Issue 93


Ableton Live 12
What’s in. What’s out. What to expect.


Review: ATC SCM20A Pro Active Monitors

ATC’s new SCM20A PRO monitors represent a breakthrough in low distortion monitoring. Greg Simmons investigates…


15 April 1998

“British acoustic engineers ATC hand-build the most accurate professional reference loud­ speakers in the world.” So says the opening paragraph of ATC’s sales brochure. It’s a lofty claim, and one that is made by many manufacturers. The trouble with claims of accuracy is that they actually mean very little unless you know how the manufacturer interprets the concept of accuracy, and you understand their design philosophy. So before looking at the SCM20A PRO, let’s take a look at ATC and some of their key design philosophies.


ATC was formed in 1974 by driver designer Bill Woodman, and are one of very few manufacturers whose speakers are revered by audiophiles and recording engineers alike. Much of this reverence is due to a single-minded pursuit of accuracy in speaker design. In Woodman’s world, there are no compromises, no half­-measures, and no such thing as ‘almost accurate’ speakers. To quote, “Either a loudspeaker is accurate, or it is not.”


ATC’s design philosophies go beyond the acoustic testing laboratory, and consider the bigger picture of how a monitor interacts with the room it is placed in. Fundamental to the design of all ATC monitors is the belief that a monitor must excite the reverberant field evenly. To this end, it needs broad and even dispersion throughout the entire frequency spectrum. This is easy to achieve for low frequencies, because their wavelengths are large compared to the dimensions of the speaker cone. It’s also easy to achieve for high fre­quencies, through the use of dome tweeters. But it’s difficult to achieve for midrange frequencies, because the dimensions of the speaker cone become significant and tend to focus the energy in the forward direction. To achieve proper dispersion, ATC pioneered the develop­ment of the soft dome midrange driver. ATC have also spent many years isolating driver dis­tortions and minimising their audibility. Their latest research focussed on eddy current distortion (a form of electromagnetic distortion found in every moving coil driver) and lead to the development of their low distor­tion Super Linear (SL) technology. Which leads us to the subject of this review…



    RRP: $10,995


    AR Audio Engineering:
    Phone: +61 (0)2 9810 5300
    Fax: +61 (0)2 9810 5355


The SCM20A is a member of ATC’s PRO series monitors, which take advantage of their new SL technology. It also represents a significant departure from its predecessor, the passive SCM20. Like the larger models in the PRO range, the SCM20A is an active monitor with built-in electronic crossover, power amplifiers and protection circuitry. Unlike other ATC monitors, it features an acoustically shaped die-cast aluminium enclosure complete with cooling fins and a carrying handle. The result looks and feels like it’s designed for heavy-duty military applica­tions. Built-in universal Omnimount fixings are included for flying or wall mounting, but you’ll need a solid wall. At 33kg each, these monitors are deceptively heavy and need proper stands. Don’t even think about sitting them on your console’s meter bridge.

The enclosure is supported on three feet — the optimum number for stability. There are two rubber feet at the front, and a single height-adjustable ball-joint foot at the rear which allows the monitor to be tilted up or down. A nice touch. All three feet are fixed in place with threaded screws, and could be replaced with alternative supports if desired.


The SCM20A is a two-way design, crossing over at 2.8kHz. Frequencies up to this point are reproduced by ATC’s 150mm low/mid driver, a short voice coil/long gap design incorporating a low frequency driver with a 75mm soft dome in the centre for broad midrange dispersion. Frequencies above 2.8kHz are reproduced by a 25mm soft dome tweeter manufac­tured by Vifa to ATC’s specifications. Collectively, these drivers produce a useable frequency response from 60Hz to 20kHz, remaining within a ±2dB window from 80Hz to 12kHz.

The electronic crossover is a fourth order (24dB/octave) critically damped design, individually aligned and phase corrected at the factory. There’s a 250W amplifier for the low/mid driver, and a 50W amplifier for the tweeter. Both amplifiers are Class AB designs operating considerably in Class A to minimise crossover distortion. This combination of electronics and drivers produces a maximum continuous SPL of 108dB at one metre, and it stays clean.

Built-in driver protection is included in the form of momentary gain reduction and thermal tweeter protec­tion circuits. A green ‘power on’ LED, situated below the low/mid driver, glows red when the protection circuitry steps in.

The majority of the rear panel is taken up with the cooling fins — in fact, the entire enclosure gets nice and warm after a few hours of use. Beneath the cooling fins is a protruding panel containing an AC power inlet and on/ off switch, a five-way rotary switch offering from 0dB to 6dB of boost at 40Hz, and a balanced XLR input socket. The input sensitivity is rated at one volt RMS, but an input trim control on the rear panel offers ±6dB of adjustment, so operating levels from 0.5 volts up to two volts RMS can be accepted.


The SCM20A benefits from ATC’s research into an elusive form of third harmonic distortion that’s inherent in all moving coil loudspeakers, and is due to the non­-linear effects of magnetic hysteresis and eddy currents. This research lead to the development and use of SLMM (Super Linear Magnetic Material) in many of their drivers, and a corresponding 10dB to 15dB decrease in third harmonic distortion.

That’s a significant reduction. With third harmonic distortion now as low as -60dB (equivalent to 0.1%), the SLMM drivers have distortion figures more akin to power amplifiers than speakers. (For further information on ATC’s SL technology, read the following interview with Bill Woodman.)


The SCM20As have all the sonic characteristics of the ATC range, including the superbly detailed midrange and the tight, well-defined bass. In comparison to their passive predecessors, the SCM20A’s low frequency performance is notably stronger and warmer. These terms may seem at odds with the concept of accuracy, which is why it’s important to understand how a manufacturer interprets that concept.

ATC’s design philosophy of exciting the reverberant field evenly, and their corresponding development and use of soft dome midrange drivers, were mentioned earlier in this review. To listeners who are unfamiliar with these concepts, ATC monitors initially sound bass shy — the result of a perceived increase in overall midrange energy combined with ATC’s characteristic highly damped bass performance. This is especially the case when comparing ATC monitors against other brands. But if you agree with ATC’s philosophies, you’ll quickly realise that the midrange is actually in its proper direct and reverberant relationship to the lows and highs — something many monitors have a hard time achieving. A few minutes listening to well-recorded performances of acoustic music will confirm this.

The benefits of ATC’s Super Linear technology are clearly audible — the SCM20As have a remarkable clarity, precision, neutrality and attention to detail about them. Their ruthless commitment to accuracy reveals every detail of each sound played through them, for better or worse. Tracking with the SCM20As is an absolute pleasure, and provides a great way to make sure you’re capturing good sounds from the very beginning — whether you’re fine tuning the position of a microphone, modifying a synth sound, or cleaning up a sample. Mixing is also a pleasure, for the same reasons. As an added bonus, their ultra-low distortion figures mean you can concentrate on the sound for extended periods of time without feeling fatigued.

But there’s a side to the SCM20As that can be quite disturbing, because they clearly reveal the sound of your equipment itself — sound that is often buried beneath the non-linearities of lesser monitors. Unpleasant harmonic distortions generated by low cost circuitry, phase problems related to EQ, even the ‘sound’ of different cables. If that level of detail is going to upset you, stay away from these monitors — as Jack Nicholson said in A Few Good Men, “You can’t handle the truth!” Alternatively, you could consider the SCM20As as an upgrade to your hearing.


Whether you agree with ATC’s philoso­phies or not, one thing cannot be denied. If you’re looking for absolute accuracy in a small monitor, I doubt there are any on the market that will show these up. There may be some equals, but that depends on your definition of accuracy. I personally agree with ATC’s philosophies, and this particular pair of SCM20As will not be going back to the distributor. In fact, they’ve become my new reference for recording, mixing and reviewing equipment. They’re going to cost me an arm and a leg, but my ears reckon they’re worth it.


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Issue 93


Ableton Live 12
What’s in. What’s out. What to expect.