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The Celestion SL-600, first of the luxury compact monitors

Earlier, I wrote about the ProAc Tablette mini-monitor. This was a tiny speaker that used generally tried and true technology to produce a fine miniature loudspeaker. A bit later, Celestion came up with a completely different approach to the small monitor, the SL-600. The difference was the technology that was brought to bear. The acquisition of Laser Interferometry equipment allowed Celestion’s designers to examine the action of drivers and what was discovered was that most speaker drivers operated in “break-up mode” through significant parts of their ranges where different parts of the diaphragm were moving independently rather the ideal of “pistonic action”, that is, the driver moving as one piece.

This resulted in development of all new drivers for the SL-600 (and its little brother, the SL-6). The woofer looked fairly conventional, but it did benefit from the new measurement technology. But the really obvious thing was the tweeter. It sported a copper (!) dome and did indeed act much more pistonic than typical domes of the day, at least up to about 19 Khz, where its fundamental resonance resulted in a large peak. In practice, a notch filter was introduced into the crossover to remove this. The copper dome also had another effect. Since it was relatively heavy, it lowered the sensitivity of the tweeter. Enough that, had the woofer sensitivity been lowered to match, the system would have been almost impractically inefficient. So what was finally accepted was a tweeter that was around 2 db down from the level of the woofer. Keep this in mind for later.

For the premium SL-600, there were more tricks up Celestion’s sleeve, mainly involving the cabinet. It was always known that resonances in speaker cabinets had an effect on the sound, but around this time speaker designers started to seriously attack this problem. Within a few years, the Wilson WATT would come out which used methacrylic material to deaden the cabinets and cabinets generally became thicker, heavier and better braced.

Well, Celestion took a different tack. Unlike the SL-6, which employed a more normal MDF cabinet, they decided to design a LIGHTER yet still stiff cabinet using a metal honeycomb material called Aerolam, commonly used in airplanes. The idea was that the material would not store and re-release energy and what resonance they had would be high enough in frequency to be out of the most critical range. They also employed mounting plates to attach the drivers to the front baffle, further stiffening the cabinet. These plates were serrated vertically with small channels to break up sound diffusion off their surface. All in all, it must be said that they started this design off with a clean sheet of paper. So how does this speaker sound today and how successful was its design in retrospect?

One thing the SL-600 was always celebrated for was its disappearing act and disappear it does. The image is vertically challenged as are most small speakers (though not as much as the original ProAc Tablettes), but, beyond that, the images are quite holographic. Mainly, I suspect, because the boxes radiate almost no sound from themselves to confuse the imaging issue. These cabinets also do not have serious resonances that color the sound of the drivers in tonal terms, either. And this results in a clarity through the midrange that is good even by current standards.

But there is another factor, controversial at the time, less so today, that factors into the sound of this speaker. That speakers should not measure dead flat in the treble is a commonly held view today, but back in the ’80’s this was not so much the case. Remember the less sensitive tweeter we talked about earlier? Well this resulted in a response tilted down somewhat from the mids into the treble. So you had a speaker that sounded less bright and more velvety than the norm of the day due to this tilt (and truthfully, by the lack of break-up artifacts in the Celestion tweeter that most tweeters added that contributed spurious treble energy to most conventional speakers). Many listeners of the day rejected the SL-600 for this characteristic, feeling they were slow and dark. Ironically, that “problem” with the tweeter resulted in a speaker more in keeping (in my view) with a natural tonal balance on the largest variety of recordings.

Of course, there are problems here, the worst of which involves the bass/midbass. It’s extended enough for the cabinet size, but sounds a bit vague, as though the woofer runs out of low-distortion steam as you go into the mid-bass. This has the effect of making a discontinuity in the sound, instruments whose fundamentals are higher sound very clear while the lower voiced instruments can sound less so. This is not a world-shattering effect, but it’s there and the louder you play the SL-600, the more this effect can be noted.

Due to the small size of the drivers and the cabinet, dynamics are good rather than great. Here it’s hard to single out the SL-600, most small speakers suffer from this. But some newer designs its size do somewhat better in this area. There is also a bit of a sense of hollowness to the sound, I suspect from the residual cabinet resonances. This is not a severe effect, fortunately. And there is not quite the silken beauty of the best of today’s drivers here.

The intent of the SL-600 at the time was a no-holds-barred assault on the ideal small monitor. It retailed for a high price for its time, but, especially in the context of the mid ’80’s, it was pretty much a successful design. And if you aren’t too troubled by its modest failings, this is still excellent performance in many ways for a small monitor speaker even by today’s standards. Of course, if you are willing to go for the throat, you can certainly outdo the SL-600, but not for near their price on the current used market…


The littlest Pro Ac, the original Tablette or how much bass is enough?

We take for granted the number and variety of small monitors available in the hi-end market today, but it was not always so. Back in “The Day”, small speakers also had to be cheap (who would pay big bucks for a small speaker, the logic went), so you generally didn’t get well-built cabinets or premium drivers and the speakers weren’t usually very good. Unless…

You looked into the pro market at the BBC-designed LS3/5A, which was designed to be, as the Brits’ would say, a “high quality miniature loudspeaker”. It was also designed for what we would now call near field monitoring, but worked OK as a home speaker and increasingly found favor in the hi-end home audio market.

Eventually, the home audio oriented speaker companies noticed this emerging market and started to develop products to cater to it. The ProAc Tablette came along at the right time (early ’80’s) and happened to catch the eye (ear?) of The Absolute Sound magazine and its legendary editor Harry Pearson. Harry gave it a quite complimentary review and ProAc, which previously had only a minimal presence in the US market, was off and running.

After the reviews came out, I made it my business to hear them for myself. I worked in NYC at the time and took the train to the nearest ProAc dealer, a now long gone store in New Brunswick, N.J. Sure enough, quite a nice sound came out of those diminutive boxes. I wanted a pair, but never bought them for reasons lost in the mists of time. A few years ago, a pair of the originals showed up on Audiogon for a reasonable price so I righted that wrong from long ago.

The reason to talk about this speaker (besides its good sound) is its status as one of the first and best of these early mini-monitors (as they came to be called). It can be fairly said that the BBC monitor and the Tablette opened up this market to the mainstream. The premise was that they offered competitively hi-end sound for relatively low-end dollars, assuming you were willing to give up bass and high output levels.

Today, like back in 1981, it’s still amazing to hear what comes out of such small boxes, especially in terms of dynamics and loudness in a reasonably sized room. Obviously, they will not out do larger speakers in some ways, but to me they play just loud enough and have just enough bass to be fairly satisfying on these fronts.

The glory here, as you would hope, is the midrange. Even now, it does a fine job, better than many modestly priced speakers on the market today. As with most small monitors, the imaging is fairly precise, but limited in size and height. The treble is a bit accentuated, but not so much as to scream, at least when played at reasonable playback levels. The drivers show their age a bit, being a bit more rough and ready than the better drivers today. But you don’t get that quality of drivers in modestly priced speakers and you can pick up a pair of used Tablettes for around $300. This math looks good to me.

With mini monitors, bass is always the elephant in the room. Here is where their enclosure size works against the Tabs. Listening to the Decca/Fruhbeck de Burgos Albeniz Suite Espanola, the music is accented by tympani strikes. On a larger speaker (or live music) the tymp powerfully cuts through the orchestra, on the ProAc it sits back as just a component of the orchestra. But it’s there at least, and you can understand the composer’s intent. The lowest open string on the Bass is around 40 Hz and the lowest on the Cello is around 60 Hz, so the ProAc’s in a sympathetic room can at least begin to reproduce these instruments (without really capturing them in full). But it must be said that, bottom line, their approximately 1/3 cubic foot enclosure is just not large enough to get convincingly to the lowest string of the Bass.

A speaker with an enclosure volume of  around 1 cubic foot (3 times as much) is large enough to reasonably get down to that magic 40 Hz figure in room. Again, not to say that response even lower than 40 Hz wouldn’t be desirable, but it’s less of a limitation since that’s mostly the province of the last handful of pipe organ pedals, synthesized electronic music and movie soundtrack effects and the like that are not generally of major musical significance, for classical music especially.

But then again, having the frequencies below 40 Hz also helps bring out concert hall room ambience (which certainly does help create the illusion of a live music venue) and can add to the sense of power on rock music, so they can’t be ignored completely. The problem is, as you descend below 40hz, reproducing this deep bass well becomes expensive, requiring large speakers, large rooms and powerful amplifiers. Sometimes I ask myself if it isn’t more trouble than it’s payoff. But I used to own Infinity IRS V’s, so I guess my arm can be twisted on this point.

Oh well, enough bass theorizing, back to the Tablettes. You can hear why they made waves, both positive and not so positive all those years ago. I do believe that the original Tablette sounds better than the sum of its parts, an example of the magic a wily speaker designer can conjure up.

So I still like them, and with their tiny size it’s no problem to store them when not in use. I guess, truth be told, as my only speaker their limitations would be just too, errr… limiting. Even ProAc eventually increased the size of the Tablettes in their later iterations. Perhaps a comparison between the original and the current Tablette will be possible at some point.

At the end of the day, there are many larger speakers I prefer less despite their low-frequency advantages. It turns out that what the original Tabs do well is not such an easy act to follow.

The “Back Seat” system

I just “celebrated” my 60th birthday (BTW, it’s popular to say “60 is today’s 50”, but forget it, 60 is still 60). It’s a time for contemplation about life, love and, yes, audio and how it all fits into one’s future. Retirement looms with the life-style changes that follow along, often meaning moving into smaller quarters or even frequent moves to follow the weather, etc. So I thought it would be interesting to work on what I call a “back seat” system.

I define a back seat system as a system of very high quality all of whose components will fit on the back seat of a car. This rules out 6 foot tall speakers, 200 pound amplifiers, 4 chassis preamps and the like. Not necessarily cheap, but thrifty with regards to size, weight and complexity without sacrificing performance as much as possible.

And this may come to the fore even more as time goes on for people not of retirement age as tight economic conditions enforce smaller living spaces and may require more “following the sun” from place to place to stay employed. Not to mention that some people just prefer a less obtrusive stereo or to have music in family shared spaces where big, hot and complex systems just would not be appropriate.

So I am going to try to investigate this as time goes by. I hope, besides satisfying my own personal curiosity, this will be of interest and use to readers of this log. Equipment that is part of this survey will be tagged with “Back seat stereo”.