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LS3/5A improved    by Sergio Canini

 

photos 2 & 3  courtesy of Paul Whatton drawn from the web site www.ls35a.com

 
For those who consider these speakers some little jewels, the presence of a defect and/or a malfunction can cause a strong embarassment: you need  to make them repaired. Yes, but Where? Will they repair them well? Will my precious loud-speakers sound like before? I was asked just like these by a client of mine who has brought me these loud-speakers noting that at a certain frequence there were some problems. He retained that they didn't work and that they needed a recovery.   
But before speaking of it, let's see the story of these loud-speakers:
In the first of 70s the BBC was in possess of some big monitors Tannoy designed by Mr. Fountain for the most difficult sonorizations. Nevertheless  these speakers, both due to the dimensions and because they were'nt lacking of some defects (that mainly are:
 excessive low frequency response and  restricted high frequency radiation) weren't suitable for individual listening by sound engineers.
In particular it was needed a small speaker system in order to employ it in mobile studios (vans, shelters etc.) for external events.
.Speakers like these were already  present at that moment. Denton models by Wharfedale would have been adapted. 
But BBC preferred to design them from ground and I don't believe it was due to reasons concerning quality of reproduction, but referred to the kind of assembling instead (the latter would have simplified eventual repairs) and to reliability, all these  reasons due to professional use. One of the guys of the team who developed the project is Jim Rogers, known for his researches about phonoabsorbing materials discovering in the long-fibres sheep wool the most suitable material for the Transmission Lines (TL), that he produced and that were best sellers al the time. Rogers obtained the first license to produce the LS3/5A (this was the name assigned to those systems) . After some time, also Chartwell, Audiomaster, Spendor, Harbet and Richard Allan obtained a similar license. Kef supplied the components and the crossover. It is strange the absence of the wharfedale from this list (Wharfedale was fouded by C.A.Briggs, one of the most significative persons of english HiFi). Of course the manifacturers was allowed to sell the product also to customers other than BBC. So the LS3/5A legend was born. Jim Rogers, already a myth for the sheepwool TL, became a double-sized myth.
Afterward the same Jim Rogers designed an improved model of those speakers, that was named JR 149. They were different from the original one, beeing housed in an aluminiun round cabinet. Who listened to them says they show a better dynamic range and a lower bass ( the limit of LS3/5/A). In 1988 BBC itself modified the crossover's design. The nominal impedance passed form 15 to the current 11 ohms and bi-wiring was easily allowed. This puts some shadow in the design ability of BBC, considering the absolute absence of usefulness of such a technic.
Coming back to our time, what my customer showed was the limit of these speakers, indeed. In fact they was created to work at low power levels in vans and shelters, and also had an internal volume as great as a shoe box. Looking at the problem in my laboratory I saw in the range 70-140 Hz, the little Kef B110 was overloaded with signals exceding 15 volts, The result was the production of a sound like a noisy flatulence, an offence that the poor Jim did not deserve indeed, also if he had been at fault as the designer. Probably, in order to give  some more consistence to the sound reproduction, Jim set the quality factor at resonance to a value of 1.3. This quality factor is considerably high, it deprives the woofer of any "self-control" in that range, so that distorsion arises. A high-pass filter could have fixed the problem; on the contrary it would have  lowered bass response. Surely, a high-pass filter working with strong signals only could have fixed the problem without inconveniences. 
The circuit shown at right  has proven to be very effective to the purpose.The lamp has a filament whose resistance is less than 1 Ohm (when it is cold), and more than 30 Ohm (when it is hot). By this way, the lamp acts as a by-pass for the capacitor (that forms the high-pass filter) only when the signal  is low in level, As the signal becomes higher and higher in level, the capacitor looses is function and in consequence the filament becomes hot.    The capacitor is a non-polarized device,100 uf-100V (or more), the lamp is a tubular type 18W - 12 V.   The values were chosen as specified due to the fact that the involved speaker systems   worked in conjunction to subwoofers.   The low range of the LS3/5A is a bit light, but in this case it has little importance. The applicable power without distortion is in the range from about 20 to about 90 W. 
If someone employs the speaker systems without the subwoofer and in the meantime he would like to lower the low range as less as possible, it is advisable to increase the capacity value to a maximum of 200 uF. To the same purpose the only 12V lamp can be replaced by two 24V lamps of the same type (connected in parallel) It is obvious that modifying the circuit in such a way decreases its protection against distortiont. It is also  advisable to make some trials and to  mount the circuit outside the speaker system. The connetions are made in series with the speaker system, i. e. between the + lead of the amplifier output and the + lead of the speaker system input.
  •  
    • If one so desires, after having checked the proper component values to choose (depending from listening preferences), he can build the device inside the speaker system, glued to the woofer's magnet by thermo-grip glue or by a good adhesive tape, as the above pictures show (note the lamp during the various working stages). In order to gain access to the inside of the speaker system, you can remove its front panel, on which the speakers and the filter are placed.
    It is unnecessary to remove the filter network, it is sufficient to cut the wire connected to the + lead and to the protection circuit (that is unpolarized, so you can choose its input and output).
    After that, a new wire must be connected from the device output to the + lead that was disconnected.
    You can also completely fill the inner of the speaker system with absorbing material (poliurethanic foam or high-density glass wool,  
    This is sufficient for fixing the problem when the distortion amount is low and desultory.
    In this case may I suggest to adopt this last fix first, and after to build the protection device if necessary.
     

    Sergio Canini         10/9/03


     Hello Sergio
     I just found your web site at  http://www.caninialtoparlanti.it/LS3.5A.UK.htm  It has some very interesting information about the LS3/5a. But I notice that you have used two of my photographs without asking
    my   permission. I own the copyright to both these pictures & I have attached them to this email so you can see which ones they are. I am willing to give you permission to use these pictures if you would  kindly add an acknowledgement underneath each of them to say the photographs  belong to Paul Whatton, and if you could also include a link to my web site  at http://www.ls35a.com  If you can do this in return I will be pleased to add a link to your pages  from www.ls35a.com  Best wishes  Paul

     Dear Mr. Paul,
    I beg your pardon for using  your photographs without asking your permission  and I thank you very much for your kindness in giving me it. I'm going to  make a link to your web site in my page and to declare that the photographs began to you  Very truly yours.  Sergio Canini


     Hello Sergio
    Thank you very much. I will add a link to your very interesting web site from www.ls35a.com in the next couple of days. You are very welcome to use the pictures.  My deceased father, Maurice Whatton was one of the BBC designers for the LS3/5a (see http://www.bbc.co.uk/rd/pubs/reports/1976-29.pdf). I noticed you have a discussion on your site about why the BBC chose to design the speaker in-house.  My understanding of the problem they had was that, back in the 1970s, they could not find a commercial off-the-shelf loudspeaker that was consistent
    enough.  It was also thought to be important that a small monitor should have the same general characteristics as the BBC Grade A monitor, the LS5/8. They wanted to be sure that someone listening on the LS3/5a further down the
    broadcast chain would not be tempted to adjust the programme sound balance as a result of listening to a pair of speakers with different characteristics. And the repeatability was important so that someone listening in different studios would hear fundamentally the same sound balance.  So you might argue that the LS3/5a was designed more for consistency than for absolute sound quality?  Best wishes . Paul
     

    Dear Paul:
    First, I thank you very much for your letter, from which I learnt some unknown aspects of LS3/5a. I am also very happy that you have a mind to start a comparison between our opinions on the quality of those loudspeaker systems. Really, I spoke about the quality only in order to put in evidence that bad characterisic to which I put a fix. But you were able to read also under my lines and have noticed that my absence of appreciation  towards LS3/5a could signify a bad sentence on them. Now I considered the matter again and more in deep and I can tell you my opinion. LS3/5a represent probably the best you can do at those times, nevertheless some design aspects (i.e.,  the alignment chosen for bass-response, the adoption of an extrenely complex crossover filter in order to achieve a smooth response), if could be acceptable in those days, are no longer acceptable now. I think that LS3/5a, as well as other speaker systems that audiophiles considered as myths, are outdated now. The modifications that BBC made (and, in particular, the adoption of the unuseful bi-wiring design) posed these speaker systems out of the strictly technical ambit. Personally I ptrefer to stay with my feet strictly on ground. If you can read Italian (or if you have any friend who can do), please read my webpage www.caninialtoparlanti.it/dnc.htm in which you can see my opinions. I should like to know yours. With sympathy and frienship,  Sergio.