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I've always respected ESLs. When time came to revamp my system, I spent a while looking into ESLs before deciding to go with ribbons & planars. I ran across several sites that may be of use to you:
for construction and other info on ESLs.
for information on Mylar (polyester-the most common membrane material) and Kapton (polyamide-able to withstand higher temperatures, though that shouldn't matter in an ESL application).
The gist of it is that you take a sheet of Mylar and lay it out on a *very* clean surface; then rub the bejeesus out of it with a cotton ball & a lot of graphite to make it conductive. That gives you a membrane. This part is pretty standard. Then you attach that to a framework and suspend wires or metal plates on both sides. This is where you can be just as crazy and creative as you want. Keep in mind that Acoustat used the white plastic grids from fluorescent light fixtures for their ESLs with wire glued to the inside. (It's cheap, fast, and looks okay, but the plastic can resonate. Thwack one of those plastic grids with your fingernail to see what I mean. If you can figure out a way to kill the 'plastic sound' you've got a winner.) Several companies have used perforated metal sheets, usually painted or coated to insulate the surface in case of accidental contact (overdriving) with the membrane. The problem here is that the holes are punched, which tends to leave small, sharp raised lips that can cut your membrane in the event that they contact one another.
There are books out there. One is by Gayle Saunders (the Martin Logan guy--I think I spelled his name right). The book is reputed to be informative, but biased (kinda like Self & Slone's books on building amplifiers). Do not expect a balanced overview of the topic with contrast and compare between different contruction methods. He shows you how to build a functioning ESL, but there is one-and-only-one-way to do it, and that's his way. Disclaimer: I have not read the book, myself.
Be aware that you're going to need an extremely stable amp, as ESLs represent a highly capacitive load & many amps come unstuck when presented with such a load...particularly at high volumes.
For a transformer, buy a tube output transformer and run it backwards in order to step up the drive voltage. Some people report having used power transformers in this part of the circuit. They claim to achieve high fidelity, but then so do Bose and Klipsch. To each his own. Bear in mind that power transformers are optimized for 50-60 Hz and act accordingly. It might be possible to do some equalizing to smooth out the frequency response should you decide to use a power transformer, but it would be easier to buy a wide bandwidth transformer in the first place.
The power supply will be an extension on the basic voltage doubler--just a string of diodes and caps (not necessarily very large caps, mind you). You can start with wall voltage and multiply more times, or use a power transformer to get up to some arbitrarily high voltage, then use a shorter string of diodes and caps to get to your target operating bias. Hint: diodes and caps are much cheaper than power transformers. On the other hand, there's virtually no current draw from a properly operating ESL, so you can get away with a small transformer.
Here's an idea that I toyed with, but never used since I went ribbon/planar: Neon signs use very high voltages. You might try to locate the transformer from a neon sign. This would give you *really* high bias voltages with next to no effort. I don't recall seeing anyone ever mention this idea before; there may be some hidden problem, but I wasn't able to see anything wrong with the concept. If you try it and it works, let me know.
One last thing. Due to the necessary high drive voltages for the speaker, it might occur to you to try to run a speaker straight off the plates of a tube amp. 6550s will easily give you several hundred volts of swing. It can be done. It is tricky, due to the capacitive loading. I've done it myself on a small scale, but again, dropped it in favor of my current setup.
I've got a fairly extensive set of notes on this if I can only remember where I put them. Hopefully, this will be enough to get you started.
Good luck.
Think safe thoughts while working on the bias circuit.

follow up to els,

You ever find what you asked for? In the last 3 years there's been a lot show up on the net. How did your plans go? The most appropriate film looks like the film offered at, Their's is the thinest I've heard of.

I'm in the design stage myself. Besides the design of what I think may work, there is still the problem with the output transformers, which seem to be the real stickler in the last 3 years too. It appears that a lot of wrangling, nearly arguing, have been going on the in forum about transformers, without resolve.
I actually seen a hand wound transformer on the net, but it looked huge, and I bet you may have passed beyond this stage by now.

Blow up any amps? I hope my HarmonKardon has the billoks.

Well, I thought I'd ask!
I've actually got a pair of ML Ascent panel from a friend, so I'd just need to build the mdf box and the electronics.
I've been asking for some specification on the audio transformer used by ML and the exact voltage for the film without result so far.
I'm going to buy the transformer and high voltage parts from just real music. But that will be to after the summer.

Good luke with you building..

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