The ATC C4 Sub Mk 2: New Driver, New Amp, New Sound?

Although they don’t come cheap, the speakers and subwoofers built by the UK-based Acoustic Transducer Company (ATC) have major pro-audio cred — you’ll find them in many of the world’s best recording studios and mixing/mastering houses, thanks to their high output, low distortion, and rock solid build quality. The company also makes active and passive speakers for the home audio market. ATC’s newest product is the C4 Sub Mk 2 active subwoofer system ($5,999), designed to “unite the highest standards of driver and electronics engineering in a single system,” according to the company. It combines an all-new, proprietary 12-inch driver with a newly-developed 300-watt class AB amplifier, both of which are hand-built at ATC’s factory in England.

The new driver, which was given the catchy, easy-to-remember designation of SS75-314SC, is apparently the first purpose-built sub-bass driver developed by ATC’s R&D team to reproduce only the lowest four octaves of the audio band. The design uses ATC’s signature long-gap/short-coil configuration, but pushed to the extreme with a 30mm magnetic gap, surrounded by a 6-inch N48M Neodymium magnet that is more powerful than anything the company has assembled before. According to ATC, the new sub also boasts a completely redesigned motor system featuring a new rubber roll-surround for high excursion capability, a new spider for optimized compliance geometry, and high-flow motor venting for low noise and compression. This “unprecedented” short-coil long-gap configuration reportedly ensures that the voice coil remains within the motor structure, even at high drive levels, thus maximizing cooling, reducing power compression, and improving reliability, all while minimizing distortion. A large, flared vent running through the center of the motor structure is designed to minimize turbulent air noise within the driver while simultaneously increasing voice coil and motor cooling. As a result of these design elements, along with the use of a reasonably large cabinet volume, the C4 Sub Mk 2 is able to deliver an extended low-frequency response without electronic equalization, according to ATC. This approach reduces phase shift and group delay, which ATC says makes the sub easier to integrate with partnering loudspeakers. 

The driver is powered by an all-new 300-watt class AB amp “optimized to the requirements of the new driver, resulting in fast, dynamic bass with minimal coloration,” according to ATC. Max SPL is listed as 110dB continuous, 116dB peak. 

Note: ATC doesn't specify bandwidth or distortion with this SPL spec.

The defeatable low-pass filters are analog 4th-order Linkwitz-Riley designs. And the presence of both balanced line inputs and outputs means that it’s possible to daisy-chain multiple subs.

The C4 Sub Mk 2 is a sealed box design, with a cabinet measuring approximately 23 inches tall, 18 ½ inches wide, and 21 inches deep — fairly big for a sealed 12-inch sub, but not enormous. It weighs a substantial 92.6 pounds, and its internal volume is 70 liters. The hand-built cabinet uses 1-inch-thick panels and heavy bracing. Styled to match ATC’s “Classic” and “Tower” series loudspeakers, the sub is available in an unusually wide variety of finishes. Cherry, Black Ash, Walnut, and Oak real-wood veneers, as well as Satin Black and Satin White paint, are standard options. Premium Rosewood, European Crown Cut Walnut, Burr Magnolia, and Pippy Oak real-wood veneers will cost extra. High gloss over the wood veneers and black or white piano lacquers are also available.

ATC says that the C4 Sub Mk2 was designed with a focus on resolution and integration, so it’s probably not going to deliver brute-force bass down to the single digits. Instead, the sub should deliver “a more balanced and coherent experience for the listener, extending the low-frequency response (down to 22 Hz) without masking details or changing the tonal character in the upper bass/lower mid-range,” according to the company.

More information: ATC C4 Sub Mk2

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