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Building an Electrostatic Speaker
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beun




Joined: 28 Jun 2006
Posts: 676



PostLink    Posted: Thu Sep 08, 2011 4:25 pm    Post subject: Reply with quote


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It's a mix between tape and screws. This is actually not that crazy, the membrane of Martin-Logans is held on the stators using 3M double sided foam tape that acts as spacer as well.

Later you will see that the membrane is taped as well.

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beun




Joined: 28 Jun 2006
Posts: 676



PostLink    Posted: Sun Sep 11, 2011 1:35 am    Post subject: Reply with quote

Today I fitted the front stator on top of the rear one as seen in the picture below. The clamp and two sticks take care of an exact vertical and horizontal alignment.



After drilling all the holes through both stators into the frame I screwed in the threaded inserts so I can use nylon bolts to clamp the front and rear stators together.


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beun




Joined: 28 Jun 2006
Posts: 676



PostLink    Posted: Sun Sep 11, 2011 7:46 pm    Post subject: Reply with quote

Managed to solder the HV-leads to the stators, this went a whole lot easier than I expected most likely because steel is actually a bad heat conductor. The remaining cutout is filled up with epoxy which is now drying.


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beun




Joined: 28 Jun 2006
Posts: 676



PostLink    Posted: Mon Sep 19, 2011 5:33 am    Post subject: Reply with quote

Lots of progress this weekend but no pictures worth showing. I now have two frames with threaded inserts and another set of finished stators.
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beun




Joined: 28 Jun 2006
Posts: 676



PostLink    Posted: Fri Sep 23, 2011 2:07 am    Post subject: Reply with quote

Just ordered all the parts for the High-Voltage supply and when it is put together I can precisely measure it up and design the panels for the enclosure.
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beun




Joined: 28 Jun 2006
Posts: 676



PostLink    Posted: Mon Sep 26, 2011 4:18 am    Post subject: Reply with quote

I have now mounted the spacers on one of the back stators. The width of the stator is divided into 9 sections where the width of each section follows an exponential relationship. Tomorrow the second stator will be completed with the spacings in a mirror image of the first one. I will make some pictures as soon as I can.
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beun




Joined: 28 Jun 2006
Posts: 676



PostLink    Posted: Sun Oct 02, 2011 7:29 pm    Post subject: Reply with quote

Finally some pictures worth showing. One frame has been fully buffed and the rear stator with spacers mounted. The finish doesn't look like a piano finish everywhere but overall it looks quite acceptable.

I had to rearrange the spacers a bit because there wasn't enough rear clearance for the membrane in a few places. The reason is that the curvature is greatest in the center and tapers off at the edges. So technically you would want the largest spacing at the outside if it wasn't for the fact that the lowest tones should have the longest distance before the waves destructively interfere. This would suggest to put the widest spacing in the center which as said before isn't possible. My original had some membrane clearance problems but this one should work.







The tape here only holds down the bared wire to contact the membrane, this is temporary.











All trim goes on at the very last minute, I wouldn't want to scratch it now.

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Last edited by beun on Mon Oct 03, 2011 3:31 pm; edited 1 time in total
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beun




Joined: 28 Jun 2006
Posts: 676



PostLink    Posted: Sun Oct 02, 2011 10:16 pm    Post subject: Reply with quote

And now with an initial mount of the membrane. The rest consists of very carefully stretching it tight.


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garyfritz




Joined: 08 Apr 2006
Posts: 12023
Location: Fort Collins, CO


PostLink    Posted: Mon Oct 03, 2011 2:52 pm    Post subject: Reply with quote

Kim, designing and building electrostats like this strikes me as being incredibly specialized knowledge. Whereinhell did you learn it? Have you been fooling around building stats for yourself for years and now you're building them for others?
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beun




Joined: 28 Jun 2006
Posts: 676



PostLink    Posted: Mon Oct 03, 2011 3:54 pm    Post subject: Reply with quote

Gary,

Yes that is basically correct. I built my first one in the early 80's and it took me about 1 year thinking about it and doing design and about another year to actually built them. Back then I would have to go back and forth several times before I was happy with the result. Even now I am still learning because the exponentially changing distance between the spacers is actually new (to me at least). I used to have the spacings unequal from the beginning but this distributes the resonance frequency a whole lot better. I have to give credit to the customer here who initially came up with the idea of making the spacings much more unequal than they used to be.

I even designed and built a direct drive HV-amplifier back then capable of generating a 1200Vpp audio signal, one that I still have, but it was a little bit too weak to give the best results. I will revisit that soon and expect to be able to generate 3000Vpp this time.

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Curt Palme
CRT Tech



Joined: 08 Mar 2006
Posts: 24296
Location: Langley, BC

TV/Projector: All of them!


PostLink    Posted: Mon Oct 03, 2011 4:15 pm    Post subject: Reply with quote

You should leave the speaker like that. It makes wives and girlfriends look skinny. WAF: HIGH!
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beun




Joined: 28 Jun 2006
Posts: 676



PostLink    Posted: Mon Oct 03, 2011 11:24 pm    Post subject: Reply with quote

Another step in stretching the membrane, more tension.


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garyfritz




Joined: 08 Apr 2006
Posts: 12023
Location: Fort Collins, CO


PostLink    Posted: Tue Oct 04, 2011 4:25 pm    Post subject: Reply with quote

How do you stretch and attach the membrane? I'm assuming Scotch tape wouldn't cut it... Very Happy But I'd think any kind of tape would have limited useful lifespan.
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beun




Joined: 28 Jun 2006
Posts: 676



PostLink    Posted: Tue Oct 04, 2011 5:47 pm    Post subject: Reply with quote

Gary,

You would be surprised how strong Scotch tape actually is, the first one was put together using regular Scotch office tape and they lasted for almost 20 years before I took them apart to remove the rust of the stators as those weren't powder coated yet back then. The tape stretched a bit but not that much.

Currently I use something better, it's 3M Super Bond Film Tape 396. This stuff sticks really well, The ESL-0.5 speakers for example have been sitting in my garage for over a year now for lack of space in the house, at well over 110F in the summer and below freezing in the winter and the tape hasn't stretched yet. The first place to notice when the tape stretches is always in the corners where it start to wrinkle a little bit and with this tape there is no wrinkle in sight.

Do remember that commercial Martin-Logan speakers use double sided 3M foam tape where the tape tape acts both as a spacer and the glue. Have you ever tried to remove that tape from something, you tear the foam apart but you cannot remove the damn glue? The same holds for those speakers, the glue lasts forever but the foam disintegrates in about 10 years and you have to re-membrane it or buy a new panel.

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garyfritz




Joined: 08 Apr 2006
Posts: 12023
Location: Fort Collins, CO


PostLink    Posted: Tue Oct 04, 2011 6:27 pm    Post subject: Reply with quote

Cool, interesting.

I'm puzzled/fascinated by the shape of the membrane and stators. The fuzzy understanding I had of electrostatics like e.g. the Magneplanars was that the membrane was one big flat surface. Obviously that would contribute to the "beaming" the Maggies were so famous for.

Your curved membrane should produce a much wider dispersion, and presumably the height takes care of any vertical beaming. I assume that's why you went with the curved membrane? What was the design intent of the variable spacing between stators and membrane? You said something about the lowest tones &etc but I didn't quite get the concept.

But with the spacers breaking the membrane surface into many narrow/tall sub-panels, you lose the large surface area of a flat stat. How does it handle bass with all those small independent panels? I'm a little bit surprised it can handle the 80Hz you spec it for. I guess those panels are big enough for 80Hz?

You have a high voltage between the stators to move the membrane, right? How do you keep from electrocuting cats? Mr. Green

Thanks for answering all the curious questions!
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beun




Joined: 28 Jun 2006
Posts: 676



PostLink    Posted: Tue Oct 04, 2011 7:09 pm    Post subject: Reply with quote

Gary,

You are right, one big membrane beams terribly, therefore the more expensive Maggies use a separate tweeter ribbon and a crossover to get around that. Quad used a flat membrane but installed a delay line and ring shaped segments in the voltage drive to create an artificial point source. Other versions of a stat use a wire stator and they limit the high frequencies to just a small center portion in order to reduce beaming.

A curved membrane like Martin-Logan gets around the beaming by spreading the sound over a curved area, the downside of their method of a perfectly curved membrane and horizontal spacers is that they can only have vertical tension in the membrane otherwise it would want to flatten out. I don't like asymmetrical tension so I didn't go that route.

An other big upside of the curved stator is mechanical stability, a large flat piece of metal is rather floppy but curved in one dimension makes it rather strong.

Now lets talk about the biggest downside of one large membrane, its resonance which creates a sharp peak in the frequency response at low frequencies that can easily be 20dB higher. The original Martin-Logan CLS was infamous for that 'one note bass', it may sound impressive but it gets tiring easily. You can try to dampen it, some manufacturers use silicone dots on the membrane to make it heavier or you can try to avoid it. This is where the spacers with exponential distance comes into play. Each strip has its own resonance but each strip has a different one thereby spreading it out over a much larger frequency range and avoiding the one note bass.

Lets take a look at the low frequencies for a moment, a 12" woofer with a displacement of around 1cm max can move about 0.7 liters of air, the widest strip of the ESL-0.88 can move 0.4 liter (198cmx11cmx2mm), the next widest about 0.38 liters. Together that is already more than a 12" woofer and I am only talking about 2 of the 9 strips yet. Granted, there is destructive cancellation of the sound wave and therefore it doesn't go lower than around 80Hz but it can move some air.

The electrocution risk is really non-existent, the powder coating isolates very well and you can lay your hand on one stator without feeling anything, don't pinch the two together though, the field will make your fingers tingle (I tried it) Laughing

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garyfritz




Joined: 08 Apr 2006
Posts: 12023
Location: Fort Collins, CO


PostLink    Posted: Tue Oct 04, 2011 7:31 pm    Post subject: Reply with quote

Ahh, I didn't understand that the spacers had varying distances. So you have different-width strips giving you different resonance points.

Sounds like a really cool design! Too bad you couldn't scale it up and become the next Martin-Logan -- but you'd kill yourself if you tried to make more than one set of these every couple of months. Laughing And I suspect the headaches of Chinese manufacturing are probably not worth it...
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beun




Joined: 28 Jun 2006
Posts: 676



PostLink    Posted: Tue Oct 04, 2011 7:52 pm    Post subject: Reply with quote

I have some ideas for a new frame design that will go a whole lot quicker but it has to be fabricated here in the US. I don't believe in all that Chinese outsourcing.

Like a manager of a large electronics firm once said "We can get nothing done a whole lot cheaper in India than we can get nothing done here"

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mightym




Joined: 04 Jun 2011
Posts: 6
Location: Okrahoma


PostLink    Posted: Tue Oct 04, 2011 11:46 pm    Post subject: Reply with quote

Kim, I'm a little fuzzy on exactly how you're stretching the membrane, and how you then maintain the tension. Would it be possible to see some more detail pictures? or how bout' a clarification from you regarding your process?

Thanks

John
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beun




Joined: 28 Jun 2006
Posts: 676



PostLink    Posted: Wed Oct 05, 2011 5:10 pm    Post subject: Reply with quote

John,

I made some closeup pictures this morning which I will upload tonight. While doing the second speaker I will make some more pictures of the initial steps as well.

Basically the steps are as follows:

1) Put the big sheet of mylar on the speaker and
2) Carefully cut out what you need with a VERY sharp X-acto knife, try not to create any nicks.
3) Start by taping the four corners and slight tighten them.
4) Start taping at the center top and bottom in the middle of a section and pull tight
5) Work your way outward in a left-right and top-bottom motion, towards the sides slightly angle your tape outward.
6) Once in a while while doing step 5 tape the sides as well again starting at the center but this time keep dividing each remaining section in two.
7) Now and then you will notice you will have to re-tighten every piece of tape because the mylar will start to stretch
8) When on the top and bottom every center of a section is taped start dividing the untaped portions of the membrane in two.
9) Keep on doing this and keep on re-tightening every piece of tape until the distance between two pieces of tape is about 3/4"
10) Use some 3/4" wide copper tape to solder the bias wire on and tape it on the membrane, make sure this tape has conductive glue.

http://www.mouser.com/ProductDetail/3M-Electronic-Specialty/1181-TAPE-1-2/?qs=sGAEpiMZZMuwwZaQzCsHbDCPeWTWqAfGO97VdYSMSwo%3d

11) If you want you can use 1/4" wide copper tape to built a complete charge ring around the edge. With a fully conductive membrane it shouldn't be needed but it doesn't hurt either.
12) Cut off the tape flush with your spacers and using 2" wide tape go over everything

As I go along I will post more pictures of this process. A word of warning, be VERY careful stretching this stuff, mylar is incredibly strong but even a small tear will kill you like what happened to me last night. It ripped right through and I had to start over.


Kim

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