KEF LS50 Anniversary Model loudspeaker

The tale's been told many times: Back in the early 1970s, the British Broadcasting Corporation needed a small nearfield monitor for use in remote-broadcast trucks. A team led by T. Sommerville and D.E. Shorter, both of the BBC's Research Department, developed the two-way, sealed-box LS3/5, based on a small monitor they'd designed for experiments in acoustic scaling. The speaker showed much promise, but problems with the drive-units—a woofer with a doped Bextrene 5" cone and a 1" Mylar-dome tweeter—led to a detailed redesign, the LS3/5a, carried out by Dudley Harwood, also of the Research Department (and later to found Harbeth), and Maurice E. Whatton and R.W. Mills, of the BBC's Designs Department (footnote 1).

The only limitations of the LS3/5a were intended to be those arising from the necessarily small enclosure and the absence, under nearfield monitoring conditions, of the need for a wide dynamic range. Despite its intended use as a nearfield monitor close to a boundary (the studio mixing console), the LS3/5a proved equally effective used as a conventional stand-mounted speaker in free space. Not only did the rise in its upper-bass response give the impression that there was more bass than there actually was, it also provided a degree of baffle-step compensation that resulted in a neutral in-room midrange balance. (The "baffle step" rise in a speaker's on-axis freefield response is due to the size of the speaker's front baffle, which is much smaller than the wavelength of the sound the speaker emits at low frequencies, becoming equal to or less than the wavelength in the midrange. Although the speaker puts out the same energy in the midrange as it does in the bass, the restriction of that energy to a narrower window in the midrange results in a rise in response above the baffle-step frequency.)

More than 60,000 pairs of the LS3/5a were manufactured under license from the BBC between 1976 and 1988; J. Gordon Holt reviewed the original LS3/5a for Stereophile in the spring 1977 issue (Vol.3 No.12), and his report, along with subsequent reviews of the Spendor, Rogers, Harbeth, and KEF versions, is available in our free online archives.

Although it wasn't an original licensee for manufacturing the complete loudspeaker, the British manufacturer KEF had an intimate relationship with the LS3/5a from the start, as it supplied both of its drive-units: a B110 woofer and a T27 SP1032 tweeter. KEF was founded in fall 1961; as well as publishing a beautiful, and beautifully informative, book on its history, written by Ken Kessler and Dr. Andrew Watson. KEF has celebrated its 50th anniversary with a special-edition loudspeaker, the LS50 Anniversary Model.

The 50
Costing $1499.99/pair—affordable, considering its "flagship" status on KEF's website—the stand-mounted LS50 is an unusually styled minimonitor. Though its height and width are almost identical, it certainly doesn't look like the LS3/5a—not with the rose-gold color of its solitary Uni-Q drive-unit, its convex profiled baffle, and the subtly embossed KEF logo on its top panel. The appearance doesn't tell the whole story, however. From KEF's white paper on the design of the LS50: "The LS50 is a two-way loudspeaker system, inspired by the LS3/5A. . . . Like the LS3/5A, the LS50 has been developed with the extensive application of the latest engineering techniques, along with meticulous attention to detail. Extensive listening tests were performed to ensure the right engineering choices were made to achieve the best possible balance. Both systems could be described as 'Engineers loudspeakers,' [sic] where the design has been determined by engineering parameters and sonic performance, rather than marketing requirements."

This white paper is worth studying as a casebook for modern loudspeaker design, in which designers have full control over all aspects of both the drive-units and system. (The references listed at the end of the paper comprise a history of modern loudspeaker design.) In the LS50's design, considerable use was made of numerical techniques, such as Finite Element Analysis (FEA), Boundary Element Analysis (BEA), and Computational Fluid Dynamics.

The Uni-Q drive-unit was developed from the 5.25" driver used in KEF's lower-cost Q- and R-series speakers. A 1" aluminum-dome tweeter, derived from the HF driver used in KEF's second flagship model, the Blade, and using a neodymium magnet, is mounted on the woofer motor's central pole-piece. A version of KEF's patented "tangerine" waveguide is on the front of the dome, which allows a deeper, stiffer dome to be used without interfering with the desired spherical radiation pattern, which pushes the dome resonance up to the region of 40kHz. The profiles of the woofer cone surrounding the tweeter, the ribbed long-throw woofer surround (which KEF calls Z-Flex), and the convex baffle, all smoothly continue the waveguide to give optimal high-frequency dispersion. The woofer section of the Uni-Q driver operates below 2.2kHz and uses a cone formed from an aluminum-magnesium alloy. A proprietary method is said to eliminate the high-Q resonances that would otherwise mar its upper-frequency output. The voice-coil former has an aluminum shorting ring to reduce flux modulation, hence midrange distortion.

The woofer is loaded with a large reflex port with an elliptical cross-section, flared at its inner and outer openings and offset to one side on the top of the rear panel. As you can see from my measurements that accompany Stereophile's reviews of reflex-loaded, stand-mounted speakers, the ports in these speakers often suffer from midrange resonances well above the port's nominal passband. KEF's engineers came up with a unique solution for the LS50: the middle section of the port's internal wall is flexible, made of closed-cell foam. At midrange frequencies this port wall is claimed to allow sufficient sound to escape for resonances to be reduced by as much as 15dB, but with little effect at low frequencies.

The enclosure was modeled with BEA and FEA analysis to produce a final design that radiated from its surfaces as little sound as possible, along with optimal suppression of internal standing waves. A combination of vertical and horizontal cross-braces with a compliant layer between these braces and the enclosure walls proved to be the best solution. While the LS50's enclosure is made of MDF, the convex front baffle appears to be molded from high-density structural foam with a ribbed internal surface. Though the front of the Uni-Q driver is flush with the baffles, it looks as if the driver is mounted from behind the baffle. Four rubber grommets at the corners of the rear panel cover the heads of the bolts that secure the baffle to the enclosure. Electrical connection is via a single pair of high-quality binding posts.

Although the LS50 was designed and engineered in England, it is manufactured in China by Gold Peak, the Hong Kong firm that has owned KEF since 1993.

Listening
I set up the LS50s on 24"-high Celestion stands, the central pillars of which were filled with a mix of birdshot and dry sand. These placed the KEFs' tweeters 31" from the floor, a few inches below the height of my ears in my listening chair, which meant that I had to slouch a little to get the optimal top-octave balance. I placed the speakers at the positions in my room that have worked well for all the minimonitors I have reviewed in the past few years, with the woofers about 3' from the book-and-LP–lined sidewalls and 6' from the wall behind the speakers. The LS50s were toed-in to the listening position.

It is a cliché to describe a minimonitor as "sounding larger than it looks," but that was the case with the LS50 once I'd optimized their setup. With classical orchestral music—I've been listening to a lot of Sibelius recently, the Barbirolli/RPO Symphony 2 in particular (Apple Lossless, ripped from the Chesky reissue CD of the original Reader's Digest LP)—the little KEFs produced a big sweep of sound, but without the lack of low-frequency definition so typical of the LS3/5a and some of its descendants. The half-step–spaced low-frequency tonebursts on Editor's Choice (CD, Stereophile STPH016-2) spoke very cleanly, with no emphasis of the tones. The pitches of the occasional double-stopped notes from the double bass in "Both Sides Now," from Herbie Hancock's River: The Joni Letters (24-bit/96kHz Apple Lossless file transcoded from FLAC, Verve/HDtracks), remained clearly defined, as did Leonard Cohen's gruff baritone in "The Jungle Line" from the same album, which didn't sound too "chesty."

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Footnote 1: A BBC white paper by Harwood, Whatton, and Mills, "The Design of the Miniature Monitoring Loudspeaker Type LS3/5a," report RD 1976-29, is available here. The full history of the LS3/5a can be found here.
Source : stereophile[dot]com