Review: ATC SCM 20sl TA loudspeakerReview: ATC SCM 20sl TA loudspeaker

Originally printed in the April 1999 issue of Gramophone.

Despite a lifetime's involvement with loudspeakers, their design, modus operandi, application and performance, I still find myself hesitating when asked to prognosticate on how a newcomer will be regarded ten years from its birth.

Of course it's easy to see that some will have no more than a brief moment of glory, but who would have been bold enough to predict that Quad's first 1957 electrostatic would run and run, substantially unchanged, for over 30 years? The exception that proves the rule? Well possibly; much more common are the basically sound designs which lend themselves to a continuing process of development. Consider for example, B&W's original Model 801 of 1979 which finished (or has it?) as the Mark III version, to be eventually outclassed by the Nautilus 801 reviewed only last November. It could well be that we have a further example in the ATC model which is at the heart of this report.

The ATC marque was firmly established as a source of large professional monitor loudspeakers by the 1980s. Not surprisingly their outstanding performance had led a few well-funded individuals with suitably extensive domestic accommodation to have a pair at home. Often these were people who had come across the monitors in their professional careers as musicians, or elsewhere in the entertainment industry, and it was not long before we at Gramophone became interested and ran an early review on what was then the smallest model in the range, the SCM 50 (John Borwick, March 1987). 

The people at ATC were not slow to realise that although their products were, and always would be, at the top end of the market, there were many music lovers who would appreciate their enviable standards in a smaller compass, more domestically acceptable. By the beginning of 1990 the first

SCM 20s were coming out of the factory. Incidentally, SCM stands for Studio Control Monitor (even this smaller model has professional applications) and the numerals always refer to the internal volume of the cabinet in litres. The SCM 20's first appearance at a public show was at the Sound & Vision exhibition in Bristol in February 1990 and it was an immediate success, even at its then price of £1,320 per pair. Having given it a thorough work-out I concluded a report in July 1990, saying, 'ATC want to make the best loudspeakers in the world. On this showing they have made a great deal of progress along that path'.

A problem faced in that 1990 report was the difficulty in finding suitable stands to support the exceptional mass of the SCM 20, some 23kg. ATC subsequently came up with a special pair of its own manufacture but also offered a different version of the loudspeaker with a 'T' (for tower) suffix in which the cabinet was extended to floor level. This extension took no part acoustically: the working loudspeaker only occupied the upper region, remaining at 20 litres. A further development of the SCM 20, and one of some importance, was reported in February 1997. This dealt extensively with a pair of non-conducting rings included in the magnetic circuit of the main driver which effectively cut the ratio of harmonic distortion. Loudspeakers with this improvement gained the suffix 'sl' for super linear.

The majority of ATC's larger professional loudspeakers had always been produced as active versions containing their own power amplifiers, although passive crossovers could be fitted as an alternative to special order. SCM 20s, which were primarily aimed at domestic users, had always been fitted internally with passive crossovers, ATC having observed that ventures into the active field by other makers had met with very limited success.

However, requests for an active version were being received and that empty space in the bottom of the tower version beckoned. Tim Isaacs, ATC's electronics guru had always been a great believer in the benefits of the active approach, mating dedicated individual power amplifiers to each driver unit. Over the years he had developed some rather clever circuit twists to permit phase alignment between units and sonically benign protection arrangements which safeguarded both amplifiers and drivers from the worst excesses of careless studio engineers. By the end of 1997 a compact two channel amplifier pack which fitted neatly into a cavity formed in the base of the tower versions was completed.

A pair of these amplified SCM 20s1 TA loudspeakers costs £3,995 in standard finish which brings them nicely into the select group of rather upmarket transducers which have been entertaining me these past months. Of course it is necessary to point out that this is the only pair which includes four channels of power amplification, totalling 700 watts, preceded by active crossovers.

Note 'preceded'; this is a very different application to the more common bi-amping followed by existing passive crossover sections which many people practice. It is necessary to draw this distinction in view of what follows.

ATC's Alan Ainslie who brought me these loudspeakers and helped set them up in my listening room had listed a few of what he considered the main advantages of active operation. Here are the main ones:

 • Accurate electronic frequency division, not at all affected by drive unit characteristics such as voice coil temperature.

• Direct control of individual drive units, negating production variables.

• Accurate phase correction through the crossover region, improving imaging and dispersion.

• Reduced power amplifier distortions, especially intermodulation, because of benign load and reduced bandwidth.

• Improvement in dynamic range by eliminating driver interdependence.

• Cost efficient, power amplifiers included and no expensive casings.

The SCM's amplifiers are built on a thick blackened aluminium plate which is flush fitted into the lower rear of the cabinet and retained by eight bolts which engage captive nuts: these have to carry the substantial weight of the assembly. The upper half is taken up by a heavy vaned heatsink; on the inner face of this area are clamped the power MosFet devices and the single large printed circuit board which carries the many small components. The lower half of the plate displays a standard three-pin fEC mains connector, an on/off switch with indicator light, a fuse holder and an XLR input socket. Connectors are protected by a pair of guard rails.

Inside this lower section is a very large shielded power transformer,  with multiple output windings to feed the two power amplifier sections separately and a lower voltage regulated supply for the early stages and the electronic crossover. The very plain cabinet is constructed of veneered MDF and the two drive units are fitted on to their own thick baffle plate which is surface mounted to the top front face. A black cloth covered grille frame exactly fits over this protrusion and carries the ATC logo. There is a small angled plinth and spikes can be fitted. At this point I might mention a further variation, the SCM 20A for professional users, which houses similar active operation in a cast metal casing.

The input XLR connector can be wired for either balanced or unbalanced feeds; a one volt signal is required, easily obtainable from a preamplifier or direct from a CD player if the latter has a volume control. I tried both with no detectable difference, both were equally enjoyable. Of course the foremost thought in your reporter's mind was to question Ainslie's list of active advantages, for although the sounds I was hearing were of a very high standard indeed, comparing them with my notes on the 1997 passive version was not going to be very illuminating.

However, I had underestimated the cunning of the ATC lads: this particular pair of loudspeakers had been fitted with an additional normal passive crossover and a changeover relay wired in parallel with the amplifier on/off switch. So with the switch off I could feed it as a normal loudspeaker from my own amplifier and with the addition of a second parallel input connection immediately change to active operation simply by switching on. Alan Ainslie sensibly suggested that I did not do this until I had immersed myself thoroughly in active operation for several days and this was no hardship as I revelled once again in the clean response of ATC's remarkable midrange dome, which retains its unusually constant dispersion pattern although here it is part of the main driver. Piano, as usual with ATC designs, appeared solidly at the end of my room and the wizard Alfred Brendel gave me one of his rare excursions into Bach (Italian Concerto, Chromatic Fantasia and Fugue etc., Philips D 442 400-2PM). And then, because I had just been reading his obituary, Robert Shaw conducting Verdi's Requiem with his mighty Atlanta forces (Telarc D 80152, 3/88). The ATCs took it all in their stride.

Dedicated crossover and power amplifiers

After the prescribed few days I switched on my usual external amplifiers and turning off the ATC's internal ones sat down again to listen. Frankly, based on previous experience, I had not expected to notice much difference but in this case there certainly was. As far as I could tell the high frequency end of things was unchanged but the main drivers were now behaving in a far less controlled manner. Bass had become noticeably more flabby although it remained tuneful and the once commanding midrange had lost a measure of definition and was no longer the decisive and authoritative factor which had previously been so outstanding.

By my reckoning only two significant things had changed: the inclusion of the high level passive crossovers and the loss of the phase correction built in to the active amplifiers. Being of a suspicious nature I decided on a further test; I placed both loudspeakers side by side in the centre of the room and fed one in the active condition with a screened lead direct from my preamplifier and the other in the passive condition straight from a spare Esprit power amplifier and about a metre of 79-strand wire. Switching the preamplifier to the mono mode it was possible to sweep from one loudspeaker to the other with the balance control. The result was exactly the same: a clear victory for the active version. I remain somewhat puzzled by the degree of difference obtaining but on the present evidence if you are equipping or re-equipping your music room with a system of some class ATC can make a good case for investigating the active approach.

Specification Frequency range 60Hz-2OkHz (-6dB cut-oft frequencies) Amplitude linearity ±2dB, 80Hz-1 2kHz Horizontal dispersion ±80° coherent Vertical dispersion ±10° coherent Maximum spl 108dB continuous, free field Crossover frequency 28kHz, phase corrected Amplifier power bass/midrange 300W rms; tweeter 50W Input sensitivity/impedance 1 v/i Ok ohms Dimensions 1,023H x 239W x 355D mm including grille and spikes Weight 40kg Manufacturer Loudspeaker Technology Limited, Gypsy Lane, Aston Down, Stroud, Gloucestershire GL6 8HR Telephone 01 285 760561 Fax 01285 760683 UK retail price £3,995 in black ash; cherry, walnut and rosewood extra

Geoffrey Horn

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