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Linn Sara DMS to PMS Procedure - Printable Version

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RE: Linn Sara DMS to PMS Procedure - skott2772 - 2011-07-09 10:10

I'm really enjoying this thread Dr Eddie. There has been a lack of DIY posts on this forum recently.

I recently purchased a pair of Sara's which have been put a side for the time being as they are not sounding their best. I think it may be the crossovers but was told they were a nightmare to get to.

I'll keep watching to see how to get at them and who knows, I may change mine to actives.

Keep up the good work Smile


RE: Linn Sara DMS to PMS Procedure - Dr_Eddie - 2011-07-12 06:40

Hopefully, it will be a compromise in the airtightness of the pressure chamber somewhere, looking at the crossovers they appear to be of very highly rated components that I feel are extremely unlikely to give problems ever. The main thing to remember with Saras is they put a very highly stressed load on a power amp and also shine a spotlight on the preceding system parts. They will never cover up a front end that is inherently bright or edgy. They tell it how it is. It is certainly not a nightmare to get into them if you really want to as you can see, but to rebuild them properly will take a lot of care and careful application of patience as it did when Linn built them.
If you have heard them in the past with your current system sounding better then something may be amiss, it is not unknown for the rear bass driver's dust cap to become unglued which will destroy the Isobarik principle and the extended bass response with it. They need to be on their dedicated stands too, and are very fussy with room positioning. It can take a long time to find their sweet spot. If they are shouting, try to get them further apart at least 6 feet or more , mine are 8 feet apart. When it's right you will know. It all snaps into place.
Keep trying all avenues before you decide to break into them because it might be your system or it's set up.
They really work best with an LP12 and vinyl , I wouldn't think digital is really what they shine with to be honest.
Unless it's an absolutely impeccable source.


RE: Linn Sara DMS to PMS Procedure - Dr_Eddie - 2011-07-12 07:42

The next stage is to remove all the glue to reveal the fixing nut and 3 fibre washers
like so.

http://postimage.org/image/av3oy8kk/

Then chip away all the rest of the glue sealing the wire holes etc. It should be possible to remove the nut and washers so that the baffle will be able to come up leaving the bracing rod in situ.

It should by this stage look thus :

http://postimage.org/image/ay1jcg6c/

and these are the fixings removed :

http://postimage.org/image/ayd43vms/

Before the baffle is removed, we need to dismount the tweeter. This is fairly easy to do. First remove the 3 fixings on the tweeter faceplate. Now using one of the baffle fixing screws removed from under the Velcro strips, screw this into one of the three tweeter fixing holes and with a claw hammer and a small section of wood lever up the faceplate to break the seal. Remove the tweeter and about an inch back from the original soldered joints cut the cables with side cutters. Stagger the cuts so that they are not opposite each other. We’ll solder the joints to remake the connections, and use heat shrink wrap to insulate them.

Now the baffle is free to come up with some judicious tugging at the tweeter hole first. There is a lot of sealant applied to the baffle rebate , so this can be a bit of a sweat but it will come in the end.

http://postimage.org/image/ax4glias/

When the baffle does come you can now see where Linn hid the crossovers, 2 boards both identical each side of the tweeter.

http://postimage.org/image/aza6utic/

http://postimage.org/image/2b9izvblw/

http://postimage.org/image/2batax1fo/

I cut the wires to the XLR input and removed the two crossover boards with all internal wiring complete. I wrapped each drive units paired wires with masking tape and wrote with magic marker which drive unit it fed. This will be installed in two non magnetic aluminium cases with an XLR input and 4mm banana outputs to the active speaker which will have 4mm inputs to the two bass drivers ( paralleled ) and tweeter separately.
Here is the fully stripped out speaker after a long day.

http://postimage.org/image/32myzpr5w/
The next instalment will begin the long road to the finished article.
I am awaiting cable and 4mm sockets to arrive before I can start the rebuild.


RE: Linn Sara DMS to PMS Procedure - Tony Tune-age - 2011-07-12 13:02

Nice pictures Dr. Eddie, it will be fun to see pictures of the finished product. You're correct about the Sara speaker sounding great with a Sondek turntable. Looking forward to reading how your Sara speakers sound after your upgrades are completed.


RE: Linn Sara DMS to PMS Procedure - Dr_Eddie - 2011-07-13 00:17

Incidentally Tony , many people have asked how the Sara 9 got its moniker. I believe it was simply an obvious choice since the original version saw the light of day in 1978 ( when the baby called Sara was born,) and in 1987 when the updated version appeared she was a 9 year old schoolgirl, hence the Sara 9. By this stage gaskets were being used to seal the cabinets instead of silicone sealant and the inputs were of 4mm banana sockets, which is how I intend to build up this pair.
Thanks for your encouragement guys, I'm now chasing up my Scanspeak tweeters which have been on order for a week without appearing yet. Good old Royal Mail eh?


RE: Linn Sara DMS to PMS Procedure - Turntable Fan - 2011-07-14 12:49

(2011-07-08 18:58)Tony Tune-age Wrote:  Very detailed information Dr. Eddie, look forward to reading about the remaining steps of your projectCool.

The first set of Linn speakers I had the pleasure of hearing was the Sara 9, which was bi-amped with two Linn LK280 power amplifiers. We conducted a listening session with two different Sondek LP12 turntables, one had the Ittok tonearm, and the other had the new Ekos tonearm. Needless to say, both turntables sounded great to everybody in the room.

Cheers

(2011-07-13 00:17)Dr_Eddie Wrote:  Incidentally Tony , many people have asked how the Sara 9 got its moniker. I believe it was simply an obvious choice since the original version saw the light of day in 1978 ( when the baby called Sara was born,) and in 1987 when the updated version appeared she was a 9 year old schoolgirl, hence the Sara 9. By this stage gaskets were being used to seal the cabinets instead of silicone sealant and the inputs were of 4mm banana sockets, which is how I intend to build up this pair.
Thanks for your encouragement guys, I'm now chasing up my Scanspeak tweeters which have been on order for a week without appearing yet. Good old Royal Mail eh?

Good historical information thanks for sharing. Post pictures of the completed project.


RE: Linn Sara DMS to PMS Procedure - Dr_Eddie - 2011-07-21 10:34

Well, the cable has turned up, and this morning the tweeters arrived by courier. Now I await the 4mm Chassis and Panel sockets for the crossover units and speaker cabinets. There is still much we can do in the meantime. For instance the method of sealing the drive units into the cabinet is worth considering. Why did Linn change over to gaskets on the later models instead of using Silicone sealant? I believe the answer to that is that Silicone sealant is TOO good. Effectively you end up with a hermetically sealed enclosure. The reasoning behind this being that any loss of air from the inside to the outside will cause a loss of bass response, In principle this is sound reasoning, but in practice there is one huge flaw in this idea that Linn eventually realised.

The whole point of the Isobarik principle, where you have a pressure chamber that allows the front bass unit to move without pressure effects exerted on it from the rear of the cabinet, is that the pressure behind the unit is equivalent to the room pressure in front of the unit. This allows the cone to move unhindered ( therefore more linearly ) and hence without compression distorting the audio signal. This provides an extraordinary bass response that seems impossible for a cabinet of this size ( or even twice this size ). The lower limit seems almost subterranean but as tight as a D.A. if you get my drift. This is bass that is unbelievably fast and well defined, tactile and textured, starts and stops on a sixpence. Absolutely zero overhang. This provides rhythm and drive aplenty.

If you seal the cabinet too tightly however such that is hermetically sealed ( which is what the silicone sealant actually does ) then the pressure INSIDE the cabinet will only equal room pressure outside the cabinet on a rare few days of the year when the weather is so inclined as to coincide with the day the speaker was assembled in Drakemire Drive 30 years ago. This pressure difference will cause the front driver to be offset from its rest position depending on the scale of difference. It will definitely affect the ability of the speaker to perform properly. It also explains why Isobarik DMS and PMS systems could sound great one day and poor the next.

This is where Peter Belt got his foot in a very large doorway with his mumbo jumbo potions and creams. The answer was a simple change in the weather could wreak havoc with such a perfectly sealed system. Since temperature and pressure go hand in hand the chances of them ever being the same as day of manufacture is pretty remote.

What is required therefore is NOT a perfect seal but one which is good enough under dynamic conditions (which is all that is required ) to allow the pressure chamber to perform as intended. The motor industry is geared towards making seals that function dynamically without ever being perfect. Hence Linn eventually realised that gaskets were the way to go. These will allow the air pressure inside the cabinet to equalise with room pressure as these changes occur, whilst still being effective whilst the speaker is being used.

So now the question is what materials do we select for the task? Since the pressure chamber seal is supposed to be gastight under dynamic conditions I reasoned that manifold gasket material would be a good choice and purchased two large 1mm thick sheets of such a material.

This was used to make suitable gaskets for the 4 bass drivers as the pictures following show. We can use a bass unit to scribe the outside shape of the gasket like so.

http://postimage.org/image/bcr7ihus/

The baffle extension ring comes in really handy to hold the gasket in position whilst you mark the centre cut out for the spider. In the end I got 8 gaskets from the one sheet with enough material to produce gaskets for the baffle edge and tweeters left over.

http://postimage.org/image/bdb1yns4/

http://postimage.org/image/bdy7hip0/

http://postimage.org/image/be6h695w/

http://postimage.org/image/beequzms/

http://postimage.org/image/beoo32lg/

The whole idea of these is to produce a speaker that is sealed sufficiently well under dynamic conditions such that the Isobarik principle is effectively working whilst not being so tightly sealed that the chambers can’t equalise with the air in the room as fluctuations in the barometer will dictate. We do not want any temperature and pressure driven offsets to spoil the linear motor response from our Active speaker.

In order to effect this I intend to leave a pinhole in the glue sealing around one of the cable holes in the pressure chamber and the use of gasket material around the baffle edge ( instead of copious amounts of silicone sealant) will help here as well.

The next item I am awaiting before I can start the rebuild is a Hot Melt glue gun. Since it is only going to be used for this one job, it is hardly worthwhile buying an industrial grade tool such as Linn no doubt used. Loctite sell a domestic model which with 6 sticks of adhesive costs around £22. This should be perfectly adequate for our purposes which is mainly sealing around the pressure chamber and the retention of the muslin pockets of wadding around the periphery of the pressure chamber.


The next update will look at the manufacture of the external crossover boxes and how they were made and assembled.


RE: Linn Sara DMS to PMS Procedure - Tony Tune-age - 2011-07-21 12:35

Very interesting update Dr. Eddie! You are certainly putting a great deal of effort into this speaker project, and they will essentially last forever...once completed.


RE: Linn Sara DMS to PMS Procedure - Dr_Eddie - 2011-07-21 18:22

You're right Tony, if only because I believe the end result will be something I can live with for ever. It is very rare to find a speaker that is so satisfying to listen to playing music. I've always hankered after an Active pair of Saras but never had the budget 30 years ago. The cost of the Naim electronics was well beyond my means.
About the only other speakers I could ever envisage satisfying me now would be the legendary big Isobariks, Naim SBL's or DBL's. Once you've heard any of these nothing else comes close, except maybe Active Saras. I've been listening to my DMS pair, recently with much material from the Chris Frankland playlist and the results have been sensational.
Albums such as Chuck Mangione's " Save Tonight For Me " Chicago's "V" Ben Sidran's " A Little Kiss In The Night " and Tower of Powers " Bump City " have all graced the Linn. and the effect of that is that the Saras just melt away leaving the artist in the room. It suspends belief. I don't need a pair of Sonus Fabers at 140K to light my fire, thank goodness, I'll take any of the above.

Rest assured I'll compare my homebuilt Saras with my Linn built DMS pair ( DMS on both ) before I embark on the Active project. They have to perform at least as well as the DMS pair before I commit finances to the acquisition of further Naim electronics.
I will be brutally honest in my review of these which will be tunedem in the best CF traditions which you will read about in due course. Thanks for your kind comments Tony and to everyone for your interest.


RE: Linn Sara DMS to PMS Procedure - Tony Tune-age - 2011-07-21 18:47

Good luck, and may the force be with you Dr. EddieCool