. That’s it -- understated elegance. And for those who miss rack handles, they can be added if desired, though they’ll contribute nothing sonically.

Around back, things are just as economically laid out. You’ll find the inputs -- gold-plated RCAs for single-ended connection or gold-pinned XLRs for balanced. There’s also a nice set of gold-plated five-way binding posts for speaker connection, a fuse holder, and an IEC power-cord receptacle (cord included, of course). Nothing frivolous is added, and nothing required is ignored.

The top plate contains three vent openings, below which are the 150.2's only heatsinks. In all the time I used the 150.2, it never got warm enough to do anything besides melt ice cubes (slowly) -- not that I tried. This is one cool-running amp, a quality that will assist in keeping your power bill low and be especially attractive if you are one who leaves your amp powered up 24/7.

System info

I used the Audio Research 150.2 amplifier in a system that includes a VPI HW19 Mk IV turntable, modified Rega RB300 tonearm, and Clearaudio Aurum Beta S cartridge for analog playback, and a Marantz 8260 SACD/CD player and Panasonic F65 DVD-V/DVD-A player for digital. The preamp is an Audio Research SP16. Speakers are Magnepan MG1.6/QRs. Cables included Alpha-Core Goertz MI2 biwire speaker cables and Alpha-Core TQ2 (mainly), DH Labs BL-1, or Harmonic Technology Pro Silway Mk II interconnects. Everything except the amp sits on a Target TT3 rack, homemade sandboxes and DH cones and squares. The amplifier for comparison is my Sunfire Stereo.

I encountered one small glitch while using the 150.2. I use rabbit ears to grab the local CBS affiliate (channel 3) off the airwaves, and the 150.2’s digital circuitry interfered with signal reception on every TV in the house. Turning the amp off solved the problem, but keeping it on all the time improves its sound. In my case, I simply didn’t watch CBS for the review period! Audio Research says that the problem may have been eliminated due to CE testing for European sales, however.

What digital does

The first thing that grabbed my attention was how the 150.2 seemingly wrung the best out of my system. The only limitations were those of its surrounding components. Place the 150.2 in a quality audio system, and no matter what upgrades you choose to make, the 150.2 will continue to make beautiful music. Part of the reason for this is its neutrality, an Audio Research byword. It faithfully passes along the signal sent through it with little or no editorializing. The music that brought this aspect to my immediate attention was my old favorite "Blues for Bighead" from Andy McCloud's Gentlemen of Jazz [Mapleshade CD 07832]. I’ve heard this track so many times, with so many different systems, that I can almost hum it note for note. As played back via the 150.2, I heard the pluck of Andy McCloud's bass, the snap and crackle of Victor Lewis’s drumset, the breathiness of Joe Ford’s soprano sax, and silver shimmer of Steve Nelson’s vibes reproduced as though I were hearing the master tape. The 150.2 re-created a wonderful sense of tactile realness, which in turn allowed me to suspend disbelief far more easily. End result? Greater enjoyment.

The 150.2 played music -- pure, natural, honest-as-the-engineer-captured-it music. Perhaps it is a bit on the lean, laid-back side of neutral -- but only by an iota. "Thinness" is definitely not a word I’d associate with the 150.2's sound. The 150.2 fleshes out images far better than I anticipated. It certainly doesn’t have classic tube warmth and roundness, but it always portrays a sense of three-dimensional people in real space. For instance, I slipped in the new MoFi Blood, Sweat & Tears 3 SACD [Mobile Fidelity UDSACD 2013] and was impressed by the choir that wraps around the band during the song "Hie-De-Ho." I could identify each of the singers as an individual person, discern how their voices were positioned behind the band, and hear how they interacted with each other and lead singer David Clayton-Thomas.

Along the same lines, the Audio Research amp has the ability to separate musical lines, even among instruments played in the same register. Roxy Music’s "Avalon" from the SACD of the same name [Virgin ROXYSACD 9 7243 5 83871 2 4] has the bass guitar and bass drum playing one on top of the other, but via the 150.2, it was easy to differentiate the two. And not only could I tell the difference, I could follow the fingers as they ran up and down the bass strings as well as the "whomp" of the mallet hitting the bass drum.

Another of the strengths of the 150.2 is its ability to handle dynamic contrasts. Going back to Blood, Sweat & Tears 3, toward the end of the band's cover of Steve Winwood's "40,000 Headmen," the group as a whole launches into a few bars of Thelonious Monk’s "Well, You Needn’t." When a group as powerful as BS&T plays music as dynamic as Monk’s with as much gusto as they do on Blood, Sweat & Tears 3, it’ll pop you right out of your seat, which is what happened to me with the 150.2 in my system. Hand in hand with the 150.2's dynamic prowess is its resolution. After all, one without the other equals canned music, and "canned" is another word that doesn't describe the sonic qualities of the 150.2. All of the instruments used by BS&T were clearly identifiable as exactly what they were and how they were played, whether plucked, beaten, blown or fingered.

All the typical audiophile standards are met (or exceeded) by the Audio Research 150.2. Soundstaging -- both in terms of width and depth -- is first rate, tonal qualities are very well handled, and the frequency extremes are not emphasized or diminished. On the soundtrack for the movie Kansas City [Verve CD 314 529 554-5], the final track is a duet (well sort of) between bassists Ron Carter and Christian McBride. Both basses are mixed front left/right and offer the full, deep, woody sound you would expect from a large stringed instrument. I could easily follow either musician’s plucking or bowing as well as tell who was playing what and when. The 150.2 laid out the technique and playing style used by McBride and Carter, letting me hear when each held the floor. Set well back behind the bassists, Victor Lewis and his brushes caressed the drumheads, and his cymbal work shimmered. And when Don Byron floats in between and behind McBride and Carter on clarinet, the tonal purity is a sound to behold.

One sonic aspect that impressed me greatly was how the 150.2 sounded with vinyl. You might think, as I did, that analog might be an area in which a digital amp could be suspect. Not the 150.2. Cueing up The Soulful Side of Gene Ammons [Analogue Productions/Moodsville APJ 039], I heard every ounce of physical effort, and the thought process, that Ammons put into these oh-so-soulful tunes. The 150.2 has the ability to shine a spotlight on each musician, once again portraying him as a person in space. This talent allowed me to tell when Ammons was moving around the microphone as he played during the recording session. It didn’t matter what LP I chose to listen to -- the 150.2 was up to the task, reproducing the music as separate parts making up a greater whole. And this quality kept me reaching for more and more albums to listen to -- and staying up later than I should have.

What digital doesn't

If you rigidly align yourself with either the tube or solid-state camp, the 150.2 may not float your boat. The 150.2 lacks the rich, shining-light-from-within sense that tube lovers crave. At the same time it’s missing the laser-like precision and cleanliness that many solid-state amps demonstrate. It doesn't grab hold of the bass frequencies with the iron-fisted grip of more powerful transistor amps -- although it does handle the lower frequencies much better than any tube amp I’ve heard. And I'm not going to say that the 150.2 bridges the gap between the two camps -- that wouldn’t be true either.

What the Audio Research 150.2 does do is get itself out of the way, and allow the signal to be reproduced as sent by your front-end and preamp. When I inserted my 300Wpc Sunfire Stereo amp ($2295 when still available) back into my system, I got to hear the other side to the same sonic coin. The Sunfire amp brought back a bit of bass control and improved dynamics -- which is probably attributable to the Sunfire amp being twice as powerful as the 150.2. As well, images seemed to come across with a bit more definition than through the 150.2.

But there were a couple of areas in which the Audio Research amp clearly excelled. One was overall coherence -- music just seemed to flow from the 150.2 in a manner that the Sunfire amp couldn’t touch. The second was the 150.2’s superb neutrality -- it got out of the signal’s way, not adding or subtracting anything, just faithfully passing on whatever it was fed in a better manner than the Sunfire could. One final difference between the two -- and in this regard neither can be considered right or wrong -- is that they handled transients differently. The 150.2 was far crisper, defining the leading edge of each transient and making it stand out, perhaps as a result of its wonderful way with both detail and dynamics. The Sunfire amp, on the other hand, seemed to blunt the initial edge but flesh out each note’s overall character better.

Final considerations

There has always been a certain synergy between Audio Research electronics and Magnepan speakers, and the 150.2 continues that tradition in spades. It locked onto my MG1.6/QRs and seemingly helped fuse speaker and amp into one. It’s hard to imagine a better match. But as good as the ARC/Magnepan combination is, I have no doubt that the 150.2 will bring out the best in whatever speaker, dynamic, electrostatic, or planer-magnetic you choose to use with it. The 150.2 offers plenty of power and wonderful sound in a cool-running, compact, reasonably priced package. There's also the reliability of Audio Research products to keep in mind.

Another point to consider -- the future of music reproduction is headed increasingly and unalterably in the digital direction. Kudos to the Audio Research design team for both recognizing this trend and using cutting-edge technology to produce an amplifier that demonstrates ably just how good digital amplification can sound.

"There you have it, folks, a glimpse of the future."

...John Crossett
johnc@soundstage.com

Audio Research 150.2 Stereo Amplifier
Price:
$2495 USD.
Warranty: Three years parts and labor.

Audio Research Corporation
3900 Annapolis Lane North
Plymouth, Minnesota 55447
Phone: (763) 577-9700
Fax: (763) 577-0323

Website: www.audioresearch.com

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