Chord LS5/8

While sourcing the LS5/8 loudspeakers that formed the basis of Paul's system, we learned that some of the last LS5/8 systems made used Chord amplification. But until a system appeared on eBay recently, I'd never seen one. And by coincidence, while initially drafting this page a friend found an old copy of ENG INF (an in-house publication for BBC engineering, technical, and operational staff) which contained details of this system. See below for scans of the relevant pages.

Normally, I wouldn't devote a page to a speaker that I've not had the chance to listen to, but these are so rare that they deserve to be documented. And having heard the "standard" LS5/8s so frequently, I feel that I'm almost qualified to write about these. But, please don't email me to ask what I think of the Chord LS5/8 because, again, I haven't heard them!

I'm indebted to Mike at the Audio Toy Shop, whose eBay name is "atoyboy", for allowing me to use the following pictures. And should the new owner of these want to contribute to this page, perhaps by sending me his impressions of them or maybe some more pictures, then I would be very grateful.

Overview

I'm assuming you're familiar with the standard BBC LS5/8, which shoehorned an active crossover PCB into a Quad 405. The Chord versions were produced because of the non-availability of the 405, and were chosen after listening tests detailed in the ENG INF article.

Here is a view of the Chord amps and the LS5/8s.

Front view of LS5/8s and Chord amps (56k)

Apart from the addition of a Neutrik connector on the rear panel, the loudspeakers themselves are entirely standard.

Rear view showing Neutrik and XLR inputs
       (58k)

Here we see the amplifiers and their Neutrik interconnects.

General view of amps (63k)

As you can see, the modified Chord SPM800s have been designated "AM8/20".

Close-up showing BBC designation (63k)

Inside the Chords

These pictures seen so far were included in the eBay listing, and were tantalising enough. But the amps were what interested me, and lots of questions needed answering, including what crossover PCBs were being used, and was the circuit redesigned in any way? Fortunately, Mike was happy to supply some internal shots.

Chord amplifiers are famous for using switched-mode power supplies, and this is no exception. The front half of the amplifier is given over to the power supply, and as you can see, the internal partitions appear to isolate this PSU from the rest of the amplifier. The outputs from the supply are further supported by the 6 electrolytic capacitors you can see in the centre-rear of the chassis.

General internal view of the AM8/20 (73k)

The audio circuitry is confined to the lower section of the case:

General view from beneath (75k)

And here you can see that the BBC crossover is a standard late-version PCB. For reference, the two different versions are shown below - the early crossover (left picture) is what we cloned for Paul's LS5/8s.

Early AM8/16 crossover
Later crossover PCBs

While later versions have some mechanical differences, they contain the same circuitry, as far as I can determine. The main change is the bass-boost switch; on early versions, an unusual 3-position toggle switch is used (blue square, bottom right) - this switch is in fact a 2-pole, 3 position switch (normal 3-position toggle switches have a centre-off position). On later boards, a 3 position slide switch is used, mounted onto the metal bracket that supports the crossover PCB in the Quad 405.

As you can see, the Chord has more internal space than the Quad 405, which makes installation of the crossover PCB somewhat more straightforward. Here we can see the PCB and wiring:

Closer view of the BBC crossover PCB (76k)

The volume control has been mounted off the PCB on the rear panel, and it appears that the bass-boost switch has been abandoned. I assume it's hard-wired to the "flat" position. Very sensible - the last thing these loudspeakers need is bass-boost!

And this close-up of the crossover PCB shows a few small changes:

Close-up of the crossover PCB (69k)

The resistor in series with the voltage regulator transistor is a different value, and a 2.5W vitreous enamel wire-wound device. Standard crossovers use a 1/4 watt metal film here, although I normally upgrade this to 1W as a matter of course. Of course, the Chord uses higher supply rails than the Quad, so this is no surprise. Note the red wire bringing in the +HT from the adjacent PCB that carries the output transistors.

A much larger input transformer is used, mounted on the rear of the PCB. The ENG INF report mentions that the original transformer caused some HF rolloff, which is exactly what I found when cloning the amps for Paul. We used different transformers to overcome this, and I'm glad to see that this is also the case in these models.

Finally, a couple of views of the switched-mode power supply:

Close-up of the SMPS (65k)

Note the large input filters - the two torroidal cores, top of frame, which look as if they might be common-mode chokes - and the black thermistor (next to the 5A DC fuse) used to limit the inrush current. The bridge rectifier is mounted on the side of the chassis for cooling. There is a small conventional (50Hz) transformer nearby to power the control circuitry. This is good, as all attempts to design-out this component usually reduces reliability (just ask any service engineer!).

It's impossible to guess without access to schematic diagrams, but it could be a half-bridge topology judging by the two switching transistors and noting that the collector/emitter junction of these switches are AC-coupled to the transformer via the large orange capacitor. I would imagine that a forward or flyback converter would be inappropriate here because of the large power requirements of the output stage (which is rated at 160 watts per channel into 8 ohms).

Close-up of the SMPS (63k)

The small vertically-mounted PCB contains the control circuitry - note that there are two pulse transformers (beige rectangles, to the left of the main switching transformer) to carry the base drive waveforms across the isolation barrier to the switching transistors. Given the age of the unit, I guess these are bipolar, but it's possible that they could be MOSFETs.

If anyone can shed any more light on these fascinating amplifiers, I'd be very interested.

BBC ENG INF (Winter 1992/3, No 51) report

As mentioned above, discovering this article was total serendipity! I've scanned the relevant pages - click on them to enlarge, or use the the OCR'd version of the article.

Please note: The copyright of this article belongs to the BBC, and while I believe that the inclusion here constitutes fair usage given the historical value, please be aware that it may be removed at any time.

Reading this article, it seems that someone got a real bargain. These fetched £1120 on eBay, and I'm sure that there were worth every penny! As I said above, I would be delighted to hear from the purchaser of these. I would also love to learn more about the Chord SPM800's.

Update: The AM8/21

Amazingly, there was another itteration of this concept - Chord managed to fit a stereo version of the LS5/8 amplifier into one chassis! This was designated AM8/21.

This was announced in the quarterly in-house magazine BBC ENG INF in spring 1993 - I don't have a paper copy to scan, but thanks to the efforts of BBCeng.info, you can view these publications on-line. Look for "STOP PRESS..." on page 2 of Issue 52. Yes, that's just after issue 51, cited above, so Chord weren't exactly resting on their laurels! I have seen so few of these that I can't give comment about which version was more widespread; I'm guessing that these were only ever made in small numbers anyway. Any data would be very welcome.

Of course, this requires the original amplifier to have 4 channels of amplification. Two obvious answers spring to mind: perhaps the original amplifier was 4 channel originally or, more likely perhaps, it was a stereo unit that was bridged - not uncommon at the higher powered end of the market. If anyone knows anything about these, please let me know.

I haven't yet found the emails in my archive, which were kindly submitted for publication a few years back - but as soon as I do, I will provide an appropriate credit for the photographs. In the meantime, if you recognise these, please do get in touch.

The Chord AM8/21 (25k)

I'm no expert on Chord amplifiers, but this looks like one of their professional "SPA" series units.

Rear view of the Chord AM8/21 (33k)

This rear view shows the XLR inputs and the Neutrik loudspeaker outputs. There are blanking plugs are covering holes left by phono sockets and conventional 4mm binding posts.

Internal view of the Chord AM8/21 (69k)

This internal picture shows relatively little of the "host" amplifier, but look at the crossover PCB. It's a new design - both channels on the same PCB. Let's take a closer look...

Close-up of the crossover PCB mounted in the
      Chord AM8/21 (95k)

And what do we see? Op-amps! This is a completely new design. I wonder how this came about, and I wonder what involvement the BBC had with this, if any? Absolutely fascinating...

Sadly, like the AM8/20, I have never seen one of these in the flesh, let alone heard one, but if anyone can offer any more information, I'd be delighted to hear from you.