Celestion SL600si loudspeaker & DLP600 digital equalizer
By John Atkinson   •   May, 1989 Following my reports on 13 mainly inexpensive loudspeakers that have appeared in the last four issues of Stereophile, I thought I would give myself a treat this month by reviewing the latest incarnations of a model that has stood the test of time: the two-way Celestion SL600Si...This is a carefully tuned infinite-baffle design, sacrificing ultimate extension for upper-bass and lower-midrange quality. Its crossover is conventionally British in that it puts flatness of on-axis amplitude response ahead of time coherence, while everything about it, from drive-units to the cabinet itself, is flagrantly "high-tech."

The SL600Si
It is almost six and a half years to the day that I first heard the prototype SL600, brought to my then house in Sussex, England, by Celestion's then heads of engineering, Graham Bank (still with the company after a stint at Wharfedale), and marketing, David Inman (the latter now with KEF). The wooden-enclosure SL6 was the first dynamic speaker to have wooed me away from the old Quad ESL, its copper-dome tweeter at last enabling a moving-coil design to have some of the treble magic that is routinely bundled in with electrostatics. But then to hear, back in the Fall of 1982, how the same drive-units could be made to produce a true high-end sound by replacing the SL6's conventional chipboard enclosure with one fabricated from aluminum honeycomb, was a mind-blower.

As soon as Celestion had solved the production problems associated with the new cabinet material—and there were many problems—the SL600 became my reference speaker of choice, surviving two house moves in the UK, one transatlantic translocation, two further changes of listening room in the US, and many, many, many system changes. Other speakers beckoned siren-like for a while, particularly the Magnepan MG2.5/R, Monitor Audio R952/MD, Acoustic Energy AE1, and Thiel CS1.2, but I kept on returning to the sound I liked the best.

And then, in the summer of '88, the '600 was replaced in my system—by Celestion's SL700, a speaker that finally made the old lady seem a little old-fashioned, even dowdy, by outdoing her in every area where she had previously starred, and adding higher levels of midrange and treble transparency to boot. My pair of SL600s went on to provide sterling service chez Olsher, where Dick used them for the subwoofer survey that appeared in the January 1989 issue, and I enjoyed music via the upper-class '700s (between batches of review speakers).

While I waited for a pair of the newly revised SL600Si's to be delivered to Santa Fe.

Ostensibly, apart from a more granular matt-gray paint finish, the Si appears identical to the old '600. The rear panel, however, sports a whole new set of 4mm input sockets, two for the HF leg of the crossover and two for the bass/midrange, to allow bi-wiring or bi-amping. The second-order, 12dB/octave crossover has, in fact, been redesigned and now features star-earthing. (One of the factors that led to complaints from some UK critics—that the SL600 was congested in the midrange—was because the ground returns from the two drivers shared a pcb track.) The notch filter used to lower the amplitude of the tweeter's oil-can resonance has apparently also been redesigned, with better-quality components used.

The drive-units appear identical to those used in the older speaker: the 32mm copper-dome tweeter is formed in one piece with the coil former, the latter then acting as a shorted turn in the magnet gap to damp the driver's low-frequency resonance. The woofer, constructed on a diecast basket, is also a high-tech device, its flared PVC cone being developed with the aid of laser interferometry to ensure good pistonic motion throughout the driver's passband, the radiating area being said to decrease smoothly with increasing frequency. There is no separate dustcap as such; the cone is of a piece with an inverted dome, this again determined, via laser interferometry, to be optimal.

The cabinet is unique to the SL600Si (and similar to the SL700) in being fabricated from a 0.5"-thick, metal-honeycomb aircraft flooring material. While low in mass, thus minimizing energy storage, it is sufficiently stiff enough for the resonances of its panels to be pushed almost two octaves higher in frequency than with a conventional wooden cabinet. The contribution to the overall sound from the flexing of the enclosure walls will thus be moved away from the region where instruments and voices have their energy maxima, and will also be lower in level. The result should be a low delayed-resonance signature, with correspondingly low levels of midrange coloration. Because the walls of the enclosure will now be virtually transparent to midrange sound, it is filled with carefully graded foam to absorb as much of the woofer's backwave as possible.