The ATC SCM150 ASL is a ‘proper’ loudspeaker; a foursquare block that makes no concessions to the contemporary fashion for slim baffles and minimal domestic intrusion. Instead, it is an unpretentious, call a spade a spade, no nonsense loudspeaker. The bass driver is a full-fat 380mm driver that takes up the majority of the nigh on half metre wide front baffle. There are studio and domestic versions of this model; in professional circles they are often soffit mounted, but I wouldn’t want to be installing all 75 kilos in a wall, especially any wall higher than the 250mm high steel stands they come with. Even that’s a two‑man job. Although this model has been in the catalogue for sometime now, ATC’s development of its own tweeter means that the SCM150 ASL is not the same beast as it once was. In fact, it's a lot better. ATC has 'form' in drive unit making: the company has been making its ‘super linear’ bass drivers and midrange domes for many years, but only recently has a tweeter gone into production at the facility near Stroud in Gloucestershire.
There are two ATC tweeters both called SH25-76, the standard version being fitted to models up to the SCM40 ASL and the S version in those above it. The SCM150 ASL therefore has the SH25-76S, a 25mm soft dome with higher efficiency and lower distortion than its counterpart, which offers 25% more magnetic flux density and 3.5dB greater sensitivity. When I asked ATC engineer Ben Lilly why they felt the need to build their own tweeter, he explained that it’s the only way to get precisely the driver you need rather than a third party manufacturer’s idea of how it should be made.
The amplification is also designed and built in-house and consists of three channels per speaker with 50 Watts for the tweeter, 100 Watts for the midrange, and 200 Watts for the bass. This is combined with an electronic crossover that hands everything below 380Hz to the bass driver and everything above 3.5kHz to the tweeter. Connection is by XLR only, as befits a speaker that comes from the studio world.
As you might imagine it’s the bass that first grabs you with this system, but it doesn’t do so in an overpowering or excessive way like a subwoofer turned up to 11. Instead you can hear all the timbres of bass instruments and all the nuances of the way they are played; it’s almost the opposite of bass created by smaller speakers where definition is exchanged for thick, heavy, and unnatural lows. This ATC is so clean and even handed that only truly low notes produce real gravitas, there is no excess, no flab, just girth. When you connect 200 Watts directly to the driver that it has been made for, you get astonishing control and that means extraordinary detail, coherence and, importantly, extension. This speaker does make you start looking for albums with serious low end on them, but it takes no prisoners in revealing which albums actually go down deep, and which are just tweaked to sound that way on most speakers.
Put on Koyaanisqatsi [Philip Glass, Nonesuch] for example, and the organ notes are deep and clear, the male voices full of tonal subtlety and the soundscape expansive. More important is that the music makes sense, the absence of thickness in the bass allows the tune to take precedence over the sound, it allows the message in the music to be communicated in a way that most systems struggle with because they don’t have the resolution and control that this one does. It’s not just the speaker either; the PRE60 is a critical part of the result. It is extremely quiet and produces a far more expansive image than others of its ilk. The high notes are really well extended, but clean and revealing, while cymbal harmonics are brought out of the mix alongside other quiet sounds because the noise floor is so low.