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|24th September 2018, 02:29 PM||#2|
Join Date: Jan 2016
Over the years I have rebuilt a few Stratherns. The foil and mylar supply houses will sometimes give you a sample you can work from but it is getting harder to find that kind of help.
However I have found Ebay to be a good source for the work Ive done over the years.
heres a couple examples...
NEW 24" wide x 10' long x .0005" Thick Annealed Aluminum Foil ASTM B479 Sheet | eBay
Electrostatic Speaker Membrane Dupont Mylar C 6um 40M | eBay
I have ordered mylar from that supplyer with no problems
Thinner foils do come up available from time to time. Ya just have to watch for them.
Food for thought...
The original Stratherns came with .00025" thick foil and .00025" thick polyester film (mylar).
At some point they switched the foil to .00035" thick. I believe for reliability reasons.
I have rebuilt Stratherns using the following foil/mylar thicknesses...
foil / mylar
.0005"/.00025" and .0005"
The thinner foil / mylar combinations were a bit more extended in high freq response , However to my ears the .00035" and even the .0005" foils sounded better. They were a bit more rolled off above 10 khz than the thinner stuff BUT they seemed better balanced overall and I really didnt miss the higher frequency loss.
I even tryed .0007" thick foil (standard foil from the grocery store), and .001" ( heavy duty foil from grocery store). Much to my surprise even the .0007" foil sounded good. The .001" foil however sounded less convincing. I wouldnt go that thick.
Also tryed .0005" mylar with a few of the foils. This too didnt sound bad, but there was a slight loss in sensitivity due to extra weight.
In the end I liked .00025" mylar and .00035-.0005" foil best.
|26th September 2018, 02:25 PM||#5|
Join Date: Jan 2016
Yep good question.
So theres probably a thousand ways to do this but the following is the way I like to use best for quick prototyping...
1- use a plate of glass as your work surface. Any plate of glass thats large enough will do. I usually go to a store and buy one of those cheap 5-10 dollar mirrors. I got last one at Target. It was 48 inches long and about 15 inches wide. Cost 5$
2- lay glass on a flat work surface so glass is not warped or bent. this will be foil cutting table.
3- Next make a tensioning frame for mylar. I just use 1/2" plywood or particle board. Cut out a "frame" that looks sort of like a picture frame. The inner open area needs to be large enough so that the ribbon speakers frame will fit inside the opening.
4- now cut a sheet of mylar that is about 2 inches smaller than the outside dimensions of your tensioning frame, lay mylar on top of frame and attach mylar to frame using many short pieces ( only about 2 inches between tapes) of scotch tape all around edges of mylar. Pull tapes tight all around to tension film.now go around to all the pieces of tape one more time and put an even tension on them all so the entire mylar sheet is evenly tensioned and flat. we will talk about how much tension later. If tapes will not stick well enough to wood to hold tension then I sometimes will paint the tension frame or put some varnish on it. This gives a good surface for tape to stick to. The tapes need to hold tension while you work.
5- now ether wipe glass surface with soapy water OR spray one VERY light coating of 3M super77 spray on glass surface. This to hold foil tight to glass surface while you cut and work. The soapy water doesn't hold great but sometimes works. The 77 spray works well but is a bit messy to clean up with solvent like acetone. Use only one VERY light spray coat.
6- roll foil out onto glass surface
7- using a soft tissue paper rub the foil down flat over entire surface. Start from center and work out. This is to flatten and smooth foil out.
8- carefully mark were you want to cut foil with a thin tip marker, then use a long ruler or straight edge of some kind, and a fabric cutting wheel like this
to cut the foil.
Amazon.com: Tailor #39;s Scissors - 28 45mm Stainless Steel Patchwork Roller Wheel Round Knife Cloth Fabric Leather Craft Cutter Diy - Round Roller Fabric Wheel Stainless Bladeds Cutter Printing And Cloth: Home Improvement
9- Now roll the contact cement out thin and even over the foil in area where you have cut.
10- Immediately after coating foil with contact cement, carefully peel up all foil thats not going to be transferred to the mylar. Only the strips of foil that will be used on ribbon will be left attached to the glass.
11- now coat the tensioned mylar with contact cement only where the foil strips will be attached. Dont worry if you get extra cement on here. A thinn coat of cement on mylar in areas where there will not be foil is OK.
12- at this point you have tensioned mylar film on frame with contact cement applied, AND cut foil strips laying flat on glass with cement applied. Allow the cement on both to dry per adhesive directions.
13- once adhesive is ready, align the tensioned mylar over top of the foil strips and lay tensioned mylar down over foil strips.
14- using a soft tissue or cloth firmly press (dont rub yet!!) mylar down against foil untill you have good contact. Now you can rub if you want to get a good connection. Idea is to make a good a connection as possible. Sometimes I will even use a soft plastic or even backside of my fingernails to lightly rub it all down and get a solid connection between foil and mylar.
15- once a good connection is made you can slowly, carefully pull tension frame up at one end pulling away from glass surface and peel the whole tensioned film/foil assembly up. Go slow so it realeases easily. If you pull up too fast you can stretch foil out causing wrinkles.
16- at this point you have a tensioned film/foil assembly ready to attach the speaker frame. Contact cement again works well here. 1-2 thin coats around perimeter of speaker frame AND mylar where the two will meet.Let dry, Then carefully lay mylar/foil assembly onto speaker frame with attention to alignment of foil strips with magnets positions. Its best to practice this step a few times before foil is actually attached to mylar so you know how it will go.
17- press firmly all around to attach mylar/foil assembly to speaker frame, then use a razor blade to cut away excess mylar from speaker frame and thats it! You have a new diaphragm ready to go. Finnish up with re installing magnets and foil connections.
How much tension??
You can get serious here and use a small hand held scale to tension each tape around mylar perimeter . You can even tension different sections of diaphragm to different tensions to differential tune and spread out the resonances.
I have tryed these BUT I find that they made no noticeable difference in performance. In the end I simply put enough tension on each tape by hand to make sure the mylar is flat AND it transfers to the speaker frame without loosing tension or wrinkling.
Yes I know thats not very romantic and technical BUT it works in a design like the Strathern where most of the diaphragm is actually clamped anyway.
A word of encouragement...
This is a method developed over years of prototyping. It may seem intimidating BUT all steps are practical and use practical materials/tools . If you follow the steps and practice it will only be hard the first few times as you learn.
Getting a good connection to the ribbons foil can be a problem. Aluminum foil has a thin layer of oxide that makes for a poor connection to whatever clamp is being used. In the stratherns the foil is clamped between two brass strips. I like to clean and then rough up the brass strips with 220 grit sand paper. this makes a rough surface on the brass that seems to break through the AL foils oxide layer when the screw is tightened. I also like to spread a thin layer of oil on brass before connection. This to avoid oxides forming over time in the connection. you can use vasilene here or the special contact "sauce" on the market.
Last edited by lowmass; 26th September 2018 at 02:42 PM.
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