2) Materials Required to Make Electrostatic Speakers

by Mark Rehorst

November 2006

Here's a descriptive list of the materials and some sources for them.  You can literally use almost any plastic for the insulators, any film for the diaphragm, and any metallic thing that full of holes for the stators and your speakers will make sound.  If you want to experiment make small drivers using everyday materials you have on hand.  Once you have some experience handling the materials you can order the exact stuff you need to make that super high performance driver.

You'll need some tools, of course - saws, screw drivers, paint brushes, drills, soldering iron, and etc.  Nothing exotic.

A lot of the materials come in quantities much larger than you'll need to make a couple speakers.  You can minimize waste by talking some of your friends into making some speakers with you.  Then you can split the cost of some of the items and save some money.

1) Audio Transformers- one or two per speaker. 

There are two ways to go.  You can get audio transformers made for vacuum tube amplifiers or specifically for ESLs, or you can use low voltage toroidal power transformers.  The power transformers are MUCH cheaper - you can often find suitable transformers at surplus shops for $5-10.  Look for transformers with dual 115VAC windings and a low voltage winding such as 3-9V.  Alternatively, look for any 30-100VA rated toroid with 115V or 230V windings that is not potted (embedded in plastic resin) and add your own low voltage windings by simply wrapping a few turns of wire through the core.

Transformers made for audio use may ultimately provide better performance, but cost a lot more.  If you want to use audio transformers, look for tube amplifier output transformers for tubes such as 7189, 6BQ5,6GW8, 6V6.  They will have low impedance windings (4 Ohms) to connect to speakers (you will connect them to your amp) and high impedance windings (8-10K Ohms) to connect to the tubes (you will connect them to your speakers).  The transformers should be rated for around 30W or so at 30 Hz.  They will have to be made for use in push-pull amps (such transformers are center tapped) or you will need two transformers per speaker.  Tube amp output transformers are available from Antique Electronic Supply, and other sources. Expect to pay at least $50 each. 

Transformers made specifically for ESLs are also available but they tend to be VERY expensive.  Check suppliers like Amplimo, Plitron, or Sowter.

2) Plastic Film for Speaker Diaphragms- large enough to make the size of driver you want to build, plus at least 6" extra all the way around to allow for a mechanical stretcher.

If you just want to make sound, you can use almost any plastic film you can find, including Saran Wrap.  If you want to make hi-fi speakers, thin Mylar or other polyester (5-6 microns) is the way to go.  It can be obtained from companies that make plastics for industry- this film is commonly used to make capacitors (don't get metalized film!).  I bought a roll that is 1200 m long by 1 m wide for about $85 in Japan about 15 years ago.  It can be hard to find, but there are some hobbyist sources and you can even get it from McMaster-Carr.  If you buy stuff specifically for ESLs, expect to pay too much.  Buying stuff specifically for ESLs is like buying parts for Ferraris.  The seller knows you expect to be robbed so they try not to disappoint you.

3) High Resistance Coating for the Diaphragm.

I used to recommend things like powdered graphite to make the diaphragm slightly conductive.  It is still a good, cheap option if you are just trying to make a science fair project, but you cannot get very high resistance using graphite.  It will work, but at very low frequencies you can expect the speaker to distort a bit.  Powdered graphite for lubricating locks is available from your local hardware store for about $2 for enough to make about 50 speakers. Graphite has to be rubbed into the film using cotton balls.  It is a messy and tedious process.

If you want to make a hi-fi speaker that you will use for years, use Licron.  It is a "permanent" spray-on anti-static coating that adheres to low surface energy plastics such as polyester.  It provides a much higher resistance than you can ever hope to achieve with graphite and it is 100X faster and easier to apply.  It costs about $40 for a 10 oz spray can but you'll be able to make more than 100 speakers with it.  It is NOT clear, and tends to make the diaphragm look cloudy, so if your diaphragms will be visible, Licron may not be the way to go.

I have seen others claim that diluting white glue about 1:5 and doping it with a few ions (antistatic fluid or some salts) makes a very high resistance diaphragm coating that sticks to polyester.  I have not tried it yet, but if it works it will be an ultra cheap way to coat the diaphragms.  They tell me it is practically invisible and weighs less than a Licron coating, too.

4) Perforated Aluminum or Steel for the Stators

You need one piece for the front and the back of each driver. It should be flat and have about 60% or more open area (holes). Hole size? The stuff I use has holes that are about 3 or 4 mm diameter. The "rules of thumb" say don't use holes larger than about 1/4". Check your local Yellow Pages phone book for listings under Perforators, or Sheet Metal.  Your local hardware store may have some available also.  Aluminum is much easier to cut than steel, and it is much lighter weight, but costs more than steel.  If you buy from a perforator you can get them to cut the metal to size and roll it flat for you.  You can order perforated sheet metal (and most of the other materials to make ESLs) from McMaster-Carr.

You can use aluminum window screen glued to a support frame such as a plastic fluorescent lamp grid.  Some people make stators by wrapping wire around a wood or plastic frame.  I don't think it provides better sound and it is a lot more work, but it looks different from perforated sheet metal and that is sometimes reason enough to go to the extra trouble.

5) Insulating Material for the Driver Frames- this is used to hold the two stators apart.  The diaphragm will be sandwiched between and attached to the insulators.  For most ESLs, you need insulators about 2-4mm thick.

Use PVC, acrylic, fiberglass PCB stock, or almost any other insulating material you can get readily and cheaply.  Fiberglass is hard to cut (you need a carbide saw blade), and the dust from sawing is a health hazard, but epoxy will bond to it and the stators very well.  It will make a very stiff driver, and stiff is good. 

Acrylic or other plastics are easier to work with, but you may have some trouble gluing the stators to it (try contact cement). I have used both acrylics and PC board and for all it's trouble, I prefer the PC board material.  You can get fiberglass from a PC board company- try to raid their scrap pile- and get them to cut the pieces to size for you.  We'll talk about thickness later.

6) Adhesives - this is the trickiest part of making ESLs.

You will have to do some experimentation and research to select adhesives that work with the materials you use to make your speakers.  Epoxy will bond to metal stators and to fiberglass insulators, but not to other types of plastic, and forget about using it to attach the diaphragm to the insulators - it will not bond to polyester at all.

Contact cements are quite versatile and the right one can be used to bond almost everything in an ESL.  One that I have had a lot of success with is 3M  Scotchgrip 4693 or 4693H.  It is formulated to bond with low surface energy plastics such as polyester.  You put a little on one or both surfaces to be glued and let it stand for a few minutes. Then you put the two surfaces together and it bonds instantly.  The bond is so good that the film will tear long before the glue lets go.  The only disadvantage is that once you've assembled the driver using contact cement, it is hard to tear it apart and rebuild it, should the need arise.  You can get 4693H in 5 oz tubes from McMaster-Carr for about $8.  One tube will be enough to make several large speakers.

7) High Voltage DC Bias Supply (1000-5000VDC, almost no current).  One for each speaker you build.

I recommend using a DC-DC converter that uses a low voltage input supplied by a wall-wart.  Such converters are usually very small and very safe because they can't supply much current.  They often provide output voltage that is proportional to input voltage so you can adjust the bias to suit your speakers.  See the bias supply design page for more details.  Several companies make the DC-DC converters or you can use a surplus power supply from a copy machine, though they are usually larger and not as flexible in terms of output voltage adjustability.  Small DC-DC converter modules come from companies like Emco High Voltage and Pico Electronics.  Check on-line electronic surplus dealers (Herbach & Rademan, All Electronics, Alltronics, C&H Sales, Electronic Goldmine, and etc.) for copy machine supplies.

8) Metal Tape - to connect the bias supply to the diaphragm.

Use about 2 feet of copper tape or other metal foil tape.  You can get it at McMaster-Carr.

9) Optional- insulating coating for the stators

If you use very high bias voltage to make your speakers sensitive, you will sometimes hear whining due to corona discharge around the sharp edges of the perforations in the stators.  This can be reduced by insulating the stators.  I've heard that latex house paint works fine, but be careful not to plug up the holes with it.

10) A Diaphragm Stretcher - build it yourself

You need a bicycle tire tube, a piece of plywood a little bigger than your drivers will be, some wood strips to screw to the perimeter of the plywood, and some double-sided adhesive tape.

About 18 years ago, when I made my first ESLs, I invented a very simple diaphragm stretcher.  It uses common materials and is very quick to build.  Best of all, it will put as much tension on the film as the film can handle and stretches the film more or less evenly.  It is so simple its a miracle that no one thought of it before me.  You simply stretch a bicycle tire tube around the perimeter of a table.  Next, you apply double sided adhesive tape to the underside of the table, then lay the diaphragm film out on the table and fold the edges under and attach them to the tape.  Finally, you inflate the tube.  It works every time. If your drivers have a perimeter greater than a tire tube, just stretch the tube until it fits.

You'll find more information about making the stretcher is on the Diaphragm Stretcher page.




exploded view of an ESL
 

Exploded view of an electrostatic driver.  Blue are stators, green are insulators, orange are copper traces on PCB or metal tape, pink is the diaphragm.

 

Comments or questions about this article should be sent to

The views, opinions, and errors expressed in this article are entirely my own.

1) Principles of ESL operation

2) Materials Required

3) Diaphragm Stretcher

4) ESL Driver Construction

5) Support Frame Construction

6) Bias Power Supply Construction

7) ESL Bibliography and Links

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