10-May-2012

Landmark loudspeakers



Steve Harris from HiFi News looks back on some key developments, with PMC's Peter Thomas

PROLOG
Electronics have changed the world, and that includes the hi-fi world, out of all recognition in the last 50 years. But what about loudspeakers? They've got bigger and better, or more often smaller and better. They are sleeker and shinier, with some fantastic new materials technology. But when it comes to basic principles, most of the drivers and boxes we listen to could have been designed 30 or more years ago.
And it can be argued that the foundations of modern speaker design were well and truly laid back in the late 1960s and early 1970s. That's the view of one of today's most successful speaker builders, Peter Thomas, owner of PMC.


RADICAL CHANGES
A major factor in those years was the influence of the BBC, but speaker development was also driven by radical changes in technology and in the market. It was, in fact, the period of the great hi-fi boom. Peter and his business partner, the late Adrian Loader, both left senior jobs at the BBC to set up PMC in 1991. So Peter, who'd joined the BBC in 1977 as a trainee engineer, is well placed to appreciate the earlier work of Shorter, Harwood and Hughes. He has also amassed a fascinating collection of vintage speakers, which offer living proof that the best designs from the 1970s can still sound great. 'You realise how few of the early speakers are accurate,' he says, 'They're very differently voiced, all of them. But the best ones still sound stunning, really.


'With some of the big speakers which were often criticised for being a bit slow and ponderous, I think that was the amplifiers, and not the speakers. Because they were producing incredibly low bass, the amplifiers of the time didn't really have any control. Now, when you plug them into one of today's systems, they sound quite tight and in fact quite modern. 'In the 1960s, you'd had valve amps which had no damping factor at all. You had to design the drive unit in the cabinet to have its own damping, so its bass performance was determined solely by the speaker, not by any control coming from the amplifier. Then of course, transistor amps came along. The early ones had a capacitor in the output to get rid of the DC, and that could roll the bottom end off, and also change the damping factor with frequency. Then, with DC-coupled amps, the damping factor went from something like 20 to over 100, and the whole character of the speaker changed again. 'And then you had turntables, with masses of subsonic rumble, so a lot of power was being used up driving the woofers into flapping!'


NEW GENERATION
In earlier years, hi-fi enthusiasts had been mainly classical music lovers, but now there was a new generation who wanted their systems to play pop and rock music. 'I always remember, at a hi-fi exhibition in the early 1970s, going into the Quad roo~i with a Black Sabbath album and asking them to put that on.
They said no! They wouldn't put it on! Of course, up until then, the majority of people who could afford that sort of equipment were into classical. 'But rock music put greater demands on the speaker because you tended to listen louder and also there was an awful lot of compressed bass, which you didn't get in the classical world. So the whole design approach changed. 'I've got three criteria for the speakers I've collected. Either they're classic designs, or they sound amazing. Or they're interesting, they might not be a very good speaker, but use an interesting technique. Not all of them sound great, certainly. Some of them are dire! 'I've got some early moving-coil units from the 1920s and 1930s, and acoustic labyrinth speakers from the 1940s, but the Quad electrostatic is the starting point. 'I wish I could have heard the Quad when it came out in 1957, it must have blown people away. It must have been such a leap. Because even now they sound good. In 1957, moving-coil speakers were pretty crude. I mean, even the BBC LS5/1, which came out around 1960, was not what we would call hi-fi.' So what was the next leap after the Quad?


'Well, there's a little cluster of them really. There's the LS5/5, which is the KEF-built BBC design. When you look back, you realise how far ahead KEF were with their drive units. Most other people's looked like fuzzy-felt drive units, a bit home brewed. Whereas the KEF units always had nice frames and beautifully glued surrounds.


LITTLE LANDMARK
'Then there's the original Goodmans Maxim, which was the first really small speaker, with a little 3in woofer. It was designed by Laurie Fincham, and it was a real landmark. That little driver had a heavy cone but it had a massive magnet to get the sensitivity back. Very compliant, so a nice low resonant frequency. Quite a big surround, actually, so its travel was very, very long. 'This was probably one of the first small drivers that had a very low resonant frequency and large cone excursion, and he tuned it quite low so it gets quite a nice bottom end from a tiny little cabinet.
Amazingly, it uses a paper-cone tweeter, but it works. A very clever design. 'But the next real big one was the Spendor BC1, in 1969.1 always think of it as "LS3/6 stroke BC1" because it was based on Spencer Hughes' work at the Beeb on the LS3/6.' So Spencer Hughes took all that and used his own Bextrene driver? 'And off he went. And of course he added the supertweeter.' Mainly because with a three-way there would be no purchase tax? 'It certainly was a factor. But remember that the BBC only transmitted up to 15kHz, so they weren't really bothered. The Celestion HF1300 tweeter rolls off around 14kHz or 15kHz, but Spencer Hughes put the Coles on to take it up to 20kHz. 'At that time, you could say that some of the KEFs were really good, but in my opinion the only one that came near the Quad was the BC1. What stood out was its sense of three dimensions in its imaging, something that seemed to elude most designers. Even now, if you listened to it and you didn't know, you'd say it was a modern speaker.'


FRAGILE WOOFER
Of course, the BC1 had its drawbacks, notably the inadequate power handling. With a revision to the bass unit, the speaker seemed to lose some of its sonic magic. 'I'm not convinced it was just the change from the Alnico magnet, they changed the construction too. The same thing happened with the LS3/6, which had quite a flimsy woofer. They changed that and wrecked it as well. But that was a very fragile woofer. After a while they'd start rubbing, and you'd have to turn the speaker upside down so that gravity pulled it back into the centre of the gap. You'd have to rotate them every couple of years.' And the Spendor BC3? 'Because the BC1 was criticised for not going very loud, they did this turbo version. It's basically a BC1 with another woofer, a 12in, at the bottom. It was never regarded as well as the BC1. But that was probably down to the amplifiers of the day, because it does sound very'good.'


GALE AND DOVEDALE
Unmistakable among Peter's collection of big veneered boxes, are the unique chrome-ended cabinets of the Gale 401, first made in 1972. Now, I think this is one of those designs that got overlooked as a serious speaker because of its aesthetics. People seem to think that if a product is stylised, it must be rubbish. Gale had it spot-on, because they used two Peerless bass units either side, and put the mid and treble in the middle. They are sealed cabinets, which are small for two woofers, and yet the tuning is absolutely perfect; they really do go very flat down to quite a low frequency. Also from around 1972, if seemingly mundane compared with the Gales, is the Wharfedale Dovedale III. They were very good, and a bargain in their day. The big discounters used to sell them for 60 quid - about ?1 200 now. 'They had that big old 12in bass unit, and a nice midrange, which looks like the original Spendor bass unit. Then they'd added what they called their chemical dome, a plastic tweeter. There's something about the low end - it has that free, relaxed feeling.
A bit later, KEF's 104 was a huge seller, with the passive bass radiator. Then they brought out a new crossover, the AB or Acoustic Butterworth. Which was quite a nice idea, because the crossover was in the front so you just unscrewed it and plugged in the new one. 'It certainly produced a mammoth amount of bass for such a small box. Funnily enough I like the KEF Cadenza, which was really the 1960s version. It was basically the same speaker in a bigger box.'


BEXTRENE QUACK
By the mid-1970s Peter.had been seduced by the possibilities of transmission-line bass loading, thanks to the IMF Professional Monitor [see HFN April '12]. 'I remember hearing these and thinking "Bloody hell! That's incredible, what they've done with that!" The thing I liked about them when I first heard them was not the bass so much, but the mid and top, which was much more neutral.
John Wright of IMF was one of the first people to realise that the KEF B110 had a nasty Bextcene "quack" in it, which was what the BBC spent years trying to sort out with the LS3/5A. The B110s in the IMF have masses of doping all over them. You can still just hear it in the voice, but it's nowhere near like a plain Bextrene B110. Of course, KEF went to polypropylene B110s and that solved a lot of the problems.' Another transmission line, the Cambridge Audio R50, was highly prized in its day, but Peter finds it a less successful design. I don't know what they did to them, but the bass is really overdamped and very, very dry.
They looked nice, but of course that grille ruins the dispersion! If you move your head it all goes quite phasey.' prized in its day, but Peter finds it a less successful design. 'I don't know what they did to them, but the bass is really overdamped and very, very dry. They looked nice, but of course that grille ruins the dispersion! If you move your head it all goes quite phasey.' the concept of the drive units needing to be in a line vertically.
The IMFs have the tweeter and the super tweeter above each other, and all the units are bunched together around the woofer. But I think those complications led people to switch to two-ways, as they're simpler. One thing that did simplify construction and maintenance was a move to fixing drive units to the front of the baffle rather than the back, as had been the norm in 1960s designs. But the Celestion HF1300 tweeter used on so many 1970s speakers was fed in from behind.
Changing one of these on an IMF is a fiddle. 'There's a box behind it and it's held in by one screw facing the back. So you have to take the woofer out, and then reach into the hole and unscrew it, blind. The worst thing is that they're full of fibreglass, so while you're doing it your hand suddenly comes out in red blotches!' Inevitably, Peter's speaker collection is biased towards the high-end models of the 1970s. But he can also wax lyrical about a best buy of the early 1980s, the DM 110. 'When I worked at the Beeb most of the studios had BBC designs, but the offices had ordinary speakers. So every four or five years you would do a cull of all the speakers out there, listen to them and decide what you were going to buy.
We're talking 100 or 120 a pair, back then, for a reasonable pair of speakers. We must have had 20 pairs in and we just went through the lot, until these B&Ws came on and everyone went "My Cod!" We put them up against some BBC monitors, and they were bloody good. 'And the whole thing is what you expect for 100, it's very thin board, it's black vinyl wrap, the drive units are a plastic dome, not even a soft dome, with a cheap paper bass unit. But the whole is greater than the sum of the parts. The tuning of the bass reflex is perfect, it puts to shame, really, a lot of other speakers that cost twice as much.


HIGHER PERFORMANCE
'The biggest thing that's changed with speaker design isn't anything to do with the actual parts,' says Peter. 'It's how they are designed, and measurement using computer systems. That's what's really accelerated speaker design. 'But what has also changed hugely is the quality and consistency of components. It's the same with materials. We still prefer doped paper, but doped paper is much more consistent now as it's all done automatically. There are no little old ladies painting the cones any more! 'And in real terms, you get much, much higher performance for less money. Definitely.


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