Celestion SL600si loudspeaker & DLP600 digital equalizer 1992 part 4
To finish my auditioning sessions with the DLP600, I dug my 1982 pair of original Celestion SL6s out from storage. (I never discard anything—you never know when it will come in useful.) Though these had not seen the light of day since 1985, and had had many the long boat trip, along with the rest of my household goods, when I moved to Santa Fe from the UK, they sounded fine right out of the box, sitting on the same 18" Celestion Si stands I'd used to audition the SL600Sis. The SL6 (but not its 1985 replacement, the SL6S) used exactly the same drive-units as the '600 and the later '600Si in an identically sized enclosure, the only difference being the use of veneered fiberboard rather than aluminum honeycomb for the latter's construction. In theory, therefore, the DLP600 should provide the appropriate correction for the older speaker.
I found this to be the case. While the SL6 doesn't have anything like the degree of lower-midrange transparency of the SL600—its cabinet is quite lively, even hooty, in this region—it was always a very musical, if rather dark-sounding speaker. Not only did the DLP600 usefully lighten the SL6's tonal character, it opened up the rear of the soundstage in quite a dramatic manner. On good recordings, recorded hall ambience sounded more "coherent," in that it seemed to bear a more genuine spatial relationship to the direct sounds of instruments and voices. The DLP/SL6 combination was still outpointed by the corrected SL600Si—there's no substitute for that inert Aerolam cabinet—but it sounded remarkably satisfying. The SL6 cost $800/pair in 1982; if you have a pair, get them out of storage or back from your brother-in-law and bring them back from the dead with the $800 DLP600.
If you got the impression from my listening notes that I am very enthusiastic about the improvement in sound quality offered by the DLP600, you're right. It turned a respectable but somewhat dated loudspeaker into a leading contender for high-end honors, seemingly without extracting any penalty other than the inability of its owner to get the best sound from analog sources. (It should also not be forgotten that the SL600Si is still a minimonitor: those looking for ear-blasting levels of gut-wrenching lows should look elsewhere.) The improvements in the SL600Si's image focus and palpability are, in my opinion, of equal value to those in the areas of tonal balance, the result being a significant increase in musical enjoyment.
The DLP600's market, of course, is limited: owners of Celestion's SL600, '600Si, and original SL6 who happen to use separate CD transports and processors. But to such people, I have to say that there is no better way of spending $800 on improving the sound of their systems.
It is also as a portent for the future of loudspeaker design that Celestion's DLP600 excites me. DSP won't transform a poor-sounding speaker into a Class A recommendation. Outside of Disneyland, Cinderella remains in the kitchen—a truly Zen situation where the speakers that most benefit from the use of DSP will be those that need it the least. In addition, it must not be forgotten that such time-domain equalizers will only work their magic over a carefully designated listening area. I also note that the ultimate target response for the DLP600's correction was not flat, which suggests that simplistically aiming for textbook behavior may not give optimal results for any given loudspeaker.
The DLP600 does demonstrate, however, how an already excellent speaker can be made even better. While I am sure Celestion's introduction of the DLP600 was a toe-in-the-water exercise, I predict the appearance of more general devices, where the filter coefficients for any particular loudspeaker will be stored on a plug-in EPROM module. And several companies, including Snell and B&W, are developing digital equalizers intended not only to correct the speaker's failings but those of the room. Exciting times, gentlemen; exciting times.