I've written and rewritten this review, but still feel that I've failed to capture my experience with ATC's simple but satisfying SIA 150 integrated amplifier. Products that are immediately and obviously impressive are easier to write about, and products that suck are easy, too. ATC's SIA 150 doesn't declare its virtues. I won't say that this is the most transparent amplifier I've heard, nor is it without character, but it tickled my fancy without letting me know how and where I'd been tickled.

After listening to the SIA 150 for about four months, I found that it had gotten under my skin in a subtle, welcome, and satisfying way. This is a self-effacing amplifier, sort of like its name. I presume that SIA stands for Stereo Integrated Amplifier, and the 150 reflects its output. This says gobs about ATC. The simple nomenclature mirrors the amp's performance. It allows you to listen and listen to music without calling attention to itself.

To paraphrase ATC's web page, the SIA 150 has four RCA line level inputs, as well as tape record and monitor functions. The only aspect of the design that might seem the slightest bit tweaky, for ATC at least, is that there are no phase-distorting integrated circuits. Also, ATC employs a protection design called a Momentary Gain Reduction circuit, which prevents damage to speaker drivers if the amplifier is overdriven. This probably explains why the amplifier behaved itself with everything I threw at it. At no point did I hear ANYTHING like clipping, even at very high volumes with complex music. I was not nice to the amplifier or my speakers in testing it out. One of the great things about ATC gear is that it's built like a brick mailbox. These products are designed to last and last, take whatever you give them, and sound good doing it.

The SIA 150 always sounded rich, and I prefer a richer sound. Above all things, I listen for a natural tonal balance. I don't care about imaging, bass, or the rest until I hear a realistic rendering of timbre. If the tone of a violin (or piano or guitar or human voice) is off, there's no point for me to continue listening. You'll be hard pressed to get a screechy sound from an ATC product. I like their ability to produce realistic timbre while still delivering detail. It's possible to have both with ATC products, and this is rare in my experience. The SIA 150 operates in a low-distortion, Class A state up to two thirds of its maximum power output. This means that it runs quite hot to the touch, and requires plenty of room for ventilation. This is my most serious complaint about the amp.

The SIA 150 proved to be an excellent match for my ATC SCM 35s—little surprise there. The $3300 SCM 35s and $4200 SIA 150 were oblivious to any difficulty in filling a 5500-cubic-foot room with music. I wasn't immediately aware of an increase in detail or any other audiophile concerns. I just heard excellent system matching. Ultimately, I found the SIA 150 to be more detailed than my reference tube amplification despite its relaxed presentation. I later messed around with some NOS tubes and eliminated that discrepancy, but it took the SIA 150 to let me know that there was detail I wasn't getting, and that it could be had without the system sounding bright, forward, or hard.

With the SIA 150 powering my SCM 35s, I felt no need for a subwoofer, or for more power. If I had a good subwoofer, I might have a different opinion, but I don't, so I didn't feel like I was missing out. The speakers seemed to dig down and deliver bass with ease. The bass was initially a bit too generous for my room, given its 80Hz spike. Another amplifier that arrived after I'd had the SIA150 for a while had a better grip on the bottom end. That more expensive piece let me know that the SIA 150 had congealed the bottom end. Once I realized this, Echo Busters allowed me to tame the 80 Hz bump, and the SIA 150 was back in competition.

Nevertheless, the other amplifier couldn't touch the muscularity, ease, and sweep of the SIA 150, or its ability to play all sorts of music. The more expensive amp couldn't play Led Zeppelin's Greatest Hits with as much ease as the SIA 150. Perhaps the ATC's Momentary Gain Reduction circuit allowed Zep to blast without creating problems. The ATC cranked away, oblivious to volume level. It presented large-scale music like Grieg's Piano Concerto No. 1 in A Minor in correct scale, allowing the emotionality of the music to rise and fall with the volume level, and without turning the piano into a distorted, wavering instrument.

The SIA 150 was completely at home with the left end of the piano, and with standup bass. On Gene Harris' All Star Big Band Tribute to Count Basie, the piano on "Captain Bill" and the bass on "Night Mist Blues" were hearty, full, resonant, and controlled. Although less nuanced than my reference E.A.R. tube monoblocks, the SIA 150 sounded fuller and more powerful. Its fullness of tone immersed me in the music. Voices were especially good, with a liquidity, warmth, and emotionality that did not reveal the solid state heart of the SIA 150. Israel Kamakwiwo‘ole's Facing Future, The Indigo Girls' Swamp Ophelia, and The Story's Angel in the House are vocal-centric albums. All are on the lean side, and can sound a bit thin and edgy. All three of the amplifiers I had on hand did a nice job. All sounded realistic, organic, and true to the recording. All were liquid, sweet, and very listenable. I found the SIA 150 reminiscent of tubes in its sweetness and purity, but it doesn't have a classically tubey sound. People worried about solid state sound needn't worry about the SIA 150.

The SIA 150 is more reminiscent of ATC's passive speakers and their SPA 200P amplifier than their powered speakers or the SCA2 preamp. The latter products are quite incisive, and can sound unrelenting with the wrong source or cables. The passive speakers and 200P amp sound more relaxed, richer, warmer, and fuller, with a bottom end that is a little less controlled, though not sloppy. I'm not saying that the SIA 150 lacks transparency, or sounds homogenized, because that ain't so. It sounds dramatically different with different recordings. If the recording is great, you're in luck. If not, you're not, but when things sound less than good, they are still not bright or screechy.

I have no problem recommending the SIA 150. It's built to last a lifetime. It'll drive most loudspeakers, and it will not etch or drain color from music. I'm taking my own recommendation and keeping it. No, I'm not giving up my tube amp and preamp, but the SIA 150 has such a rightness of sound, and gives such a strong sense that it will last and last and last, that I want to have it around. Plus, switching from my $13,000 worth of tube electronics to the $4200 SIA 150 involves no suffering. The ATC amp is easier to use, and I'll never have to worry about whether NOS tubes are going to be available to make it sound good, which is something I think about with my tube gear. Larry Cox

SIA 150 amplifier
Retail: $4200

ATC USA
web address: www.flatearthaudio.com

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