A Linn Isobarik PMS clone for the 21st century?
In the late eighties, I had the opportunity to listen to one of the classic setups of the day.
This inspired me to gather all the pieces for myself. I only got to one NAP250 and had to make do with Linn Sara speakers . Life happened and I never got to own a pair of Briks, as they are called by those in the know.
The things is, I still want to. The memory of the sound they made has remained with me all those years. They are now long out of production. Some are still around in secondhand form on eBay e.a. but the sellers are invariably on the other side of the globe. Around here there are none to be had. Considering the original price, I don't think the local importer sold many.
I am therefore toying with the idea of creating a modern clone of this beast.
The first problem is that the oval woofer (KEF B139) is also long out of production. There are of course two of them, the second one being inside the cabinet in an isobaric configuration. Yes, I know it's obvious but there may be readers who do not know that.
The piston area of the B139 is about the same as that of a round 10-inch driver - a good place to start investigations. There is a Linn replacement for the B139 but the price would probably make your eyes water. The midrange (Kef B110) is also long gone but at least it is conventional in shape. The tweeters were made by Scanspeak.
The second problem is that the Isobarik does not yield its inner secrets easily. They were notoriously difficult to dismantle and most owners left well enough alone. The only clues that I have is the image attached below. I also have the patent documents and a contemporary review that goes into some detail of the internal construction, if there is any interest.
The passive Isobariks were notorious for present a very difficult load to an amplifier. Electrically the crossovers were two three-ways in parallel. I am therefore thinking of going active with this one. It would make the tuning of the crossover easier, but at added cost. Then again, the passive crossovers will be far from cheap.
Any thoughts on the viability of such a mad project?
Wasn't this a Linn Isobarik for the 21st century?
Ah yes, the Keltik. Those are the very woofers that I refer to in post 1.
I don't know much about them and have never heard them in action. From what I can gather, they never quite attracted the same fanatical following as the Bariks. Kind of like when Porsche tried to replace the 911 with the 928. Except that in this case the Isobarik and Keltik are both no more.
Unfortunately, cloning the Keltik would be quite impossible with round woofers. But if anybody has any info on them, I would like to know more. Google turns up little except second-hand ones for sale.
It will be difficult to reproduce the tonality (or your memory thereof) of the original with modern speaker drivers. But it sure is an interesting project!
Fyi Tang band produces pretty decent oval woofers at a reasonable price. It wou.d be cool to keep the same overal look and feel. Check them out.
I don't have access to winisd at the moment, but i seem to remember that this program can simulate isobarik configuration, also worth checking.
The Keltik is a right old belter of a speaker, I remember walking into a Hi Fi show in the late 90's thinking there was a live band.
It happened to be a pair of Ketliks, actively driven and fed from an LP12 filling an entire ballroom with sound, very impressive.
The dimensions are 308 x 214mm.
Here is the cutout required for a B139.
The dimensions of the hole add up to 300 x 210mm. The KEF datasheet (see below) also gives the second dimension as 210mm, although the first is not stated.
All of a sudden this project looks a whole lot less hypothetical... :cool:
EDIT: I have found this driver on sale and the price looks right:
Only one problem - it has an impedance of 4 ohm. This means that each woofer will needs its own amp, or the LF amp will have to be stable at 2 ohm and lower. Did I mention that it will be an active speaker?
Hat tip to talaerts!
I'm pencilling in this unit for the midrange. The only reason at this stage is that it is also from Tang Band. That should count for something.
I have taken a pair of isobariks apart... went on an isobarik building run shortly after.
There are no issues building something using the same concepts... you'd have to work to get a system today with the same personality/coloutful sound as these. ie you can do much better.
Personally, i would take the concepts & approach of the Castle-style microTower and add isobarik bass. I do beleive that isobarik does bring more benefit than just allowing use of a box ~1/2 the size. With the wide choice of drivers today thou, one has to carefully consider whether spending 2x as much on a single woofer can get you more.
I wouldn't get stuck on using an oval woofer.
I have owned a pair of DMS Briks for a few years that I bought from a house clearance and paid £50.00 for them; they did have all the tweeters blown and the relevant crossover parts were also fried but with a little patience I took them apart and fixed them. I cannot claim that they are as good as new but they do sound great. What were Linn thinking by putting the crossovers in the cavity between the Isobarik loaded bass drivers?!
Personally I think you should buy the actual drivers used, as they come up on ebay quite frequently; you will have to bite the bullet with the purchase and shipping costs but if you want that sound......! The cabinet is complex but not that bad and could be easily improved upon.
Go for it..............
At this stage, the most vexing question for me is just how those midrange enclosures look and how they line up. Any detail that you guys who've been there and done the Isobarik dismantling can fill in the missing gaps, it would be much appreciated.
I still have the Sara's on hand to dismantle and investigate the bass bin construction, which should be similar. Come to think of it, I've done that twice to replace fried woofers. (Start of rant) Naim amplifiers does not have any DC offset protection of any kind. Any relays in the output would degrade the sound quality in some mysterious way. And our world famous amplifiers never break down anyway. (End of rant)
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