BBC LS5/9:

The LS5/9 was designed by the BBC Research Department and manufactured under licence by Rogers. Production started circa 1983, and continued well into the 90's.

It was intended to be used in locations where the much larger LS5/8 would be inappropriate. The BBC Design Report (BBC RD 1983/10) makes interesting reading, and is available from the R&D website.

Drive units:

As one of the aims was to produce a close sound quality match with the LS5/8, they chose the same Audax tweeter - a 34mm fabric dome design fitted with a metal grill for protection and designated LS2/12. The tweeter was available from Maplin, but Maplin have cleared down their stocks of Audax drive units (along with anything else useful!). It was listed as MY22Y and sold for around £25, although it fell to £12.99 before discontinuation.

The metal grill protecting the tweeter dome is a BBC modification, and was apparently intended to support the weight of the whole speaker! Certainly it is thick and well-damped.

The bass unit was the matter of some in-house research, and the Design Report details the experimental work to determine the best cone profile and material. A full range of development techniques appear to have been used, ranging from laser interferometry to subjective listening tests. The resulting design, designated LS2/14, was manufactured by Rogers.

No-doubt Rogers benefited from their close relationship with the BBC. Certainly, the bass driver in the LS7 appears to be very similar, sharing the die-cast chassis and distinctive transparent polypropylene cone.

Observant readers might notice that the woofer appears to be wired out of phase. It isn't - I've rotated the drive unit through 180 degrees to cure a rubbing voicecoil, caused by the cone settling on the suspension over time. The wires from the crossover PCB are different lengths, so rather than replacing them, I simply swapped them at the PCB end, thus maintaining correct phase.

Enclosure:

The enclosure is made using classic thinwall practice - 9mm high-quality birch plywood, critically damped with self-adhesive bitumen pads. This method has largely passed out of favour due to the extra design work and manufacturing costs involved, but it's good to see that Harbeth for one are maintaining the tradition. All internal walls apart from the baffle are lined with Rockwool-type insulation, held in place by black cloth which is stapled to the internal 18mm batons that join the panels. Some examples are fitted with thin transparent plastic instead of cloth.

In traditional BBC fashion, the grill is held on with Velcro, and is very difficult to remove once it has been on for a few years. Access to the inside is via the removable baffle which is secured with machine bolts which connect with metal inserts in the batons.

There are similar inserts in the side panels for attaching the speakers to stands and mounting brackets. These are plugged with brown plastic caps for aesthetic reasons, and sealed internally by the bitumen damping panels. Additionally there are similar holes in the rear panel which are intended to hold an optional BBC amplifier. This is a conventional mono amplifier, and doesn't turn the design into a true active model - and in my time in the BBC I've never actually seen one of these.

None of these fixing holes are visible in the brochure pictures, suggesting that domestic models didn't have them. Also the external tweeter level adjustment tags are also not visible in the later brochure picture - again, I wonder if this was only a professional option?

Crossover:

Like the LS3/5a, the crossover is mounted on the baffle behind the tweeter, and is equally complicated! Interestingly, the original product produced by Research and Development was an active design just like the LS5/8, but this active version never went into production because the passive high-level crossover designed by Maurice Whatton and Trevor Newlin of Designs Department was found to produce results that were just as good. In fact, Trevor recently contacted me and said the following:

I am glad to read about your comments about the LS5/9. this was my major job at the BBC to design this brute.

There were several hick-ups along the way. The crossover was the first designed using SPICE. It was engineered to have a low source impedance over the entire audio range which also gave it a very even input impedance hence making it largely cable immune.

As a result of producing this speaker we heard all the imperfections of the Broadcast amplifiers then in use. My next project then became the design of a broadcast amplifier that had sufficient audio quality so you heard the loudspeaker not the amplifier. This amplifier was subsequently licensed to outside vendors. For domestic amplifiers we found the Quad 405 gave a passable performance.

Early versions of the crossover used radiometal inductors and like the original LS5/3a, tweeter level matching adjustment was done by selecting tappings on an auto-transformer rather than a resistor ladder. Many thanks to Brian Drummond for providing this picture and lots of other details that have made this page more complete. Incidentally, his crossover was home-made - production versions had the usual silkscreen markings.

I've never seen one of these in the flesh, but I've only examined models with later serial numbers. Brian thinks that this version was in production for at least a couple of years. The later design (shown below) was mentioned in the 1985 departmental annual report because it was the first BBC design to use ferrite inductors and was also the first to be designed with the aid of Spice computer simulation. Apparently the success of the design also fed into the work to redesign the LS3/5a crossover that appeared in later 11 ohm models.

I've produced this schematic by reverse-engineering the PCB layout. As is often the case, the inductors aren't marked, so many thanks are due to Marcel Janssen (from the Netherlands) for supplying the inductor values, thus saving me from having to measure them.

The tweeter level is basically matched by an auto-transformer, but note the comprehensive provision for fine level adjustment to account for inconsistencies in the Audax tweeter.

There appears to be a typo with the PCB labelling (the -2 tap that connects to the -1 position on the tagboard), but I'm reasonably sure that that's the value printed on the PCB. Next time I take one apart I'll double-check this point...

For those wondering how it works, it's reasonably straightforward. Taking the path to the woofer first, the inductor and 6u8 capacitor network form a second-order low-pass filter with a 12dB/octave slope. This determines the basic crossover frequency.

There is a frequency-response plot of the drivers driven at constant voltage (ie, no crossover) in the Design Report, and this shows a fall of 5dB or so at frequencies below 500Hz. This is caused by a phenomenon called "baffle step", where the increasing wavelength corresponds with the width of the baffle - have a look here for more information.

This is compensated for by the next inductor and parallel resistors combination. These form a mid-bass filter - essentially at lower frequencies the signal passes straight through the inductor, whereas at higher frequencies the inductor no longer passes the signal, but the resistors do.

Finally, the series combination of 30uF and 5.5Ω compensates for the rising impedance of the voice coil at higher frequencies. This is important if the preceding filter characteristics are to be preserved. Bear in mind that high-level crossovers are essentially just potential dividers where the resistances happen to change with frequency. As the loudspeaker voicecoil is the lower 'arm' of this divider, the ratios will change according to the impedance curve of the drive unit, significantly affecting the midrange response.

The tweeter filtering starts with another second-order filter. The inductor has two roles here - as well as its' inductance forming the filter along with the 4u7 capacitor, it also acts as an auto-transformer. This enables attenuation of the signal without wasting heat in resistors. The 6u8 and 10u capacitors form a further filter, and finally the level-control resistors provide fine adjustment of the tweeter. Note the 3 higher value resistors - these are to ensure that the load impedance seen by the high-pass filter remains constant in the face of differing impedances presented by varying tweeter level adjustments.

Note the phase-reversal of the tweeter. This is standard practice in this sort of situation, and is done to provide the best integration of the two drivers. If you feed in a signal that is close to the crossover point then both drive units will be contributing to the sound you hear. Now, think for a moment about the phase of the units - you'll probably know how bad your hifi sounds when you've accidentally wired your speakers out of phase... It's a similar principle here - any filter causes changes in phase response, and it happens that the sum of all the phase shifts of both filters at the crossover point is near to 180° - hence the phase reversal of the tweeter. If you get this wrong you'll notice two problems - first there will be changes in the amplitude response. Rather than the outputs of the two drivers adding together coherently around the crossover point, you'll get all manner of strange cancellation effects causing a very peaky amplitude response.

Secondly, consider feeding a mono voice to your stereo speakers - you hear the voice midway between them. If you reverse the phase of one of your speakers, the position of the voice will be anywhere but in the middle! Indeed, it will move about the room as you move your head around trying to locate it... There is a similar conflict in a loudspeaker - at the crossover point both drivers will be producing sound, and it needs to be 'in-phase'. If not, the midrange will fail to come from the baffle, and this will destroy the imaging capabilities of the system.

It's easy to look at the small collection of passive components in a typical crossover and assume that it is a straightforward business. Hopefully, this brief description will give you some idea of how involved it really is. And we've barely scratched the surface... Any competent designer will tell you how difficult crossover design can be - and that's with the range of test equipment that I can only dream about!

Sound Quality:

I've found that these speakers produce interesting reactions from people who hear them. Certainly, very few people at work like them until they've heard them properly set up in a decent system. Unfortunately, these 'speakers are much more critical and less forgiving than the LS3/5a's - I've seen LS3/5a's placed on their sides and balanced on shelves - even one sitting on top of a plastic tv set, turned upside down to minimise the effects of the magnetic field on the picture! All of these sound surprisingly acceptable, which is a credit to their basic design...

By contrast, none of our LS5/9's sound good. While some are on stands or mounts, these are designed for convenience (eg casters instead of spikes) or good looks. They are also fed from fairly mediocre amplification that in some cases would be better suited to PA rather than hifi. The first time I heard some LS5/9's in a domestic setting, they were fed from a NAD 3020 and mounted on wall-brackets some 6 feet above the ground, which hardly improved my opinion of them...

I borrowed a spare pair from work back in 2001. Trying them in place of my LS7's, I instantly thought "there's no bass or treble!". This was confusing - after all these are Grade-1 monitoring loudspeakers - surely there's something wrong, either with my expectation of sound, or these speakers...

There was one thing that persuaded me to persevere with them - the midrange. As mentioned on the LS7 page, I was having a problem with midrange. As my taste in music was drifting towards less mainstream material such as jazz and world music, I was becoming more critical of midrange response. For example, everyone's favourite Miles Davis album, Kind of Blue was unbearable on the LS7's at anything other than low volumes - on the LS5/9's the whole album was just fantastic - all of the brass pieces sounded very natural and there was freedom from the colouration that killed the enjoyment of the piece.

After getting used to them, I started hearing things that were completely new to me. For example, I hadn't really noticed that all the hf percussion tended to sound the same irrespective of the recording. This might seem like an odd thing to say, but those "in-the-know" will understand exactly what I mean. I suspect it's because lesser speakers tend to have a peakier response - especially metal-dome tweeters - caused by resonances which can colour the sound considerably. So, the subjective drop in h.f. level is mostly because you are hearing the actual signal, free from these peaks. Although it's true to say that many manufacturers like to maintain a certain 'house-sound', and use this as an excuse to wind the treble response up to disguise these aberrations - this is certainly my experience of loudspeakers from the early 90's - I'd like to think this practice is less-used these days... I remember first hearing the 'clean-treble' effect on some ATC SCM-20's a few years back. At first I thought the very expensive CD player must have had a deliberately 'sweet' top end, but substituting some 'inferior' (£600 Ruarks!) loudspeakers demonstrated the difference.

Similar comments apply to the bass. The ported-box alignment is such that there is absolutely no boom at any frequencies, and you quickly start to appreciate the clean, fast bass that results. The only slight criticism is with extension - for a 28 litre box, you'd expect a cut-off slightly lower than 50Hz, but it isn't too much of a problem with most types of music. While the LS7's went lower, they were not as clean. I've got some old Musical Fidelity MC-1's, which have a very similar cut-off point, despite having half the internal volume... (admittedly, these aren't a great product, but offer a good example of the 'dirty-treble' effect)

The only real downside is the age-old hifi problem - another large proportion of my CD collection is now unplayable! This is the problem with stepping up to another level - these speakers have such a level of resolution that the good recordings really do sound excellent.

Finally, the result of much comparisons: I think that these speakers beat the famous LS3/5a's in just about every aspect of performance, apart from size! The midrange and imaging capabilities of the LS5/9 is easily better, and the overall frequency response is smoother. They are rather more revealing, and I guess that this might cause some people to prefer the slightly coloured view from the LS3/5a's - this is quite understandable given the state of some modern recordings!

Having completed the LS5/8 project and done some comparisons, I think that these mated with a decent subwoofer are pretty much all I could ask for in a system...

LS3/5a | LS5/8 | Chord LS5/8 | LS5/9 | LS7 | dB101 | Others | Service and Spares | Brochures