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Linn Sara DMS to PMS Procedure - Printable Version

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RE: Linn Sara DMS to PMS Procedure - Dr_Eddie - 2011-09-02 17:47

I am due to begin the rebuilding process in the next week, if things go to plan. I am trying to source the Allen headed Metric bolts that Linn employed on the later models. Peter of Cymbiosis has inspected the stripped out cabinets and my home made gaskets and was quite impressed with the project. He also commented that evening on my original DMS pair.
He reckoned they were the best Saras he's ever heard, so I've got my work cut out to get these sounding as good with the external crossovers.

The new Scanspeak tweeters are an improved design on the original with enormous power handling ability now ( 150 Watts Cont RMS ) with ferrofluid cooled voice coils. This should really up the ante, so it's likely the rebuilt speakers will be better. By how much ? we'll see soon enough.

I'll post pictures of the rebuild as soon as it starts Tony, so keep checking the thread.
Thanks for the interest.


RE: Linn Sara DMS to PMS Procedure - Dr_Eddie - 2011-09-10 00:52

I've sorted out the problems with the drive unit fixings. It was my fault because I assumed that the bass unit fixings were Metric like the tweeter units are ( M3 ). After endlessly trying to measure the bass unit screws ( without success ) I decided to take the screws to Clerkenwell Screws in London WC1. The guy instantly spotted that the larger screws were in fact 2BA. These are 3/4 inch long with an allen socketed head. This surprised me until I remembered that the flat front cabinets weren't fitted with T-Nuts but embedded brass inserts. The tweeter fixings are 10mm M3 allen headed screws.

So now I have a full upgraded set of later type fixings and the rebuild can begin.

First I'll show you how I made up the crossover bricks from sheet aluminium.
Watch this space.


RE: Linn Sara DMS to PMS Procedure - Dr_Eddie - 2011-09-10 01:04

I've also broken my own rules by not being able to resist a Naim serviced pair of NAP 135's that came along a bit early. Drat, now I've really got to build these things like my life depended on it. ( I won't feel 2 Grand ie 2K. if these speakers don't wipe my DMS's ).

Now I've got a 4 pack and a lot of work ahead...............................Big Grin


RE: Linn Sara DMS to PMS Procedure - Dr_Eddie - 2011-10-13 20:13

S.A.R.A. DMS Crossover Bricks

In order to make these, I decided the easiest way to proceed would be to make a cardboard template to decide on the dimensions needed, to house the circuit boards and wiring. There were a number of issues that I needed to consider at the same time.

The most pressing of these was where to accommodate the unit. The most appropriate place being the base of the stand. Linn themselves took this route with the last of the Isobariks made in the early Nineties. It would also look quite neat if the dimensions closely mirrored the stand’s lower recess. In cardboard this worked quite well and the size was then decided upon.

Next issue was what material should be used. The casing ought to be non magnetic, since there are ferrite inductors in the circuit, so ideally no ferrous material at all. Aluminium was chosen, since it is cheap and cheerful. If it is good enough for Naim it’s good enough for me. It’s also easy to work with, in terms of bending cutting and filing not to mention drilling also. You’ll be drilling a lot of holes, before you’re done. So let’s get started.

Bend the cardboard template to shape, and trial fit the crossover boards. The fixing holes for the circuit boards can be marked out here. Fashion another piece of card for the lid.

When the template is complete the design can be transferred to a fresh sheet of aluminium.
The folds can then be put in for the ends, sides etc. It should look something like this………………………………..

http://postimage.org/image/rtynyb1g/

This next picture is of the crossover assembly all wired up still as it was removed from the speaker. Note that I have identified which wires are connected to either the XLR input socket or the various speaker drive units. This is exactly how I am going to install the crossover into the chassis. The original input socket made by Switchcraft will be employed to do the same job in the chassis. Note that Linn only employed two makes of XLR socket, either Switchcraft ( as on the early models like this ) or Cannon as on the later cabinets which employed the Elbow socket which allowed the speaker to be placed even closer to the wall. Both Switchcraft and Cannon are the studio standard for quality of and reliability of connection in a professional studio environment. Here then is the jumble of boards and wiring I need to get into that chassis………………………………………………………

http://postimage.org/image/rzucqq90/

Note the large rubber tap washers which will need to be used to isolate the circuit boards from the chassis. There are numerous soldered connections under the boards which would short out otherwise. The Naim NAP 135’s would not like that one little bit, so this is a must. The RS Panel sockets for the drive unit connections need to be mounted in the chassis BEFORE any heat from soldering is applied. The Plastic thread tends to get distorted by the heat and the nut will refuse to go on otherwise.
The two connections to the Switchcraft socket can more easily be made out of the chassis though. Before this construction took place I had completed the chassis builds, drilled all holes and checked everything fits together before painting in Hycote black wrinkle finish paint. After paint hardening, the boxes were ready for the installation.
I trial fitted the various sockets as in the next picture. The 4mm banana sockets are at the Standard 19mm spacing between centres allowing the use of Naim speaker connection plugs.

http://postimage.org/image/1w4ey3tz8/


The speakers will have rectangular panels of 12 swg aluminium mounted over holes in the back of the cabinet and sealed with both gaskets and hot melt glue for airtightness, employing the same connectors and spacing. This should look neat and be robust and reliable in use. My standard ethic is to have HF on the right, and LF on the left. Although I will also label them for the sake of completeness too.


The next stage is to mount the boards into the chassis using the fixings employed originally, but I had to remove about 10mm of screw thread as they were too long and would protrude further than the rubber feet from the base.

This is how it looks now……………………………………

http://postimage.org/image/1w55epdvo/

http://postimage.org/image/1w644zo90/




Once it is all in place you can identify the connections that go to the input, and by removing the socket and holding it firm with some Blu-Tak, it is easy to get a good reliable solid soldered connection. Note the earth connection is at the base of the socket and the hot live connection (red) is to the right of the socket looking at it from the plug side. Some insulation is applied to ensure these connections don’t ever touch. They are of necessity here quite close to each other.
Let the socket COOL after soldering BEFORE you touch it. OUCH. Then it can safely be replaced in the chassis.

These two connections are of critical importance so make sure they are beyond reproach. Now, though, we’re getting somewhere…………..

http://postimage.org/image/1w6e27r7o/



By dressing the wires around the outer edges of the chassis the build can be kept fairly neat. Not that easy, since there is an awful lot of wiring in there and it is heavy gauge and bulky too. I felt it was important not to shorten any wires since this may alter the way the crossover performs. So the wiring is exactly the same as in the cabinet but displaced into these chassis units………………..
http://postimage.org/image/1w6mbwhok/


Finally the last 4 connections are made. The two bass driver wires are connected together , ie. Both red and black are doubled together and connected to the two panel sockets closest to us.

Then the red and black single wires to the tweeter are connected to the two sockets furthest away. This is how it finally looks inside the box.
Tinning both the connector spade and wire end makes it a lot easier to get a good joint.

http://postimage.org/image/1wkrpfds4/




And here is the finished article along with one I made earlier. Neat ? I’m pleased with how they look. How they perform is another issue, but if I’m going to make something that is going to be seen then it needs to be of a certain quality in my book.

http://postimage.org/image/1wl4xq5qc/


This where they will live at the base of the stands………………………………….
http://postimage.org/image/1wmaa5u2s/


Of course ultimately they will be redundant if the Active project is successful but I’m not taking any short cuts. Just in case……………………………………….

Next instalment is to start the speaker rebuild. Stay tuned.


RE: Linn Sara DMS to PMS Procedure - salamonscream - 2011-10-14 09:08

Hi

Many thanks for a very interesting and informative thread. I have no wish to criticise as you have taken on a project that I would throught twice before taking on, but your latest pics brought a couple of things to mind.

1 Were you have stripped and twisted the cable together, i would have prefered to see solder run the whole lenght of the exposed copper, as this would avoid any chance of the copper oxidising and reducing performance at a later date. I note that you have taped the joints presumably to avoid shorting, but that will not provide a airtight cover.

2 Yes they look very nice in the stands, but how are you going to make your connections as it looks as if the stand will prevent this?

Please do not take any offence at my coments, It's me just being pedantic!!!!

regards


RE: Linn Sara DMS to PMS Procedure - Dr_Eddie - 2011-10-14 23:56

No offence taken salamonscream, taking your last point first, the position of the cable connections is cunningly designed to run under the stands crossbrace at the rear. Naim cable fits the bill well, although the pic really doesn't show the almost 1.5 inch clearance. I'll post pics with all connections in place at a later stage.

The point you make about the stranded connections is a valid one, but Linn actually used less solder than me in this case. Also worth bearing in mind is that I plan to use these boxes mainly for testing the Active speaker with the current passive system, and after this the boxes will only be used when I send my 135's back to Naim for servicing ( in pairs ) and can then run the system in passive mode. In the actual speaker the similar joints will be made as in the chassis boxes but I will be using 50 amp Litz transformer cable which is already tinned over the copper so the connections should be durable, also I plan to use shrinkwrap here, not tape.

I like the way you think, I'm pretty much the same too.


RE: Linn Sara DMS to PMS Procedure - Mirage F1 - 2011-10-15 14:04

Really really impressive bit of craftmanship.

Im sure they will work well in active-mode.


Regards


RE: Linn Sara DMS to PMS Procedure - Dr_Eddie - 2011-10-15 15:14

Thanks for your kind comments Mirage F1, I am quite pleased with how the crossover boxes turned out. I have only heard Active Saras once, at the Audio T Hi Fi show at the Holiday Inn Swiss Cottage Jan 1983. The Naim system driving them was Chrome Bumper stock. The deck was a Valhalla LP12 with Black Ittok LV11 and Asak-T cartridge, feeding a NAC 32.5/ SNAPS/ SNAXO Chrome version/ and 2 X NAP 250's. all of that into Flat fronted Active Saras on the new (then ) fully spiked top and bottom stands as seen in my pics
The effect was shattering. The system produced a sound that still haunts me when I think back to it. It was like listening to a Live rock group in a hotel bedroom. You literally felt the music inside you. But it didn't hurt, it was exhilerating with a boogie factor off the scale.
People were heard muttering "....kin" ell " on the way out.

It's a place I'm hoping to revisit soon, only now I can just about afford the Naim electronics and the room is a more decent size. The Linn has improved vastly since those days, so this could really motor if all goes well. The price of the Naim gear was well out of my league then but that kick started my love affair with the Sara Isobarik, The only other speaker that nearly drew me in was the big Isobarik. Sheer size was the drawback then. Saras fitted into my room, briks didn't, shame really, but, they share many traits, and a good pair of Saras can really give a lot of the clout of the big briks. Once heard never forgotten.


RE: Linn Sara DMS to PMS Procedure - Tony Tune-age - 2011-10-15 20:20

Great progress indeed Dr. Eddie, the pictures show a lot! Your Sara speaker project requires a significant amount of patience and skill...which you clearly have!

Good luck with everything, I hope they sound the way you want them to soundCool Keep us posted...Wink


RE: Linn Sara DMS to PMS Procedure - Dr_Eddie - 2011-10-16 11:22

Thanks Tony, I will be starting the rebuild any day now, and I have sourced at long last a Snaxo 2-4 ( 2006 model ) for the project as well. Picking it up from Grahams ( thanks Paul, you're a star ) this week along with a 7-1 Hydra ........yegods. I'm beginning to get sweaty palms now.

The only thing left to source now is a Supercap and Burndy to power my Superline, because I'm nicking its HiCap2 to power the Snaxo. Another point is the crossover is actually for the SBL which crosses over at 2.7K instead of 3k as in the Sara. I intend to try it as is to start but it may need a Naim tweak to really hit the spot.

Of course that all means I need a 10 level Naim Fraim to house all these boxes properly so two 5 levels needed or a 6 and 4. That will mean about £4k to add to the cost unless I can get it ex-dem without the SC. ( Head in hands .....................) so you could imagine I agree with your sentiments entirely Tony. I'm stark staring bonkers really.