Featuring wholly ATC designed and built drivers and amplifiers, the new SCM40A system is an active version of the SCM40 three-way floor standing speaker, launched in late 2013. The SCM40A utilises ATC’s remarkable soft dome dual-suspension SH25-76 tweeter together with the company’s signature 75mm soft dome mid-range and a 164mm short coil/long gap (under-hung) bass unit. The drivers are powered individually by a proprietary ATC tri-amplifier pack. Housed in a curved, laminated cabinet, the SCM40A is the first of the current ‘Passive’ Series speakers to be made active.
Sharing technology with the renowned ATC soft domed mid-range drive unit, the new SH25-76 tweeter employs a unique dual suspension system that suppresses rocking modes even at high power output levels. The ATC configuration of a short edge-wound voice coil in a long, narrow magnetic gap ensures exceptionally low distortion throughout its operating band and removes the need for ferrofluids, which can dry out over time and compromise performance.
A complex-shaped, soft-domed diaphragm extends high frequency range and offers a smooth off-axis response. The tweeter’s 15,000 gauss (1.5 tesla) neodymium magnet has a black heat-treated top plate, which dissipates heat away from the voice coil to maintain high power handling and low power compression. A precision-machined 5.5mm rigid alloy waveguide provides optimum dispersion, a flat on-axis frequency response and resonance-free operation.
ATC’s iconic 75mm midrange dome is the latest version of the ground-breaking SM 75-150 soft dome midrange driver introduced by ATC in 1976. It uses a hand-doped acrylic diaphragm, dual suspension system and a highly linear drive system to deliver uniform dispersion, wide bandwidth and low distortion. ATC’s unique doping formula offers an excellent balance of high damping and low mass, which make it ideal for supressing energy from undesirable diaphragm resonances.
The SCM40A’s 164mm bass driver utilises a hand-coated carbon-paper diaphragm driven by an ‘under-hung’ (short coil/long gap) motor and utilising a massive, optimised magnet assembly. In this application paper pulp is selected for the ideal balance of mass, damping and rigidity. The under-hung motor topology maintains linear coil force and inductance, even at high drive levels, keeping distortion low.
The on-board, discrete MOSFET, class AB three-channel amplifier uses ATC’s Grounded Source output stage to deliver 25W to the HF driver, 60W to mid and 150W to the bass driver for a total of 235 watts continuous power. Input is via balanced XLR.
The crossovers are all active, 4th order modified Butterworth with phase compensation circuitry for each crossover point. Active crossovers have the advantage that their response is not modified by a loudspeaker’s variable voice coil impedance, and the phase compensation enables phase coherency at the crossover points, improving tonal balance and enhancing imaging for a pin-point stereo field.
The SCM40A benefits acoustically and aesthetically from a curved cabinet construction braced and laminated for high rigidity and damping and finished in real cherry or black ash veneer. It is designed for optimum performance with the grille on – although the difference in performance without the grill is minimal. The metal grille fixes to the cabinet via hidden magnets. According to ATC tradition, the model number represents internal cabinet volume in litres.