SPECIFICATIONS

C /5 (25 C200S'W

Enclosure type: two-way sealed two-way sealed coupled-cavity

Frequency response: 68 20,000Hz 65-20.000Hz 40-120Hz

Drive units: I 10mm. 19mm I 60mm, 19mm 200mm

Sensitivity: 85dB 87dB 87dB

Power handling: 50W 70W 150W

Nominal impedance: 4 Ohms 4 Ohms 4 Ohms

Dimensions (mm): 265 x 180 x 150 340 x 205 x 175 487 x 249 x 249

Weight: 3-2kg 4-I kg 7.7kg

UK retail price: £99 £139 £249

A saving is to be made when purchasing either the C15 or C25 at the same time as the C200SW as follows: CI5 + C200SW £329; C25 + C200SW £369

Manufacturer: KEF Electronics Ltd., Tovil, Maidstone. Kent MEI 5 6QP

SUBWOOFERS have been around for quite a few years, but until lately they have produced only a limited show of interest from the public. They reproduce only the very lowest frequencies, say from about 120Hz downwards, and the idea is that they can be coupled to a system where the main loudspeakers are small in size and lacking in bass-perhaps having been chosen because of limitations in room space or available cash, or both.

My own lack of enthusiasm for the earliest subwoofers centred on two areas. First, the small main loudspeakers of a few years ago were often rather poor in general quality (with some glorious exceptions such as the ubiquitous but pricey LS3/5A) and merely providing some bass extension still left the overall performance unsatisfactory. Today things have changed and many small loudspeakers, in all price brackets, do a surprisingly good job of radiating realistic sounds with good attack, smooth treble and wide dispersion. Their lack of low-frequency power may be their only important shortcoming so that the subwoofer idea could be quite attractive.

Second, the early subwoofers consisted of a single large chest-shaped enclosure in which the left and right channels were combined. The theory was that the bass frequencies carry very little stereo information and bass-only loudspeakers are strictly non-directional, so a single cabinet was all that was needed and it could be placed anywhere in the room. My ears told me different. Lumping the left and right bass frequencies together in one box noticeably restricted the sense of ambient depth and spread, and any gross separation of the bass source from the main loudspeakers always sounded unnatural.

Today several subwoofer designers have met this criticism. They provide two subwoofers instead of one and make these small enough (using clever drive unit and enclosure designs) to permit precise positioning to suit the user's taste and listeningroom constrictions. 1 reviewed a distinctly up-market manifestation of this philosophy back in September 1987. This was the B&W Loud speakers Concept 90 system, which comprises a pair of mini CM! loudspeakers (costing £345) to which can be added (as bolt-on supports) a pair of column-shaped floor-standing CM2 subwoofers costing a further £550.

Here we have KEF addressing the more affordable price bracket with cleverly shaped subwoofers which lend themselves to a wide choice of placements, including straightforward use as supporting pillars for the main loudspeakers. The KEF C200SW costs £249 per pair and, although the manufacturers say that they may be used with other main units, including most of their own early models, they are "designed specifically to operate with the C15 and C25".

Therefore, since these two current KEF models can be expected to provide optimum electrical and acoustic compatibility with the C200SW subwoofers—and since it seems ages since I reviewed anything from this cornerstone British manufacturer—I propose to describe my responses to the C15 and C25 first, and then report on their performance when coupled to the subwoofers. The CIS and C25 come at the bottom end of a new range of six C-Series loudspeakers launched about a year ago, the other four more expensive models all featuring KEF's innovative Uni-Q coaxial drive unit.

C15

The pint-sized C15 is a straightforward two-way system with tweeter, main driver and polyester damping material crammed into a closed box of only 3.5 litres internal volume. None the less the components and construction are of good quality and the claimed performance is a real improvement over run-ofthe-mill rack system loudspeakers.

The 19mm tweeter is metal domed (aluminium foil) with the kind of tangentially pleated integral surround that I first remember seeing used very successfuly about 30 years ago, and ferrofluid damping. The 110mm main driver has a diecast chassis, polypropylene cone and 32mm diameter voice-coil designed for up to 60 Watts power handling and sound pressure levels (SPL) up to 100dBA.

Electrical connection is via substantial recessed gold-plated terminals on the rebated back panel. The nominal impedance is given as 4 Ohms. Sensitivity is below average at 85dB for 2•83V at one metre. The cabinet is solidly constructed and finished in simulated black ash or walnut vinyl, with a matching grille cloth stretched over a rounded-edge moulded plastics frame. The user instructions recommend placing the loudspeakers against a wall to avoid time-delayed reflections. Of course this will also introduce a (deliberate) degree of bass boost, which can't be bad for any mini system—but may need a rethink when the C15 is used together with the subwoofer.

1 began my listening tests with the C ISs mounted on stands at my usual one metre or so out from the rear and side walls. The lack of bass was obvious but there were compensations in terms of the bright tone without undue emphasis, overall smoothness, good presence and excellent stereo imaging. Some bass reinforcement could be obtained by simple shelf-mounting against a wall, and indeed inserting the loudspeakers about two-three metres apart in bookshelves produced an 'invisible' and musically satisfying sound source. Volume was adequate and could be increased to any reasonable domestic listening level without serious overload side-effects.

C25

Moving up to the C25, we find an internal volume increase from 3-5 to 6.8 litres and marginal specification increases under all headings: sensitivity up to 87dB, power handling up to 70 Watts and maximum SPL 106dB. Apart from the larger enclosure, all the beefing up of performance can be attributed to the change from a 110mm main drive unit to KEF's new 160mm unit with a lightweight polymer diaphragm. The tweeter is the same as in the CI5 and all other constructional features are as near as I can see identical.

The same operating instructions are supplied for both models so I followed the same test procedures (including a room plot of frequency response which confirmed the C25's higher sensitivity and slightly delayed bass roll-off). Used at a good one metre from the walls, the C25 gave a little more than the C15 in terms of lower midrange power, and was noticeably more sensitive. Yet music with any important bass content— classical or pop—had less impact than most people would like, and signs of cabinet resonance could be detected. Again, moving the loudspeakers close to the wall, as KEF recommend, produced a better balance and feeling of depth. As the same tweeter is used, the presence and treble smoothness were again very satisfactory.

C 200SW subwoofer

The new KEF subwoofer enclosure is a five-sided short column designed for floor-standing or horizontal positioning if preferred. It contains a single 200mm bass driver using K EF's "single coupled-cavity" configuration, which means that the driver is mounted inside the cabinet, out of sight, loaded by a tuned cavity and thence radiating through a contoured duct let into the narrow fifth side. This in effect forms the rear of the cabinet and also carries the necessary two pairs of gold-plated terminals, one to accept the signal from the amplifier and the other to relay the suitably high-pass filtered signal components to the main (satellite) loudspeakers. The coupled cavity construction is claimed to combine the higher effficiency of a normal bass-reflex enclosure with the superior transient response of a sealed box. It certainly gives the designer fine control over the various parameters and, since the subwoofer has to cope only with frequencies from about 120Hz downwards, its design can come close to the ideal targetted response extending down to around 35Hz.

The built-in crossover network shelves off some of the low-frequency energy on its way to the C15, C25 or other chosen satellite, thus allowing the complete three-way system so formed to attain slightly higher power handling capability than the satellite alone. KEF recommend that the cable connecting the subwoofer to the satellite should be of the same length for both the left and right systems, even if the physical separations are different (to maintain matching and level equality). This cable can be quite short of course if it is the intention to stand each satelite on top of its subwoofer. However, depending on the shape of the given room and furniture, there is scope for experimenting with several layouts.

The satellites should naturally be placed at a little below ear height and the conventional two—four metres apart for optimum stereo listening. Then the subwoofers may be tried at a narrower spacing, or even close together, or at a wider spacing, vertically or on their sides, close to the wall or tucked away in any convenient space. Though it seems like sacrilege, you can even try reversing the phase (polarity) of the connections to both satellites relative to their subwoofers. There will be the expected variation in bass response close to walls or corners but also a more subtle change in stereo staging. For instance, I found that solo piano recordings came very much to life, and as if being played live in my room, with the subwoofers at a wider spacing and laid on their sides. The only important proviso is that all four loudspeakers should line up more or less in the same vertical plane, i,e, should be equidistant from the listener.

System performance

In these listening tests the C I 5s gained very much from the addition of the subwoofers but there was a certain hollowness of tone or midrange reticence. With the C25s voices remained analytically clear and there was a satisfyingly natural treble/bass balance with a realistic feel given to the music's acoustic setting. Where space inhibits the installation of relatively large full-range systems, this new KEF C200SW subwoofer can provide a neat and versatile solution.

As for the Cl5s and C25s, they are both good examples of modern minispeaker design and represent good value. The C15 sounds less like a box but cannot quite equal the C25 for dynamic range capability.

JOHN BORWICK.