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."



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.
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COMMENTS
Audio Asylum Bruce from DC's picture

Being a dedicated "stand-mount" guy and something of a cheapskate, this review got me really excited.  Maybe I should replace my 10-year old Joseph Audio RM7si Signatures!

Imagine my disappointment when I went to KEF's US website and tried to find a dealer.  Even at maximum range from my location -- Washington DC -- I came up empty.  There is a local dealer, but it stocks only KEF's low-line stuff, not any of the fancy products (including the Reference Series).

Assuming that one would agree that Washington DC is not the sticks, it might be worthwhile for Stereophile to enquire of KEF exactly how many US dealers the company has which stock this product -- or the Reference series for that matter.

Sergei's picture

The reason you can't find a pair Audio Asylum is because KEF in their wisdom decided to insult all of their loyal dealers by offering this product ONLINE ONLY!

I am a Canadian Dealer(soon to be ex-dealer now) for KEF. We had a clent want to hear a pair of these and most likely purchase due to being a long time KEF fan. To both of our surprise when I contacted my KEF rep he explained this was ONLY available through the KEF online store.

We played him a pair of other bookshelf speakers he happily purchased instead.

There is really no logic as a business man and Hi-Fi enthusiats that KEF would want to snub their dealer base with an "Anniversary" product. We were even considering displaying the Blade's in our showroom...

 

hcie95he's picture

In the review the LS50 is compared with the B&W CM5 and the LS3/5a. In the past John reviewed also the Harbeth P3ESR and the Spendor SA1, which are both more expensive. I am wondering if the LS50 is in the same league as the Spendor and the Harbeth?

By the way I am surprised that KEF doesn't sell the LS50 in the US via their dealer network. In Europe you can buy and audition them via their dealers.

norcalhifiguy's picture

 

Sad to see the previous comments, but as the owner of small audio shop in a very small town I can say with absolute authority that these are indeed available from direct dealers. (I can't speak to the Canadian issue as we are in the US)

We are thrilled to have these on display in our main listening room and must say they live up to the great reviews. These are amazing little speakers.

Please, do seek out and support your local independent audio dealers. 

These are people that love great audio as much as you do.

Hear them in person. Buy them locally!

audiodoctornj's picture

As one of KEF's highest level dealers, Audio Doctor, has the Blades, the entire Reference Series, R Series, Q Series, and T Series products as well as the LS 50, I can tell you they are available at select dealers, I don't know the issue with the Canadian distributor but they should be available to them as well.

WIth that being said, I do feel that is unfortunate that KEF does not have more stocking dealers with the LS 50, or with a wider range of their products.

The current KEF lineup is some of the best products the company has ever produced and in particular the BLADE represents $60,000.00 and above levels of performance for $30k, the BLADES are amazing speakers!

I feel that there are a few issues going on, Stereophile and the Absolute Sound do not really do comparative anaylsis of one product to another in their reviews, if they did and were not affraid to stand up to their advertisers, they  would proclaim that the BLADE is a bargin for the abolute top level of performance that they give and would recommend that anyone looking at a Wilson, Magico or YG etc should give the BLADES a listen before considering spending more on anything else.

We did the High End Audio Show in NYC last year, and our setup with the BLADES and all Chord Electronics sounded as good as any of the other setups at the show including a $300,000.00 plus YG setup and another mega dollar MBL setup and these were $30k speakers standing up agains $110k speakers!

So in summation, KEF makes fantastic products, has a weak dealer network, and needs to revamp their advertising and marketing to get more people excited by their products.

You should hear a pair of LS 50 setup with the really nice little KEF R 400 sub you have a roughly $3k package that sounds remarkable, huge soundstage, shocking dynamics and great transparency!

If anyone wants to visit our shop call 877 428 2873 and make an appointment to hear these incredible little jewels.  

10sephirot's picture

The little KEF sure is getting good reviews.  Lets face it looks count also.  It made me think about what stands would look best.  In my opinion the Usher RSW-708 in walnut would be a very special look:

http://audioscape.ca/wp-content/uploads/usher-rws708.jpg

The KEF plus the stands I subjectively think look best is a $2000 proposition. 

I'll go ahead and match that up to the Vincent Audio SV-236 in black:

http://www.audioadvisor.com/ViewLargerIMage.asp?title=Vincent+Audio+-+SV...

another $2000.

I'll go ahead and add the Musical Fidelity MMF-51SE:

http://www.elusivedisc.com/images/MMF-51SE-Large.jpg

Yeah well anyway that is another grand.   $5000 total.

I told all this to Santa and I got thrown out of the shopping mall.

But in all seriousness this is what a good review makes us do: dream. 

Thanks Stereophile.  Another afternoon wasted daydreaming:)

JRT's picture

In the measurements, the waterfall diagram showing polar response in the vertical plane passing thru the voicecoil centerline appears to be much smoother than the polar response in the horizontal plane.   So I think the first tweak for improved performance, clearly demonstrated in the measurements, would be to lay the speaker on its side.  

prof's picture

In the measurements, the waterfall diagram showing polar response in the vertical plane passing thru the voicecoil centerline appears to be much smoother than the polar response in the horizontal plane.

Umh, no. You've been fooled by the difference in scales. The scale on the vertical waterfall plot is ±45°. The scale on the horizontal waterfall plot is ±90°. At ±45°, the horizontal dispersion is as-smooth or smoother than the vertical dispersion.

techblogpool's picture
ryebread's picture

Quest For Sound in Bensalem, PA

bought them last week, epic speakers

wgb113's picture

@ryebread,

Curious what amplification both QFS and you are using.  I might try to head over there for an audition.

Bill

Gradofan's picture

 

The sound I'm getting from this set up is "stupendous!"  Don't think it can get any / much better - certainly not for less than 10X the price!

 

Using the Sony  XA5400ES SACD/CDP (the best there is), into the Anti-Mode 2.0 as a pre and DSP to eliminate the "room boom," into a tube buffer, into the Class D Audio amp into the LS50's and R400b produces real "you are there" sound.  Incredible detail, resolution, tone, tenure, texture, stage, image, bass, mids and treble... etc., etc...  

 

Hard to believe... really... 

 

And just an incredible value!

 

wgb113's picture

Everything I've read about the LS50s indicate that they're great for a small room.  My room is REALLY small @ 10'x12'x9'.  It's been acoustically treated but placement of the speakers and listening position results in a near-field setup.  Are the KEFs going to work well in such a setup?  It seems their manual recommends 6'-10' between the inner sides of the cabinets before toeing in.

Bill

Azani's picture

I've just purchased the pair after searching high and low for floor standers to replace Rogers LS2a/2. Auditioned Harbeth P3ESR, like it but thought it sounded like big voice coming out from small mouth. Haven't got the chance to audition any Spendor monitors tho. 

Anyway, my room is also small (8x13)' with acoustic treatment but still able to handle LS50 quite well. Generally, a slight enhancement from Rogers LS2a/2, livelier that is.

just yesterday, started to experience sudden annoying jerks and "tone switching" (for the lack of better word) on the right speaker whenever CD is played. It was fine the day before. Yet to figure that one out. Anyone experience it?

Cheers

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