~ BBC LS3/5A External Crossover and information ~

As well as reading this article I suggest you download and read this BBC Research Department pdf which describes "from the horses mouth" the design of the "15Ω" version of the LS3/5A Monitoring loudspeaker designed to highlight imperfections in recorded sound and was the first BBC monitoring loudspeaker to be licensed for production outside the BBC Design &Equipment Department (D&ED)

There are so many myths and legends about the LS3/5A and they are only compounded by the fact that many of them have had the wrong drive units fitted as an "upgrade" or repair and the earlier models even if untouched have tended to age "variably" which makes subjective judgement of "THE LS3/5A" very difficult

I still have and regularly use a 15Ω pair made by Goodmans and a later Rogers 11Ω Pair ~ Both sound very good but also distinctly different as anyone who hears them confirms ~ I am even considering buying another pair ~ Either Black KEFs made in 2001 (with old stock units from about 1995) or at least a later pair that will last a while ~ I also considered the Sterling Broadcast V2 which perhaps should be an LS3/5B ? but as lovely as their cabinets are the thin screw on back design was dropped by the BBC and licensed manufactures only made the fully enclosed cabinets for the Beeb

On the left one of my 15Ω BBC LS3/5As made by Goodmans around 1980 ~ Almost all the 15Ω versions of the LS3/5A appear to be "teak" finish

Black Ash - Rosewood etc. are most likely going to be the later 11Ω versions (12Ω as they were known in the BBC D&ED) made after 1988 ~ Any LS3/5A with Bi-wire inputs will be an 11Ω version or something someone has cobbled together.

All 15Ω Versions of the LS3/5A like the one on the left should have Tygan Plastic Grilles which look shiny and are hard and Nylon like ~ The Tygan plastic is hard to glue in place which is probably why it was stapled through the Velcro to the Grille surround ~ The technique was also used for later cloth Grilles

The 15Ω version was also fitted with what has become known as the "White Belly" version of the KEF B110 Driver (see below)

Picture: 15Ω BBC LS3/5A
The only problem I had with the Goodmans LS3/5As was the horrible terminals they used . . .
picture: Goodmans LS3/5A new terminals
picture: Goodmans LS3/5A old terminals

The Goodmans fitted terminals were simply very poor ~ They were just screws and the only way to make a good connection was by fitting crimp lugs to the ends of the speaker wires ~ This was not suitable for my other speakers so I fitted the Goodmans LS3/5As with 4mm posts

In order to maintain the airtight seal required of the design I filled the recess left by the original terminal plate with black resin which looks nice and gives a good seal ~ It also prevents the terminals from rotating when tightened

The small rubber feet shown above allow the cabinet flex on all sides ~ There is NO benefit fitting spikes to any bookshelf speaker ~ Unless you want to ruin you furniture or stands and the speaker enclosure ~ If you have floor standing speakers on carpet then spikes may help raise the bottom above the carpet and let the enclosure flex better but don't kid yourself that better coupling to the floor will improved the bass

Note the screw on the right has two threads ~ The coarse thread near the head is an opposite Left Hand Thread which bites into the front panel and forms a stud ~ These screws are used to hold the B110 and the crossover PCB in place ~ If you try to tighten the B110 from the front in the normal way you will actually loosen it !

The only way to tighten the B110 or PCB mount if these screws are fitted is to remove the front panel tighten the screws anticlockwise slightly and then tighten the nuts ~ Be careful not to over tighten the nuts on the B110 as the front panel can become distorted which may make the air seal worse

LS3/5A panel screws
Goodmans LS3/5A internal crossover
A weak spot of the LS3/5A especially the commercial 11Ω versions is the alignment of the battens can leave gaps at the corners of the the front panel which blow at low frequencies ~ If you do not have an audio oscillator you can make a test CD with low frequencies between 40Hz and say 200Hz to test for "blowing"
The gasket fitted to the front panel is very thin and often does not provide a good seal ~ I have made larger gaskets cut from 1.5mm neoprene or rubber cork as shown above ~ In both cases the gasket is one piece cut from a sheet

You may have read elsewhere about B110 Drivers with "off centre" cones ~ I have never experienced an "off centre" cone but a common problem is for the voice coil to rub because it has hit the back of the magnet and the former becomes folded over— During 1980s I had a pair of 15Ω LS3/5As that I used with LEAK TL50s set to 15Ω ~ The LEAKs were more than capable of making the voice coils hit the "end stop" with a loud "crack" as they were pulled in

This did not cause a problem in normal operation but the voice coil did appear to rub if the cone was pushed on one side by hand but not if moved by applying a d.c. voltage ~ The problem appears mainly with 15Ω versions and there are people "out there" who can remake the voice coils and indeed refurbish the entire unit ~ The cost may be high but having a new correct cone weight B110 with new surround can make a huge difference to a 30 year old LS3/5A

Another problem you may notice if you do the "blowing test" is that the cones of the 15Ω LF drivers (mainly "White Belly" B110s) will resonate and vibrate with a loud buzz at some frequency in the region of 80Hz but will be clear at frequencies either side ~ My speakers above do this and it can be reduced by carefully pressing somewhere on the surround ~ Pressing on the opposite side of the surround makes the vibration worse

I have never noticed the low frequency resonance problem listening to music but I once noticed a terrible shifting and spreading of high frequency notes which was due to C6 (680nF) in one speaker ~ C6 was the type with light green heat shrink sleeving ~ One of the connections had literally melted off so I took the opportunity to change both C6s along with a few other capacitors

There are reports of R4 of the 15Ω LS3/5A overheating or even burning out ~ R4 is in series with C6 and I guess a lot of energy is dissipated in these components in an attempt to control the impedance and the response of the T27

White belly B110 KEF Bass driver
On The right —A "White Belly" KEF B110 LF Driver

The name is derived from the white centre which is where the aditional PVA has collected ~ The LS3/5 (later LS3/5A) was designed around the size of the enclosure and to get the low frequency response correct one of the chosen drive units - the KEF 8Ω B110 was painted with PVA to increase its mass and so lower its resonance to suit the box size and crossover

Note: The PVA does not always look white as in the picture above ~ there are green(ish) and blue(ish) versions and the shade of the colour can even change with humidity or storage conditions ~ The process of PVA Painting was later passed to KEF to do and they produced the LS3/5A LF Driver as their "System Part" 1003 or SP1003

The thought of the gnomes in caves below the BBC D&ED carefully diluting PVA to a "specific" consistency and then painting a "specific" amount onto the cones of KEF speakers to make them resonate at a "specific" frequency required to match the little LS3/5A boxes is very appealing ~ However other materials directed at the task of making a speaker for the small LS3/5A enclosure led to the more controlled ~ less weather dependant and better aging 6Ω version of the KEF B110 ~ The SP1228 shown left

LS3/5A KEF SP1228 B110
As well as making the Drive units KEF were asked by the BBC to make the crossover ~ This was partly political and partly because the SP1228 B110 required and allowed a change of cross over design ~ The new crossover did away with the transformer HF adjustment ~ which was always awkward to set ~ and it allowed bi-wiring of the "New 11Ω" LS3/5A

~ It also allowed me to Modify my 11Ω LS3/5As with a HF level control ~

11Ω Rogers LS3/5A
These 11Ω Rogers LS3/5As were purchased from a local Hi-Fi shop for about half the price they would have been on eBay

Both speakers had the common fault where the front panel seal was blowing ~ In the case shown it is not even fitted correctly ~ As they were a bargain ~ and because I have always wanted to try it ~ I decided that I would fit the crossover outside the enclosure but to do that the drive units needed longer wires

As well as the blown gasket seal there was also some usual air leakage from the "grommet edge" seal and the 4 fixing holes of the B110 driver ~ The grommet edge was glued at the ends and as you can see above I mounted the B110 with flange nuts and fibre washers to fully cover and seal the holes
Another reason I wanted to fit the crossover externally was because the HF of these LS3/5As appeared higher than expected ~ With an external crossover I could adjust the HF without removing and refitting the front panel each time

The B110 LF Driver and T27 Tweeter were wired with 16/0.15mm PTFE wire and a new 1.5mm one piece neoprene gasket was stuck to the front panel giving an excellent seal and good mechanical relationship between the front and sides of the enclosure ~ Unlike Blutak etc. it is neater and the front panel can be easily removed at a later date

LS3/5A T27 tweeter HF driver
Rogers 11Ω LS3/5A inside
Note the T27 Tweeter ~ like the B110 ~ is a BBC D&ED modified part ~ The addition of a perforated cover not only protects the dome but also sets the HF frequency response with the crossover used ~ Uncovered T27s are not a suitable choice for replacement

With the crossover removed there was a bit more space in the enclosure so fitting a slightly thicker piece of foam seemed a good idea ~ As it turned out this packing material gave excellent results ~ Maybe not true to the original spec but somehow better bass transients than before

Fitting a single piece of foam curved into the enclosure was a deliberate attempt the break up any internal reflections ~ I and some others think it works well

Not likely to get this past security nowadays

The crossover taped to the back of the speaker for convenience ~ The adjustment for the HF is a constant impedance attenuator which is under the foam pad in the centre of the PCB

Either the extra internal volume or the new curved foam or both appear to give more LF although the HF still seemed relatively higher than it should be with the attenuator set as it was to its mid position of 0dB ~ See Schematic Diagram of 11Ω LS3/5A

 LS3/5A external cross over experiment
LS3/5A external crossover box The external crossover was certainly a useful feature and the HF level taps were adjusted several times over the period of a year in my listening room the HF could be reduced a db more

If the external crossover was going to stay it had to be fitted a bit neater ~ I settled on a large die-cast box almost same width as the speaker as it would be easier to work in and I could fit a HF level switch and later maybe larger air spaced inductors

The box was painted with black Hammerite which was left to cure while listening tests continued for a few more months ~ Once the paint was cured the box was drilled from inside with a small drill to pilot the holes without damaging the paint finish

Marking the box from the inside and referencing from the inside edges proved both easy and accurate the 2 holes drilled in the base of the box aligned perfectly with the holes in the enclosure left by the original terminals

The backs of the speakers were cleaned and painted and the boxes were fitted using double sided foam tape which was edged with a black marker pen to blend in

LS3/5A fitting crossover box
Rear view LS3/5A external cross over Although the box is nearly 70mm deep the 4mm terminals on the top enable cable entry with little more depth than before and with more positive contact

The switches I had to hand were seven position double pole make before break with very low resistance gold contacts

These switches enable a smooth break free change of the HF while on power ~ The original HF links only allowed for 5 positions in 0.5dB steps i.e. ±1dB so I redesigned the attenuator to have two more steps giving ±1.5dB with the nominal 0dB now 0.5dB lower than it was which in my present room is fine

Close up of the new attenuator components ~ note the additional 7W Vishay resistors for the extra steps ~ The nominal impedance of the later T27s SP1032 is about 6Ω and this is maintained By R13 and C9 as frequency increases

The attenuator was designed to present a constant 6Ω at its input with the load of the T27

Tests both electrically and acoustically confirmed that the steps were close to 0.5dB ~ See Schematic Diagram of this Mod

LS3/5A Crossover with new attenuator
The HF adjustment is often left at my nominal 0dB ~ Although it's a very subtle effect it's a very useful facility

As you can see there is plenty of space to fit larger air cored inductors in future but for now the boxes were packed with some foam blocks and the lids fitted with a neoprene gaskets to prevent them from vibrating

There are some Fantastic claims for some third party external crossovers for the LS3/5A [and other speakers] that have been further exaggerated by certain Hi-Fi journalists ~ Better inductors fitted to my PCBs may give better measured results into 6Ω resistive loads but when the drive units are fitted and the speaker measured acoustically the results will be similar

Beware of outrageous claims like a 50dB reduction in distortion ! ~ The "acoustically" measured "power" distortion products of the LS3/5A are very low and often better than -45dB across the band 50Hz to 15kHz ~ No external cross over will improve the distortion of the LS3/5A which is already determined by the enclosure and drive units and how old they are ~ If you calculate the improvement due to 50dB less distortion generated by the crossover alone it is lower than effects caused by changes in the weather as you listen from day to day

It is most likely reduced hysteresis due to larger air cored inductors and possibly better capacitors that accounts for an improvement in the listening experience obtained with third party external cross overs but this is hard to measure and quantify and even harder to explain away in the pages of Hi-Fi propaganda no matter how big the backhander is

I think the cabinet and the foam inserts have more effect on the LS3/5A performance than the crossover components ~ The BBC originally used thin walled cabinets with removable backs for the LS3/5 but moved away from this design because the radiated sound from the enclosure was high and the excessive flexure was noticed to cause occasional colouration on some programme material

My "rounded" foam inserts appear to improve the dynamic of the LF ~ If you want a different opinion on this there is the JR149 speaker that should offer some further insight into enclosures for the KEF B110 ~ The JR149 is a speaker that would definitely benefit from better crossover components even though the cross over is already external to the cabinet

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