BBC LS5/8:

The LS5/8 is the flagship of BBC designed monitoring loudspeaker systems, and plenty are still in use in the Beeb. Most people are struck by their sound quality but would never dream of owning them in a domestic environment. These loudspeakers are BIG!

That didn't stop Paul, which will come as no surprise to anyone who knows him! These really are his ideal speakers and he has spent the last 20 years wanting a 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!

Just recently he was able to buy a pair, but without amps. 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 at line-level. This is true bi-amping, as practised by ATC and Harbeth, amongst others.

However, as we already had one working amplifier and a copy of the schematic, making two copies was 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 405's... All Paul had to do was open his chequebook...

Important: The BBC crossover was built under licence, which makes it commercially sensitive. I'm not sure how relevant this is 25 years on, but in the interests of continuing employment with the BBC, I can not disclose any more information than is on this page. Some further information is available from the BBC, but that's your lot from me. Please do not e-mail me asking for the schematic, as I can not oblige at this time!

The AM8/16 Low-level crossover:

For some reason the designs dept preferred to use transistors instead of op-amps. Perhaps the NE5532 wasn't available or well-established at the time... Also, it appears that no-one had heard of Johnson noise, as there are some rather high impedances at various parts of the circuit, especially in the treble section. In fairness, the design brief was for a 'high-level monitor' that would be used in large studios and galleries, with noise floors much higher than the typical domestic setting...

Right at the start I had to decide on the best strategy. The engineer in me was screaming to do almost everything differently - including not using Quad 405's as the amps. Given a free reign I would have used op-amps in the crossover and replaced the input transformer with electronic balancing. However, these aren't my loudspeakers and it wasn't my money. While there was a strong chance of having a better end result, what if Paul didn't like the sound? I basically had to copy the existing amp, and then if there was any question marks with the final result, it wasn't my fault!

This even extended to the PCB layout. This obviously made this part of the project much easier - producing artwork took an afternoon, as the hard work had been done for me already. The original was produced from hand-taped mylar artwork, which looks much nicer than the stark 45° lines of my very basic CAD package...

We had a problem with the aforementioned input rep-coil - the original is a slightly odd one, being very small physically. Given that we couldn't find an equivalent, we had to raid the old stocks to find 3 identical line-level coils, and then adjust the input filters to get the best frequency response. This was the only mod to the design, but I'm reasonably sure it hasn't hurt the fundamental performance in any way - indeed the replacement transformers have a much better frequency response and saturation performance than the original.

Quad 405 Mods:

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 70's, 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 presumably 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 another 405 next to it on the shelf (albeit at a much higher price). I'm not sure what to make of that - you can draw your own conclusion...

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 for reasons that become clear later. Also, 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 - the Class-A amplifier collector load is two 560Ω resistors in series (with bootstrapping). A current source instead would have helped here...

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...

Paul didn't see these picture until the end of the project. He gets a bit screamish about this sort of carnage as he works in management these days.... But, it's the only way to clean the 25 years of someone else's dirt from the heatsinks, and I needed to get well into it to fit the transformer support.

The aluminium bracket was found in my junk store. It came from a Revox 1/4" tape machine of a very similar vintage (for added "restoration-sympathy"!). It is joined to the front extrusion using holes that held the original transformer - this avoids problems with the heatsinks (you'll know what I mean if you ever take one apart). You'll see that I've changed one of the 4mm speaker connections - the yellow one is for the tweeter. 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.

Below is a picture of an amplifier PCB from the Quad. Notice how the hot-running R30 and R31 have discoloured the PCB. This earlier example uses a glass-fibre PCB - interestingly the existing amp (circa 1980) uses a cheaper consumer-grade material.

There's a few unpleasant design-details - for example there's >100V between those two fuses. Don't use in a kitchen! Also, not obvious from the component side, the input and output nodes run parallel to each other in the bottom-right. Note the quality (or lack of) of the components - Ben Duncan refers to them as "TV set resistors"...

I replaced R30/31 with higher-rated components (making sure they were metal film and not wirewound), 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 had to be fitted if you had Quad electrostatics and didn't want them to arc over internally. It works by basically attenuating the op-amp output so that it clips before the amplifier has reached full output - nice! (R6 and C4 form a feedback loop around the op-amp, including R11, so that the overall gain isn't significantly affected by this). The BBC used this facility to protect the tweeter - I measured a limit of 10V RMS (12.5W) with the original AM8/16. It's surprising how little you need for a nominally 100W system... I would've thought that forcing square-waves into the tweeter during tape-spooling isn't such a good idea(*), but I suppose it's better than 100W of hf...

*Actually, when you work out the RMS value of the resulting square wave, you get 25W. And as the voicecoil inductance causes the power to fall with rising frequency, the actual dissipation will be less. So, when you work it out, it's perhaps not as bad as it seems. It's a myth that clipping destroys tweeters.

Here you can see the rebuilt Quad next to the original (complete with BBC Perspex cover to keep clumsy BBC engineers safe). Note that the bass amp (left in the above rear view) has turned around 180° - this is 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 the bass amp generates) to the other end of the amp where it has slightly more space to breath?

The original Quad has had the nasty spring-clip speaker terminals replaced with 4mm jack sockets to match the copy. This standardisation means that all the amps can use the same speaker leads.

The first picture shows the original and cloned crossovers. I definitely think mine are prettier! The broken thing on the original (blue, to the left of the pot) is, or rather was, the bass-boost switch. I dropped the assembly and the switch exploded - it's been hardwired to the flat position. Later, the volume control pots were changed for Bourns conductive plastic models, and the transformer was changed in the original model to match the clones.

The right picture shows how the crossover PCB fits into the amp at the right end, supported by a bracket. I fabricated my copies in 1mm aluminium, which is rigid enough for this application. The original never fitted that well due to some ribs in the casting of the aluminium end panel, so I took a file to it to remove the worst of it. The clearance between this and the bass amplifier is rather too close for comfort in my opinion... (see later)

The end panel of the (much earlier) second-hand amplifier we bought 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 above.

Note also the improved earth wiring. Before stripping the 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. 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 is 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, Pauls can't hear the buzz unless he puts his ear to the drive units...

Having written that, 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 an electrostatic screen to solve this problem, as does my amplifier now!

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

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.

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 because the designers liked them - the 1% accuracy certainly isn't required in most 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 due to 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.

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 have now been removed so that the speaker sits neatly on the top plate.

They completely transformed the sound - I was not prepared for the difference they made. At the time I was using BBC LS5/9's 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 the case - the bass was absolutely correct - only present when it should be, and relatively free from boom or overhang. It also had an interesting affect on anything that wasn't securely bolted down! Suffice to say, he won't need a sub-woofer!

In a dark room, these enormous speakers simply disappear. Instruments and voices hang in the air, and every instrument sounds natural - there are no peaks or resonance's 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 - such subtleties are mostly lost on Paul - he is still revelling in the power and dynamic capabilities of them, but I'm confident that with time he too will realise these strengths...

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 even 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. This is a slow track featuring two superbly-recorded acoustic guitars which makes a superb test track as well as being a beautiful, moving piece of music. Half way through, I cross-faded between the LS5/8's and the LS3/5a's (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 LS5/8's have the required weight and authority that the loud sections demand. "Hey You" is simply stunning - ever wondered why kick-drums are so-called? 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 LS5/8's 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 LS3/5a's. 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 (despite being a DDD recording) and air-conditioning noise. While you can tell this low frequency noise is present on the LS5/9's, when played through the LS5/8's 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 was probably never listened to via full-range loudspeakers...

Another interesting track 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. I first heard this track on a Naim demo disc which was free on Hi-Fi News a few years back - well worth seeking out. Ted Sirota'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 LS5/8's made a good job of it.

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

To be honest, the difference could easily be explained by the different amplification. The Quad 405 is known for a certain "veiled" quality, indeed there are several mods you can do to help improve on this. Similarly, the Musical Fidelity is known for being a bit forward, with good detail and imaging. I think that if I was building this system, I'd consider using the A1 as the treble amplifier, as that is well within the A1's power capabilities.

The Maiden Voyage:

For those who don't know him, it's worth pointing out 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 lots of guests and even a bottle of champagne... Of course, this was a big mistake - performance-anxiety set in and both Quad's 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. 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 rather 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 due to underrating, 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!

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