LS5/8

Naked LS5/8 and AM8/16 on homemade
     stand...The LS5/8 is the flagship of the BBC designed monitoring loudspeaker systems and you can occasionally find an odd pair still in use in the Beeb, some 35+ years after their inception. Most people are struck by their sound quality but few would contemplate using them in a domestic environment. These loudspeakers are BIG!

That didn't stop my good friend Paul, which will come as no surprise to anyone who knows him! Since first hearing a pair in TV Centre back in the 1980s, he has always wanted his own pair. He managed to find a single one some years back which made a rather effective center speaker, despite not actually being able to place it within 6 feet of his TV set! But when he finally did get the opportunity to buy a pair, they were offered without the matching amplifiers. Now, this is a problem. The LS5/8 is an active design - the bass and treble drive units are fed by the left and right channels of a modified Quad 405 which incorporates a BBC-designed assembly to perform the frequency splitting and equalisationat line level. This amplifier has the BBC designation AM8/16, and is an essential component of the LS5/8 system.

However, as we already had one working amplifier and a copy of the schematic, making AM8/16 clones seemed like a relatively easy matter, in theory at least. I foolishly offered to build a pair of crossover PCBs and fit them to second-hand Quad 405s... All Paul had to do was open his chequebook...

Background to the LS5/8

A comprehensive report on the LS5/8 prototype can be downloaded from the BBC Research and Development website: BBC RD 1979/22. I'll summarise it here:

The BBC required a new loudspeaker that could be used for monitoring music at high levels. During the 1970s, the trend was for ever louder levels and the BBC's existing larger designs (such as the LS5/5) were unable to play at the required loudness.

One of the problems with existing designs was the use of Bextrene as a cone material. With appropriate doping (always done by hand) it was capable of good results, but it was relatively heavy, which clearly affected the sensitivity of the drive unit. To achieve the required sound pressures, more power was required, and this lead to problems with distortion and failures.

Dudley Harwood proposed using polypropylene as a diaphragm material. Polypropylene is lighter than Bextrene, and has better self-damping properties, meaning that no additional damping is required.

Swisstone (Rogers) introduced a 12 inch bass driver with a transparent polypropylene cone - over the mists of time, it's not clear exactly how much input from the BBC was received during the development of this, but it seems highly likely that Harwood's research was influential - perhaps essential - in the design. Whatever the exact background, this unit went on to be the heart of the LS5/8. According to the Design Report, this driver was 4dB more sensitive than their previous Bextrene-coned 12" unit, and this, in combination with high temperature materials in the voice coil, permitted much higher sound pressure levels to be reached with relative freedom from colouration and distortion.

The BBC was keen to make the loudspeaker active, and if possible, 2-way. This meant that a conventional stereo amplifier could be modified; the left and right channels would drive the woofer and tweeter separately. This woofer was reasonably well behaved up to 2 or 3kHz, and so a search for a suitable tweeter began. A well known 1" dome unit from Audax was considered, but it was found to be unreliable. The problem was solved when Audax introduced a 34mm dome tweeter which was able to handle a higher power level, and was more sensitive. This tweeter wasn't quite as good as the 25mm variant, but was considered to be acceptable.

The low-level crossover was, as already mentioned, fitted to a Quad 405. One advantage of this amplifier was the option to limit the output voltage - originally done for use with Quad electrostatic loudspeakers. This facility was used to limit the voltage delivered to the tweeter.

The rest of the design followed well-established BBC practice; "thinwall" cabinets made from birch plywood and damped with bitumen pads. The baffle is removeable once the grille has been removed, but of course this is often easier said than done as the Velcro grips with amazing tenacity. These are further secured with screws in the bottom and dowels in the top, but if you don't know this, you assume that the resistance is coming from just the Velcro, and can end up damaging the cabinet. There is an auto-transformer in the cabinet for matching the sensitivity of the tweeter to the agreed standard so that amplifiers and loudspeakers are interchangeable. Carrying handles are incorporated in the side panels, and special stands were produced with a shelf to hold the amplifier.

The low-level crossover

For some reason R&D preferred to use transistors instead of op-amps. This is curious because the earlier LS3/7 used op-amps, although this was a Designs Department design. Right at the start I had to decide on the best strategy: as an engineer, I wanted to do almost everything differently - given a free reign I would have used modern op-amps in the crossover and replaced the input transformer with electronic balancing. However, these weren't my loudspeakers and it wasn't my money. While there was a chance of a better end result, what if Paul didn't like the sound? The only option was to copy the existing amplifier, and then if there were any question marks over the final result, it wasn't my fault!

We had a problem sourcing the aforementioned input transformer - the original is very small physically. Given that we couldn't find an equivalent, we had to raid the old stocks to find 3 identical line-level transformers, and adjust the input filters to get the best frequency response. This was the only modification to the design, but I'm sure it hasn't hurt the fundamental performance in any way - indeed the replacement transformers have a much better frequency response (especially at the high end) and saturation performance than the original.

Quad 405 restoration

The Quad 405 is an interesting amplifier, not least for the current-dumping output stage. Unfortunately, this innovative feature is rather let down by the supporting cast - especially the input op-amp stage. There is plenty of info about this on the web so there's no point going over it here. I suppose that in the late '70s, there probably wasn't much better commercially at that price point. Incidentally, the last LS5/8s were made with very expensive Chord amplification, which shows how highly the original concept was regarded...

For now Paul decided to just buy one Quad 405 - the example we found came from Affordable Audio in Northampton. Unfortunately, the amplifier wasn't quite as advertised - someone had fitted a torroidal transformer as the original transformer had failed at some point in the past. This was the first thing we noticed - the unmodified Quad weighs twice as much - when we picked it up off the shelf we nearly launched it into the ceiling! The dealer assured us that he'd not realised it had been modified, but I can't see how you could fail to notice. Especially as there was an unmodified 405 next to it on the shelf (albeit at a much higher price).

Several aspects of the mod were done badly - for a start the transformer is only rated at 220VA. Luckily, that isn't such an issue for us in this active setup because the tweeter output is limited. But, the secondary windings give 40V rather than 35V, which means the supply rails idle at ±58V instead of ±50V. This results in much increased heat dissipation, especially as the Class-A amplifier collector load is two 560Ω resistors in series (with bootstrapping) rather than a current source... Finally, and this was a big problem, the replacement transformer was bolted to the bottom panel. If you have ever worked on one of these amplifiers you'll know how dumb this is. This was the first thing to fix...

The easiest way to clean the 25 years of someone else's dirt from the heatsinks and prepare the way for restoration is a full teardown:

Exploded view of a Quad 405 (62k)

An aluminium bracket was found in my junk store. It came from a ¼" tape machine of a very similar vintage (for added "restoration-sympathy"!). The bolts joining the bracket to the front extrusion use the holes that originally secured the transformer to avoid interference problems with the rear of the heatsinks (you'll see what I mean if you ever take one apart).

Installation of transformer
       bracket (70k)

I've changed the original spring-clip speaker connections to 4mm sockets and made the tweeter output yellow - the original BBC amplifier uses a 5 pin XLR socket, but there isn't really space for it on the back panel of a standard 405, and they are relatively expensive items.

Electronic restoration

This view of one of the PCBs from the Quad shows how the hot-running R30 and R31 have discoloured the PCB. This earlier example uses a glass-fibre PCB - the existing amp (circa 1980) uses a cheaper consumer-grade material.

Quad 405 amplifier PCB (88k)

There are a few worrying design details - for example there's >100V between those two fuses. Don't use in a kitchen! I bent the fuse clips away from each other, just in case... Also, not obvious from the component side, the input and output nodes run parallel to each other in the bottom-right - luckily the amplifier is an inverting design, but converting it to non-inverting is a common tweak. Note the quality (or lack of) of some of the components - Ben Duncan refers to them as "TV set resistors" (see page 241 of his book - ISBN 0 7506 2629 1).

I replaced R30/31 with higher-rated components, and spaced them further from the PCB using longer ceramic supports... You'll notice the space for R11. This is a voltage-limiting resistor, and has to be fitted to prevent Quad electrostatics arcing over internally. It works by attenuating the op-amp output so that it clips before the amplifier has reached full output. R6 and C4 form a feedback loop around the op-amp, including R11, so the overall gain isn't affected by this. The BBC used this facility to protect the tweeter; I measured a limit of 10V RMS (12.5W into 8Ω) with the original AM8/16 - it's perhaps surprising how little the tweeter needs in a nominally 100W system... And you might think that forcing square-waves into the tweeter during tape-spooling isn't such a good idea, but the peak power should be limited to less than 25 watts, which is certainly better than >>100W. Besides, clipping is not the only factor in tweeter damage (see here and here, for more background).

This is the rebuilt Quad next to the original (complete with BBC Perspex cover to satisy "Health and Safety"). Like the clone, this original AM8/16 has had its nasty spring-clip speaker terminals replaced with 4mm jack sockets, meaning that all the amps can use the same speaker leads. Note that the bass amp (left in the above rear view) has been turned around 180° to make space for the low-level crossover. Talking of which, I find it surprising that this is the bass amp - why not move that function (with all the extra heat it generates) to the other end of the amplifier where it has slightly more space to breath?

Original next to the clone (59k)

Here are the original and cloned crossovers. Things aren't quite finished yet; all the volume control potentiometers were changed for a Bourns conductive plastic model, and the input transformer was changed in the original unit to match the clones. The broken thing on the original (blue square, to the left of the pot) is, or rather was, the bass-boost switch. I managed to drop the assembly and the switch exploded - it's been hardwired to the flat position. The bracket is attached by a pair of M3 screws, which conveniently use the two holes that secure the input DIN socket. I've simplified things slightly by removing the PO316 jack socket (which is wired in parallel with the XLR socket) from the original, having decided to not fit them to the clones. They take a lot of extra space and sit dangerously close to the bass amp PCB.

Original and cloned AM8/16 PCBs (121k)

This is how the crossover PCB fits into the amplifier. The bracket in the original is painted steel, but I fabricated my copies in 1mm aluminium, which is perfectly fine for this application. The original never fitted that well because some ribs in the casting of the aluminium end panel interferred, so I used a file to it to remove the worst of it. The clearance between this and the bass amplifier is still rather too close for comfort in my opinion (indeed, see later)... The end panel of the (much earlier) second-hand amplifier was made from some sort of glass-fibre composite material, which made it much easier to work... Just as well, as I had to remove quite a lot of material to get a good fit as the casting pattern is rather different. This is clearly visible top-right below.

Crossover assembly fitted in the
     Quad 405 (96k)

Hum...

Note also the improved earth wiring. Before stripping the second-hand Quad I noticed quite a lot of hum (and a 70mV DC offset) on the outputs. After replacing electrolytics and R30/31, and attending to some dry joints, this was much improved. Altering the earth wiring, as shown above, made an incremental difference. But some buzz was still apparent at very low level, and I spent two days chasing it, and the only cure was another mains transformer! This was most odd - indeed that was absolutely a last-resort test as I'd tried literally everything else. Unfortunately, my transformer was only 100VA, else it would have stayed in there. Luckily, Paul can't hear the buzz unless he puts his ear to the drive units...

Update

I've since come across the same effect when trouble-shooting an amplifier I was building. This time I wasn't under the same pressures (project deadlines, someone else's money, etc), so was able to think more clearly about it. Once I realised what was happening, I kicked myself!

Basically, torroidal transformers have a high leakage capacitance between primary and secondary windings, which means hum currents can be established in the ground or power supply system of the amplifier being powered. The effect isn't always noticed - the specific details of the amplifier concerned make a big difference, but with hindsight I'm sure it was the cause here. The original Quad transformer has less leakage current. To fix the problem in a torroidal transformer, an electrostatic screen normally does the trick!

Final mod...

Now, I wouldn't like to suggest that our customer is impressed by superficial details but, this was a real hit! I know that Quad purists will rightly be up in arms about it, but bear in mind this is no ordinary 405, and we felt it needed a visual cue to make that point...

Blue LED (25K)

LED technology has clearly moved on since 1976, so you'll definitely need to increase the resistor value. The original red LED is fed with 20mA, whereas this blue LED is being given only 8mA and it is still too bright.

Summary

An almost artistic shot of
     the front panel (16K)The process took a surprising amount of time and effort, as there were lots of details to attend to. We had to think carefully about component choice in the crossovers, as every capacitor below 10nF is an (expensive) axial polystyrene. I get the feeling that these were used simply because the designers liked them - the 1% accuracy certainly isn't required in many cases. By sticking to 1% resistors and using polystyrene capacitors in the critical parts of the circuit we've emulated the response of the original rather well.

Once built, I was able to do a direct comparison between the new boards and the original by feeding the same signal into all three and comparing the outputs on my DM63 'scope, which has 4 input channels. As the input frequency is changed, all three traces stay perfectly superimposed - any errors were less than the thickness of the 'scope trace. This test was originally done on the bare boards minus the input rep-coils, but was repeated once the whole amplifier had been assembled, and I'm pleased to report that the results remained as good.

Listening tests

The first listening tests were done before Paul had built the stands. I have to report that this was a real disappointment, and I had a hard job hiding this from him. Stood on the floor, the bass was massively overblown and very muddy, and the midrange was just a blur. My room is rather too small for these speakers, and I wondered if the results were at least partly because of that. Obviously, I realised that the stands would help, but as they are only a foot high, I wondered how much impact they would have.

MDF speaker stand (20K)During the next day, Paul assembled them for me to try. They are made from 18mm MDF (double-layer for the bottom), 16mm threaded rod and highly polished scaffold tubes! As you can hopefully see from the pictures, these look much better than you might think...

The threaded rods have a spike on the top that was precisely turned on a lathe - this connects to the bottom of the speaker. You can see the hardwood baton on the bottom of the cabinet - these were originally fitted to locate the speakers on the BBC designed Speedframe stands, and have now been removed so that the speaker sits neatly on the top plate. We originally planned to add spikes to the bottom plate, near the corners, but found in practice that these weren't necessary as the ends of the threaded rod made good contact with the floor.

These simple stands completely transformed the sound - I was simply not prepared for the difference they made. At the time I was using the BBC LS5/9s in my normal system - the BBC design report for these stated that this smaller speaker was subjectively very similar to the LS5/8, but with less bass extension. This was now nearly the case - compared to before without the stands, the bass was much better balanced and relatively free from boom or overhang.

In my lounge, these enormous speakers worked very well indeed. They were able to disappear, allowing instruments and voices to hang in the air very naturally - there were no obvious colourations that would otherwise cause certain instruments to attract more attention than they should. I was genuinely surprised and delighted by the detail and transparency of this monolithic system.

BBC family (20K)The most interesting part for me was to compare the BBC family. All three of them have strengths and weaknesses - some of which are relatively obvious: the LS3/5a will clearly not have the bass extension or dynamic range of the LS5/8 or the LS5/9, but given the right conditions it's surprising what they can do. I played Paul a track from a Naim sampler CD - "Snow" by Acoustic Mania, taken from a highly recommended album called "Talking Hands". This is a slow track featuring two superbly-recorded acoustic guitars which is a superb test track as well as being a beautiful, moving piece of music. Half way through, I cross-faded between the LS5/8s and the LS3/5As (driven by my Musical Fidelity A1). Paul, understandably, refused to believe it!

With something more 'full-range', such as The Wall, the differences are rather more stark. The LS5/8s have the required weight and authority that the loud sections demand. "Hey You" is simply stunning, and the obvious test sequence from this album ("Another Brick In The Wall" - Parts 1 through to 2) has never sounded to powerful - I've heard it this loud before, but normally the school choir and guitar solos are painful at those levels and the stereo image collapses - through the LS5/8s everything sounded simply right at any level...

Not my usual thing, but the album-version of Clint Eastwood (Gorillaz) is fun. Unbeknown to the majority of its intended audience, there is an amusing sub-50Hz bassline that simply isn't there with LS3/5As. Viewing it on an oscilloscope reveals that some of the bass notes are almost perfect sine waves! Suffice to say, it's well worth a listen, if only to watch the reaction of people who have not heard it properly before...

A recording of the Vivaldi Concerto for 2 Mandolins, Strings and Basso continuo in G major was interesting (Deutsche Grammophon 415 487-2). The Andante is covered with hiss and air-conditioning noise. While you can tell this low frequency noise is present on the LS5/9s, when played through the LS5/8s you can almost feel the air pressure in the room being modulated! It would have been very easy to high-pass filter this in the control room without affecting the sound of the two mandolins, but one can only assume that this recording wasn't monitored via full-range loudspeakers...

Another gem from the Naim sampler mentioned above is "Geronimo's Free", by Ted Sirota's Rebel Souls. This is a busy track featuring a full range of jazz instruments over a Ska rhythm, which borders on avant-garde. It is quite unlike anything that I have ever heard before, and I have to say that I am hooked! Check out Ted Sirota's website for details. Ted's percussion is fantastic - I defy anyone to listen to it and not tap their feet!

Whatever you might think of the music, on a good system this track sounds fantastic - all of the instruments are well separated and hang in the room in front of you in a quite breathtaking manner. It has now become one of my standard 'test-tracks', and the LS5/8s made a good job of it.

However, in the final analysis, the LS5/9s driven by my A1 won the day, by a very small margin. Until I heard the 5/8s, I believed the 5/9s to be one of the smoothest loudspeakers available but the 5/8s were even smoother. To a fault, I feel. The 5/9s showed more life, more detail and sharper better imaging, while still having the ability to 'disappear'. But these differences are relatively small, and I feel that BBC R&D were largely justified in their "5/8s with less bass" claim...

The maiden voyage

For those who don't know him, it's worth explaining that Paul is a rather excitable character. Like a child at Christmas, he couldn't wait to get these speakers home. And who can blame him? So, he arranged a big Saturday evening event at his house, complete with champagne... Of course, this was a big mistake - performance-anxiety set in and both Quads blew up!

We rigged both speakers up in the designated, less-than-ideal position, and got them playing. While they provided "Shrek" for his young children, I assembled a phono-to-XLR lead for him, allowing us to dispense with the assortment of leads and adapters currently in use. The lead was carefully threaded behind his TV/AV stack, and before disconnecting signal connections, Paul pulled the power on the first Quad (the original BBC AM8/16). He was rewarded with a large bang! I noticed that the power LED went out instantly instead of fading out gently - that was a fairly strong indicator of major trouble inside...

Five minutes later the house was full of guests who were greatly amused by all this. We were relegated to mono for the evening - the "cloned" amp continued to work well while we ate and drank, and drank and drank. Later, Paul was demonstrating the difference between the LS5/8 and his existing Mission loudspeakers, but for reasons best known to himself he was muting the LS5/8 by pulling the mains out. After a few rounds of this, and with absolutely perfect comic timing, the 405 blew its mains fuse!

By now, the guests were well-oiled and highly amused by this! Normally a mains fuse blowing is a pretty mundane event, but that's the power of alcohol for you! Unfortunately for me, I was feeling the depressive effects, and spent the evening worrying about how I was going to source output transistors on a Sunday...

It was a happy ending, thankfully. The blown mains fuse was underrated, and the first amp only required two output devices and a few passive components (and PCB track repair!). We're not sure exactly what happened, but post-mortem findings suggest that +50V got onto the signal earth of the treble amp, which could have been caused by the tight clearances inside the amplifier. As mentioned above, I took the time to improve this by hacking away at the aluminium casting of the end panel, and during this repair I also redressed the signal connections because by default, the signal wires aren't insulated, despite being very close to the cans of the output devices. This is the short version of the story - it took a whole day to do the repair because of the care needed when working with high power audio amplifiers - needless to say the bench power supplies and variac earned their keep on that day!

Update - January 2010

Not long after posting this back in 2002, I moved to a different area where there are many sets of LS5/8s still in use, amongst many others from manufacturers like ATC, PMC, Dynaudio, Genelec, etc. I felt that it might be appropriate to update this page with my current thinking about these speakers after all this time.

I'm still amazed at how good these continue to sound some 30 years after their inception, but naturally with extended exposure you will begin to notice the flaws in anything. For example, the tonal balance: they are undeniably bass-heavy. The LS5/8 was initially praised for having well-controlled bass, but over the years the opinion of Group 2 studio managers and some television sound supervisors changed sufficiently to warrant a brief investigation to investigate whether there was a change in the behaviour of the LS5/8 itself, as opposed to staff expectations in light of experiences with other commercial monitors.

The results of the investigation were reported in "BBC Research Department Technical Memorandum No. S-1149, Low-Frequency Performance Of LS5/8 Monitoring Loudspeaker", by C. D. Mathers, published on 6 November 1992. Unfortunately, because it's document that is internal to BBC R&D, I am unable to publish any part of it, not even after 18 years.

Two effects were identified: first, the port tuning, which could be altered slightly to achieve a flatter response and reduced group delay at the box/port tuning frequency of 50Hz, but it was felt at the time that the effects here were probably inaudible. But the thinking around these issues has changed over the years and I speculate that designers today perhaps wouldn't dismiss this effect quite so quickly.

Secondly, a region of bass boost was identified between 50Hz and 250Hz of about 4dB. There is a rapid slope down between 250Hz and 300Hz, followed by a relatively flat section between 400Hz and 900Hz (then things become somewhat ragged from 1KHz through the crossover region - also there is a 2dB peak at 50Hz). Apparently, this broad bass boost was present in earlier examples, but not at a level that was deemed significant enough to alter the crossover. At this stage, it wasn't possible to determine the cause of this, and I'm not yet aware of any further investigation into this.

Noticing that 250Hz is the baffle step frequency for an 18" baffle made me wonder if they'd "overcooked" the baffle step compensation in the crossover? Delving deeper, when you look at the raw driver responses posted in BBC RD 1979/22, you see that when mounted in the inclosure, the LS5/8 woofer has an rising axial response above about 250Hz that continues at roughly the same rate as the slope caused by the "baffle step", and you can see that they were able to compensate for both these features with a single filter. However, if later production woofers had a different axial response, the filter could cause the characteristic bass-boost that we see today. Certainly, I have seen at least 3 different sets of component values used in this part of the low-level crossover.

Another factor concerns the path the design would have taken from R&D to finished product. The BBC had a department called Designs Department, who would take a prototype from R&D and turn it into a product that can be successfully manufactured in volume. It is entirely possible that modifications to the basic design would have been made, and although you'd think that R&D would have been involved in validating these changes, do remember that Dudley Harwood retired at from the BBC at this point, so perhaps no-one left there knew and understood the design well enough?

Whatever the cause, this post on the Harbeth User Group is especially interesting; the last sentence of the third paragraph sums it all up succinctly - Alan Shaw independently has observed similar behaviour to that identified by C. D. Mathers, but this large area of bass boost came as a complete surprise to Dudley Harwood who had intended the loudspeaker to have a flat frequency response.

Quite how this bass boost appeared and just how much it changed over the years is liable to remain a mystery. But the key message for people who might still be using them today is to remember that they aren't a tonally accurate loudspeaker, and if you fail to account for the bass excess when producing a mix, you'll produce bass-light recordings. Also bear in mind that they were meant to be flat in "free room", or anechoic conditions, so in a typical room they will tend to be bass-heavy before you begin to consider the extra 4dB discussed above!

Away from the bass end, I've noticed that the pair matching isn't very good. Given how carefully they were originally produced, this is almost certainly an aging issue - although I haven't been able to ascertain whether the problem lies with the amplifiers or the drive units. This manifests itself as an issue with the stereo imaging; most obvious when you hit the "mono" button and fail to get a well-defined narrow phantom image dead-centre between the speakers.

In terms of detail and transparency, as stated above these aren't quite as good as an LS5/9, and my ATC SCM20SL's are much better again. While the LS5/8 makes a very pleasant sound, there are better options for critical work.

That said, I find that an amplifier service helps to clean things up. As even the latest amplifiers will be at least 20 years old, they will be suffering from worn out electrolytic capacitors. This can cause a variety of symptoms, the most common being an intrusive hum or an inability to maintain the stereo soundstage when being played at high levels. Luckily, it's not difficult or expensive for a qualified engineer to replace the capacitors. At the same time I would check the amplifier is performing correctly by measuring against the original acceptence criteria, but obviously not every engineer would have access to this. Following a soak test, a final listening test is a useful final check before putting them back in service. And aside from these obvious things, I would recommend carefully checking the wiring in the region of the crossover PCB to avoid a repeat of the fireworks we experienced above.

OB Variation

As well as the Chord LS5/8, a special version for OB use was produced called the LS5/8AL (or R). These were fitted to the Radio OB fleet that, at the time of writing, is just nearing the end of its life, and I have seen (and heard) them in action on a few occasions. Not only were the cabinets shaped appropriately (baffles face 30° in), but an additional equaliser card was designed to tame the response in the confined environment of a small OB van. The examples I've heard are surprisingly good - no-doubt the different cabinet with fewer parallel surfaces and different bass tuning help, plus the vehicles themselves were fitted with comprehensive acoustic treatment.

Picture coming soon...