ATC started out making drive units when Billy Woodman founded the company in the mid seventies yet it was only recently that they branched out into tweeters. So when Ben Lilly came to my lair in order to replace the tweeters in the ATC SCM150 ASL active monitors I like to keep handy (in case of a bass shortage) I asked him why it had taken so long. Apparently it comes down to the extra challenges involved in developing the tiny domed drivers that deliver treble and in many cases a lot of the midrange too. They are far smaller, lighter and have a far wider bandwidth than other drivers and any variations in their construction are audible. The manufacturing process has to be far more precise, ATC has a temperature controlled room for tweeter construction in order to keep glue viscosity at the right level. All of which explains why only the largest loudspeaker brands make their own tweeters, everyone else uses a specialist manufacturer like SEAS who made high frequency drivers for ATC before they started doing it themselves.
So why go to all the trouble of developing and making your own? This comes down to the fact that every supplier has their own phliosophy on how a tweeter should be made, you can specify till the cows come home but there are some things that you can only do if you do it yourself. The other important reason for an in-house build is that you will never find that the model you have come to depend on is no longer in production, a common problem with smaller manufacturers who don't buy enough units to warrant a big company keeping them in production if demand elsewhere dies off.
R&D engineer Richard Newman and the team at ATC have developed two new tweeters, a standard and an S spec model, both are based on the same 25mm soft dome. The standard tweeter is used for all the models up to the SCM40 while the top version finds its way into the rest of the range, the models that have ATC's midrange domes which are essentially the bigger and inevitably more expensive active and passive loudspeakers. The SH25-76S version differs from the standard one by virtue of a different motor circuit with greater magnetic efficiency and more power in the gap; 20,000 Gauss rather than 15,000. The dome itself has dual suspensions to reduce rocking modes and deliver pistonic motion, it’s driven by a short coil in a long gap for maximum linearity and has a neodymium magnet system with a heat treated top plate. The motor, coil and dome assembly is mounted onto a waveguide, specifically designed to fit older models such as mine. The job also involved making changes to the electronic crossover to accommodate the new tweeter and the process took well over an hour for both speakers, but they’re big speakers, just moving them is a slow business.
The resulting sound retains the effortless power and extension that I am used to with this remarkable speaker but replaces a top end that was arguably a bit recording critical for domestic use to one that’s considerably more refined. It is still uncannily revealing but distinctly smoother. This speaker has a distinctly pro heritage, it was designed to make every fault with a recording blindingly obvious, you can still hear big differences between recordings with the new tweeter but you aren’t forced to turn down the volume when a particularly hard edged piece of music starts playing. I’m talking about album’s like AC/DC’s uncompromisingly bright If You Want Blood, an album that probably reflects the band’s live sound at the time quite accurately, but it’s the soundcheck sound before the masses arrived to absorb a lot of the top end. However, if you want to hear ‘Problem Child’ as it is meant to sound it’s the best recording there is. The heavy use of compression and limiting on contemporary pop and rock could also sound uncomfortable on the old tweeters and while you can still hear a ‘square wave’ sound as they call it in the trade, it’s not quite as painful. Ultimately the ATC tweeter makes what were already stonkingly good loudspeakers even better. In truth there are few speaker and amp systems that are as good as this when it comes to reproducing the power, atmosphere and energy of a great piece of music. The bass is staggering in its dexterity and extension yet you don’t need a particularly large room because it’s so well controlled. The mid and treble are as clean and open as you could dream and I recommend that anyone considering spending large on a system auditions a pair of ATC actives, the combination of accuracy and power that they deliver is very hard to beat indeed.