Dunlavy Audio Labs SC-IV/A loudspeaker Page 2

Obtaining improved bass performance from a loudspeaker is seldom just a matter of substituting a "better" woofer. The ScanSpeak woofer apparently has great low-end extension, but it also has some nasty peaks at the top of its passband. John Dunlavy says that he almost abandoned using the woofer for this reason, but kept coming back to it as the best driver for providing the bass extension and power handling he wanted. His solution to the problem consisted of several resonant circuits in the crossover, providing electrical compensation for the peaks.

In fact, another major difference between the two speakers is the crossover. John Dunlavy has always used first-order crossovers, and he hasn't abandoned this approach—the only one that, in conjunction with a stepped or sloped baffle, can yield time coherence, the sine qua non of his designs. However, there are many ways of designing and executing a minimum-phase crossover, and the SC-IV/A's crossover uses almost twice as many components as the one for the SC-IV. Compared to most speakers, the SC-IV's on-axis frequency response was already extremely linear, but this didn't satisfy Dunlavy. First, he tweaked the crossover points, improving the blend between the drivers. He then turned his attention to the small nonlinearities in the frequency response—the minor peaks and valleys caused by the drivers' electromechanical characteristics. The SC-IV/A's crossover has tuned resonant circuits to reduce these nonlinearities, with specific trim values determined by the individual testing of each speaker.

This labor-intensive process takes several hours, and is another reason for the difference in price. Though equalization of driver frequency response is not unique to Dunlavy, I know of no other speaker manufacturer that has taken it to this extreme.

Another difference between the SC-IV and the SC-VI/A is in cabinet construction. Optimizing the acoustical loading for the new woofer meant a change in the construction of the cabinet's internal chambers and using different damping materials. The SC-IV/A cabinet is said to be generally more rigid and less resonant. Because of all these structural and electronic changes, SC-IVs cannot be updated to SC-IV/As.

Careful reading of the specifications for the SC-IV and the SC-IV/A reveals another difference: the SC-IV/A is 1dB more sensitive (92dB vs 91dB). This may not seem like much, but every bit of added sensitivity helps, especially if you're using a low-powered amplifier. Like the SC-IV, the SC-IV/A has a flat impedance curve—a real advantage if you're using a tube amplifier with a highish output impedance.

It's common practice for audiophiles to set up speakers along the short wall of a rectangular room with the listening seat in the middle of the room, the included angle being about 45 degrees. (This is described in Jonathan Scull's "Fine Tunes" columns in the July and August 1998 issues, Vol.21 Nos.7 and 8.) Dunlavy's recommendation is quite different. He suggests setting up speakers along the long wall of the listening room, widely separated (in Dunlavy's factory listening room, the angle is greater than 120 degrees), with the listener close to the back wall.

As it happens, this is just how I like to set up speakers (not quite as far apart as Dunlavy, though), and this was the setup I'd been using for the SC-IVs. The SC-IV/As were placed as close as possible to the spots previously occupied by the SC-IVs, toed-in about 25 degrees (not quite pointing at the listening seat). My listening room is 14' by 16' by 7.5'. From the listening seat, the angle to the tweeter of each speaker was about 70 degrees. Setting toe-in accurately for each speaker was made much easier with the use of the Checkpoint SA-S P-700 laser alignment device.

In my review of the SC-IV, I described the benefits of putting spikes (my favorites are AudioPoints) under the speaker and removing the grille. With the SC-IV/A, the improvements brought about by these tweaks were of the same sort—tighter bass and greater transparency—but their magnitude seemed smaller, and the untweaked SC-IV/A sounded better than the tweaked SC-IV. Whether you decide to go for these tweaks will depend on the amount of obsessive-compulsiveness in your makeup. I had the SC-IV/As on spikes for all my critical listening; the grilles stayed off most of that time.

We can think of the sound of a speaker as having two components: the musical signal and the sonic character of the speaker itself. As a device with inherent electrical and mechanical nonlinearities (produced by the drivers, crossover, cabinet, and wiring), every speaker imposes some of its own sound on the music, and we have to mentally "tune out" these distortions. But they're still there, always reminding us that what we're listening to is not the real thing. The SC-IV/A hasn't completely avoided sounding like a speaker, but, compared to the SC-IV—itself already low in speaker colorations—it sounds more like musical instruments and voices and less like an electromechanical contrivance.

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