|Mark's Project Pages/Naked Hi-Fi/Rogers Speakers/Chord LS5/8|
While sourcing the LS5/8 that formed the basis of Paul's system, we learned that some of the last LS5/8 systems made incorporated Chord amplification. But I've never seen one until a system appeared on eBay recently.
By coincidence, while initially drafting this page a colleague 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" LS5/8's 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's, because 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.
I'm assuming you're familiar with the standard BBC LS5/8's, 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.
These pictures show the Chord amps and their LS5/8's. The speakers themselves are entirely standard, apart from the addition of a Neutrik connector on the rear panel.
These next pictures show the amplifiers and their Neutrik interconnects. As you can see, the modified Chord SPM800's have been designated "AM8/20".
Inside the Chords:
These pictures, included in the eBay listing, were tantalising enough. But the amps were what interested me, and lots of questions needed answering, including what crossover PCB's were being used, and was the circuit redesigned in any way?
First, here are two general views of the amplifier, taken from above and below. In both these shots, the rear of the amp is left of the frame.
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.
The audio circuitry is confined to the lower section of the case. And here you can see that the BBC crossover is a standard late-version PCB. For reference, the two different versions are shown here - the early crossover is what we cloned for Paul's LS5/8's. While later versions have some mechanical difference, they contain the same circuit as far as I can determine. The main change is the bass-boost switch. On early versions, a strange 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. These close-up pictures show that the installation of the crossover PCB is somewhat more straightforward:
There are a few small changes that I can spot - for instance the resistor in series with the voltage regulator transistor is a different value, and a 2.5W wire-wound. Standard crossovers use a 1/4 watt metal film, although I normally upgrade this to a 1W device as a matter 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.
Note that a much larger input transformer is used, mounted on the back 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 has been corrected in these models.
While the volume control has been mounted off the PCB, on the rear panel, 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 speakers need is bass-boost!
Finally, these two show the switched-mode power supply. 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).
Note also the large input filters (the common-mode torroidal cores, top of frame), and the black thermistor used to reduce the inrush current. The bridge rectifier is mounted on the side of the chassis, presumably to assist with cooling. There is a small conventional (50Hz) transformer near this, to power the control circuitry. This is good, as all attempts to design-out this component always result in unreliability, as any service engineer will appreciate!
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. I guess these are bipolar, but 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 them. Having said that, I occasionally suffer from excess bandwidth problems, so can I ask you use the OCR'd version of the article in preference to looking at the images.
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. Meanwhile, I am actively seeking consent from the BBC for the permanent inclusion of this article.
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.
©2007 Mark Hennessy
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