What's new

Oliver Acoustic Research AR-94 specifications
Daniel Azziorri RCA new topic
Roy A. Esposito Acoustat topic reply
Joe Kasperowski Acoustat topic reply
Ed Optimus topic reply

Introduction

Mark was kind enough to allow us to publish some of his articles on building ESL's on our site. Current version date: 6/7/96

Electrostatic speakers are the just about the lowest distortion drivers that can be made. But you already know about their wonderful attributes or you wouldn't be interested in making them, so I'll dispense with the BS. I present here a simple process for making ESLs. I have not included anything about crossovers or cabinets. This is strictly a "how to make the drivers" article.

Advertisement

Warnings

Before we go any further, I want to warn you about a couple things you may not be aware of. Electrostatic loudspeakers use high voltages to operate. They need a DC bias of up to 5000 V and use AC voltages up to 5000 V. The DC bias is usually supplied by a power supply running off 120VAC electrical circuits which can be dangerous. The AC voltages used to drive the ESL are usually produced by connecting your stereo amp to a vacuum tube amplifier type output transformer. The voltages produced by the transformer are dangerous! Don't screw around! If you have little kids in the house or if anyone might for any reason touch the speakers while they are operating, design your speakers so that it is not possible to come into contact with the drivers. If you don't know how to handle high voltage circuits, enlist the help of someone who does, or buy one of the commercially available ESLs.

What you need

Building ESLs involves the use of tools and materials that if handled improperly can be hazardous. Please make sure you know how to use these things before you begin. By all means, use safety glasses at all times. If would be foolish to trade your vision for the pursuit of audio ecstasy!

What you need:

Transformers

One or two per speaker - use tube amp output transformers, 4 ohm:8K -20K ohm. I have used Tango CRD-8 ( 4:8KCT) transformers that I bought in Japan. You can use transformers by Triad, Stancor, etc. Just get units that are good for about 15-20 W at 30 Hz and give a large impedance (i.e. voltage) transformation. Expect to pay about $50 each for transformers. Tube amp output transformers are available from Antique Electronic Supply, 602-820-5411, and other sources.

Plastic film for speaker diaphragms

Mylar or other polyester, thin (5-6 microns), and large enough to make the size of driver you want to build. This 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 a few years ago. I have used about 15 m of it so far. I have heard of people using Saran-wrap, but I have never heard a driver built using it. If you're making small drivers, or experimenting, try it! It certainly won't cost much...

Powdered graphite, dish soap, or antistatic solution to coat diaphragm

Powdered graphite is available from K-mart or your local hardware store for lubricating locks. It will cost no more than $2 for enough to make about 50 speakers. Graphite has to be rubbed into the film using cotton balls. Dish detergent and antistatic solution will work also, and are easier to apply, but may not be "permanent". I use graphite. Someone in Australia suggested that drafting ink formulated for drawing on "film" (the draftsman's name for polyester) will make a good, easy to apply, high resitvity diaphragm coating. I haven't tried it yet, but applying a colored liquid ought to be easy and make it easy to verify that it only went where you wanted it.

Perforated aluminum or steel

You need a piece for the front and the back of the 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 may cost a little 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.

Acrylic or fiberglass PC board stock for driver frame

Fiberglass is hard to cut (you need a carbide blade), and the dust from sawing is a health hazard, but epoxy will bond to it. Acrylic or other plastics are easier to work with, but epoxy may not form much of a bond to them (contact cement will probably work just fine). 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.

Glue

Previously I recommended epoxy to hold the ESL together. Epoxy works fine for attaching the perforated metal to the insulator frame. The problem with epoxy is that it doesn't really bond to the mylar film. A little mechanical stress can break the very weak bond and allow the film to peel away. This can be an advantage. If you find that a driver doesn't work, if you assembled it with epoxy it will be easy to rip apart and rebuild. I have done some additional research and found a contact cement manufactured by 3M that works for attaching the film to the insulating frame. Scotchgrip #4693 is the stuff to use. You put a little on one or both surfaces to be glued and let dry for 10-20 minutes. Then you put the two surfaces together and Voila!, instant bond. The bond is so good that the film will tear long before the glue lets go. Other contact cements may work well also. The only disadvantage is that once you've assembled the driver using contact cement, you have to live with it. If the driver doesn't work, you'll have to build another because you won't be able to tear the old one apart.

High voltage DC bias supply (1000-5000VDC, almost no current.)

This can be made as a voltage multiplier that works off the power lines. You'll need high voltage diodes and capacitors, a few resistors, a circuit board and a line cord. You can get away with one supply, but one for each speaker is easier to deal with- you won't have to run high voltage wires all over your listening room. See the Bias Supply section at the end of this document.

Optional Plastic coating for the perforated metal. I've heard that latex house paint works fine...

Figure 1. Exploded view of a basic electrostatic driver.

Making the drivers

Step 1. Design your drivers

Decide on the size and make the frames for the drivers. It is generally easier to make small drivers than to make big ones, but with small drivers you will need a lot of them so mounting them can be a pain. You need one insulator for the front and one for the rear of each driver. Ideally, the insulator frames should be cut from a single piece of insulating material. But they don't absolutely have to be made from a single piece. Be sure to plan and leave room for electrical connections (3 wires per driver) and mechanical mounting. I have built many drivers using different geometries and found that the following thicknesses and bias voltages will result in drivers that closely match the sensitivity of conventional boxed bass drivers without the addition of a lot of attenuation in the low frequency section of your crossover:

ESL use total ESL area DC bias insulator thickness
mid/tweet >2 ft2 1500 V 1/16"
full range >4 ft2 3-5000 V 1/8"-1/4"

The insulator thickness to use is a function of many variables. If you want to reproduce low frequencies (down to 100 Hz or lower) you need to have room for the diaphragm to move. That means thick insulators. You will also need to use high bias voltage and high driving voltages (two transformers) to get reasonable sensitivity.

The mechanical force on the diaphragm varies as the square of the distance from the stator plates. That means that if you double the thickness of the insulators, you need to use four times the voltage for equivalent acoustic output. It isn't easy to make full range ESLs, and they almost never deliver enough bass. You need really huge surface areas to get bass, but that increases the capacitance of the driver and can limit high frequency response. You can improve the bass by using electronic equalization and mounting the drivers in the corners of a room. There is plenty of room for experimentation.

Another benefit to using PC stock is that it is usually metalized on one or both sides, a feature that can be very useful when making electrical connections to the drivers. It will be best to have one insulator frame metalized on both sides, and the other metalized on one side, but we can make due with any material, even unmetallized.

There is a "rule of thumb" about the dimensions of an ESL that relate to the insulator thickness. The rule is that the diaphragm should be supported at least every 100X units, where X is the thickness of the insulator pieces. 'Supported' means that you should put insulating strips in the driver to support the diaphragm in at least one direction. 'One direction' means that long narrow drivers are OK. If you use insulators that are made from 1/16" PC board stock, the diaphragm should be supported every 4-6 inches. If you look at Martin- Logan ESLs you'll see they have support insulators every 4-6 inches and that they are unevenly spaced, presumably to move resonances of each section to different frequencies.

Figure 2. One way to make the ESLs showing the use of PC board stock.

Drawing not to scale. Electrical connections are soldered to the copper pads labeled "A", "B" and "C". Be sure to leave room for hardware to mount the driver to some sort of frame.

Step 2. Electrical connections

You will need to make an electrical connection to the diaphragm. This can be done in any number of ways, but remember that you must maintain a high voltage potential between the metal plates and the diaphragm. That's why we were careful to vacuum up the graphite powder. You may want to clean the insulators with alcohol and a very clean cloth before proceeding.

The electrical connection is made by physical contact between a metal strip and the graphite coated surface of the diaphragm. The metal strip may be the copper on a piece of PC board stock used for the insulator (very rugged and solderable), or it can be a piece of aluminum foil, or Radio Shack burglar alarm foil tape (both somewhat delicate and not solderable). Just remember that you have to be able to connect a wire from the HV bias supply to the metal. Also, epoxy is generally not electrically conductive (there are conductive epoxies available, but they are usually quite expensive), so don't completely cover the metal with epoxy.

Here is a tip to help insure long life for your ESLs. When you connect DC bias to the diaphragm, connect the minus side of the bias supply to the driver and the plus side to the center tap of the driver transformer. If you connect it the other way around, you'll find that over time the metal electrode that connects to the diaphragm will corrode like the plus battery contact in your car.

Step 3. Stretch, coat, and attach the diaphragm to the insulators

Stretching the diaphragm can be accomplished in two relatively easy ways. One way is to use a heat gun to shrink the diaphragm after it has been attached to the insulators. People have reported good results using this technique, but I haven't tried it.

I use a stretcher table of the type shown in figure 3. The table allows you to coat the diaphragm under full tension and allows you to make multiple drivers with nearly identical resonances (by inflating the tube to the same air pressure for each driver). To use it you lay the film on the table and use double sticky tape to attach the edges of the film to the underside of the table. You then pump a few strokes of air into the inner tube and watch as the wrinkles in the diaphragm disappear. You can put extreme amounts of tension on the film using this table, so be careful. Make sure you put a small hole through the table top surface to allow air trapped under the diaphragm to escape when you start pumping!

Figure 3. View of the underside of the diaphragm stretcher table.

Figure 8

Figure 8. Cross section of the diaphragm stretcher table.

The film is laid on the top side of the table and the edges are folded to the underside and secured with double-sticky tape attached to the inside of the table edge. Inflating the tube stretches the film tight. A rectangular table works just as well as a round one and is probably easier to make.

How much tension is enough? That's a difficult question. The tension you use is a balancing act. It depends on the bias voltage you will use, the thickness and spacing of your insulators, and on the frequency range over which you intend to operate the driver. Usually you will want to operate the driver above its fundamental resonant frequency. If you want full range operation, that means you want the resonant frequency to be below 100 Hz or so. That requires low diaphragm tension but low diaphragm tension means you may have to use a reduced bias voltage or you may have the driver break into a low frequency oscillation where it pulls to one side, sticks until the diaphragm is discharged, then returns to the center until the diaphragm charges up again, etc., etc.

In reality the amount of tension you use isn't critical. Rectangular drivers have multiple resonances and you will always have some of them in your pass band. I have never been able to identify any of them by the sound of the driver when running test tones through it, and certainly never when listening to music. It may be possible in an anechoic chamber or by using a FFT analysis of impulse response, but in your listening room there will always be room mode resonances and multipath effects that will dwarf the driver resonances. If the tension proves too low you can always reduce the bias voltage.

OK, so you have the diaphragm under tension on the table. Now what? Time to put the resistive coating on the diaphragm. First put the insulators in another room. Then place a little (very little!) graphite on the film and grab a clean cotton ball and start rubbing the graphite into the film. Rub it in hard. Add more graphite as needed. You really don't need to use much. You want the film to be coated with the stuff so that it has very high resistivity. It's really not critical. After you have rubbed the graphite in, grab some clean cotton balls and rub some more. You can measure the resistance of the film by dropping a couple pennies on it a few inches apart and checking the resistance between the pennies with a DMM. You want a high but measurable resistance. Move the pennies around and check a few places. If you get resistances on the order of 100K or more, you've done a good job. If you measure lower resistances, rub with clean cotton balls some more. Get out your vacuum cleaner, put a brush attachment on it, and vacuum the entire surface of film that has been coated and the area where you were using the graphite. Now wash your hands very thoroughly! Then wipe the insulators with alcohol and a very clean rag to make sure they are absolutely clean before proceeding.

Why is the resistance important? Sooner or later, a bug will get into your speakers, or you will crank the volume a bit too high and your speakers will arc. If you use a metalized diaphragm (low resistance) there is a good chance that the entire diaphragm will flame out and you'll have to rebuild the speaker (but it'll impress your friends!). If you use a high resistance coating, the amount of current available to the arc is very small, resulting in a low temperature arc that will at worst put a pin hole in the diaphragm. High resistance coatings that I've tried do not cause the normally self-extinguishing polyester diaphragm to become inflammable. This is another reason for using a very large resistance between the diaphragm and the bias supply.

If you feel that you really need extremely high resistance for your speakers, try using dish detergent or antistatic solution to coat the diaphragm. I have built drivers using all three coatings and find no audible differences between them (but maybe your ears are better than mine).

Attaching the diaphragm is easy. You simply put glue (Scotchgrip #4693) on one of the insulators (again- don't completely cover the metal) and place it, glue side down, on the coated film. The bond forms instantly, so make sure you set the frame down on the diaphragm exactly where you want it. Once the glue has set (after about 10 microseconds), let the air out of the tube and cut the film away from the table along the edge of the insulator. Now turn over the insulator/film assembly and set it back down on the table, diaphragm up. Coat one side of the other insulator with glue, wait about 10-20 minutes, then set it glue side down on the insulator/film assembly. Be sure to align the two parts carefully before pressing them together- you don't get a second chance. You might consider building some sort of fixture to ensure accurate alignment.

Now you can epoxy the perforated metal sheets to the insulator assembly. The perforated sheets are made by running a roller with metal pins over the sheet metal. That leaves the edges of the holes on one side rounded and the edges on the other side sharp. Put the rounded edge side toward the diaphragm. Epoxy the stators one at a time and be sure the epoxy has time to set before you pick up the assembled driver.

I have done some experiments aimed at rounding the sharp edges of the holes. One of the things I recalled from high school chemistry experiments is that corrosion of metals occurs fastest at points of stress and sharp edges. I tried using ferric chloride PC board etching solution from Radio Shack. Since aluminum is more 'reactive' than copper I had to dilute the solution by cutting it with water at about 1 part FeCl to 4 parts water. This kept the speed of the reaction slow enough to allow me to observe progress of the reaction and remove the aluminum when the edges were rounded. If you try this, be sure you dilute the FeCl and then put a small scrap of aluminum into the solution to test it before you put in the pieces you will use for your speakers. If you don't dilute the solution you'll end up with a bad smelling, boiling mess!

Step 4. Testing

Stand the driver up using styrofoam blocks to insulate it or hang it from a frame using nylon cord. Connect the transformer(s) to the driver per figure 4. Next, connect the bias supply wires to the transformer and the driver. Power on! If all is well you should hear a very quiet click or nothing at all.

Figure 4. Diagram showing electrical connections to the ESL.

Warning: the voltage output from the transformer is high enough to hurt you! Be careful!

You may hear a whining sound. This is due to corona discharge which you may be able to locate by turning off the lights and looking closely at the driver. Once your eyes have adjusted to the dark you may see faint blue sparks, probably coming from edges or pointed areas of metal. The cure is to reduce the bias voltage, or apply some insulating coating (finger nail polish works) to the point where the discharge is occurring. This problem can be avoided almost entirely by plastic coating the stator plates before assembling the drivers. If you coat them with plastic, make sure you leave some provision for making electrical connections to the metal.

The other thing you may see/hear is the diaphragm flapping back and forth because of insufficient tension. This can be cured in two ways. Lower the bias voltage or replace the diaphragm using higher tension. You may try using a heat gun to shrink the film more and put more tension on it before you rip it apart to replace it.

If the driver sits quietly, connect the output of your amplifier to the 4 or 8 Ohm taps on the transformer. At this point I cannot stress enough that you should never, ever, under any circumstances touch the driver while it is operating. You will receive a severe shock, and you will suffer burns from the tremendously high voltages produced by the transformer that drives the ESL. I have experienced this and can tell you that it hurts like hell (and stinks)! Don't do it!

Turn on the amp and play a CD. Turn the volume up slowly. You should hear very low distortion music with little bass content coming from the driver. If not, turn the volume up. Sometimes the connection to the perforated aluminum is poor due to oxide on the surface of the aluminum. As you turn the volume up and the driving voltage gets high enough, it will arc through the oxide layer and suddenly you will hear the music very clearly. The newly "cleaned" connection will work virtually forever after this first "burn-in".

That's it. Wasn't that easy?

Consider this: The high voltage used to "energize" the speakers causes them to attract dust. When you're not using the speakers, you may want to turn off the bias supply to minimize this effect. You should also put a brush attachment on your vacuum cleaner and clean both surfaces of each speaker once in a while.

Roger Sanders' article included an equalizer circuit to increase the low frequency output of the drivers. The circuit amounts to bass boost similar to what you can get by using the tone controls on your preamp. Sanders suggests that even in hybrid systems, the equalization is necessary to keep the speakers from sounding too weak on bass to lower midrange frequencies. I have used the equalizers and operated without them and find that the sound without the equalizer is satisfactory. You may want to try using the drivers without equalization first, then add the equalizer if you think the bass/lower midrange is weak.

What I am about to say will be regarded as heresy but before you believe what you hear from people who claim to know everything about everything (I'm not refering to Roger sanders here), remember all the suffering that has occurred throughout human history because people blindly followed what they were told by such self-proclaimed experts. Here goes: If you want to make a hybrid system and you already have some speakers that provide reasonable bass, try using them with the ESLs before you blow big bucks or go to a lot of trouble making bass boxes. You may find that the speakers you have will work well enough, saving you a lot of money/time/effort.

Making the ESL Bias Supplies

The ESL needs a high voltage DC bias supply. Generally speaking, the higher the bias voltage you use, the higher the sensitivity of your speaker. However, there are considerations beyond speaker sensitivity. If your speakers do not have plastic coated stators, then 1500-2000 Volts is about the highest voltage you will want to use, regardless of insulator frame thickness. Higher bias than that leads to corona discharge and its attendant whining sound. If your speakers use plastic coated stators, you can probably use higher voltages, but that will depend upon the insulator frame thickness also.

If you use 1/16" thick insulators, then try 1500-2000 Volts first. If your insulators are 1/4" thick, and the stators are plastic coated, you may be able to use 5000V bias. For headphones, where the drivers are practically glued to your ears, 500- 750V is probably adequate bias.

Bias supplies are generally made using a transformer to step up your local power line voltage to 500- 1500V, then converting to DC using a voltage multiplier/rectifier circuit. Voltage multiplier circuits are used in almost everything that uses high DC voltages. You can find a good description of the operation of such circuits in any edition of the ARRL Radio Amateur's Handbook.

Figures 5, 6 and 7 show schematics of voltage doubler, tripler and quadrupler circuits. The names doubler, tripler and quadrupler come from the fact that the output voltage will be approximately 2, 3 or 4 times the PEAK level of the AC input voltage. Transformers are usually rated in terms of rms voltages.

Vpeak = 1.414 x Vrms

Figure 5. Full wave voltage 'doubler' circuit

Figure 6. Voltage tripler circuit

Figure 7. Voltage quadrupler circuit

If you have a transformer that is rated for 117 VAC in and 700 Vrms out, the DC output voltages you will get from a doubler, tripler and quadrupler circuits will be 1980, 2970, and 3960 VDC.

The components used in the voltage multiplier circuits must be capable of withstanding these high voltages, so shop carefully! Each speaker needs high voltage bias, but you really only need one bias supply. If you're on a budget or not sure you want to go whole-hog into this project, start with one bias supply. You may find later that it's more convenient to use two bias supplies (that way you don't have to string high voltage wire all over your listening room).

There are a few things to keep in mind when shopping for bias supply transformers. First, you don't need any appreciable amount of current, so get the physically smallest, lowest cost HV transformer you can find. Don't use neon sign transformers (usually too much voltage and always too much current). You can steal transformers out of old vacuum tube electronic junk, or buy them for a few dollars from companies like Antique Electronic Supply or Fair Radio Sales. If you can't find the exact voltage you want, don't worry about it- you can use the voltage multiplier circuits to get you where you want.

There are alternatives to the transformer/voltage multiplier circuit. Sometimes you can find DC supplies from photocopiers available in the surplus market. Sometimes they are even adjustable. They usually put out higher voltages than needed, but that is easily taken care of by adding a resistive voltage divider to the output.

Sources for HV parts, power supplies, etc

Alltronics

  • Name: Alltronics
  • E-mail: mark@rehorst.com
  • Homepage:
  • Address: 2300 Zanker Rd. San Jose, Ca. 95131, USA
  • Tel: +1 408-943-9773
  • Fax: +1 408-943-9776

C and H Sales Co

  • Name: C and H Sales Co
  • E-mail:
  • Homepage:
  • Address: 2176 E. Colorado Blvd. P.O. Box. 5356 Pasadena, Ca. 91117-9988, USA
  • Tel: +1 213-681-4925
  • Fax: +1 818-796-4875

Fair Radio Sales

  • Name: Fair Radio Sales
  • E-mail:
  • Homepage:
  • Address: P.O. Box 1105 1016 E. Eureka St. Lima, Oh. 45802, USA
  • Tel: +1 419-223-2196
  • Fax: +1 419-227-1313

Herbach and Rademan

  • Name: Herbach and Rademan
  • E-mail:
  • Homepage: 18 Canal St. P.O. Box 122 Bristol, Pa. 19007-0122, USA
  • Address:
  • Tel: +1 215-788-5583
  • Fax: +1 215-788-9577

Don't omit the resistor at the output of the supply! It is there for three reasons. First, it insures that you can't get enough current out of the power supply to hurt you. Second, it helps force the speaker to operate in the "constant charge" mode. Third, it helps minimize damage to the speaker in the event the diaphragm comes into contact with one of the stators.

Comments or questions about this article should be sent to mrehorst@fmi.fujitsu.com. The views, opinions, and errors expressed in this article are entirely my own. Mark Rehorst.

New ESL Bias Supply Design

2/24/99 M. Rehorst

I have always disliked the idea of having a high voltage transformer in the bias supply. Even a low current transformer can be quite dangerous, even for experienced people. Some recent, unpleasant experience has prompted me to come up with a design for a safer bias supply that is very small and easy to build. The cost is a little higher than using a transformer and voltage multiplier but if it saves you from an unpleasant shock, it is cheap at 10x the price. As an added bonus the output of the new circuit is adjustable over a very wide range.

The new circuit consists of a 12-15V, >100 mA wall wart type power supply, a low voltage adjustable regulator, and a DC to high voltage DC converter module made by a company called Emco High Voltage.

The heart of the circuit is the DC to high voltage DC converter module. It is very small, and fully encapsulated with pins for circuit board mounting. I used a G-40 module which cost about $60. This module is rated at 4000VDC out (at 250 uA) when 12VDC, 100 mA are applied to the input. Emco has modules in this series that go as high as 6000VDC out. Specs for this series of modules can be found at http://www.emcohighvoltage.com/emcoindex.htm

The output voltage is approximately proportional to the input voltage so by using an adjustable regulator circuit ahead of the module you can make a bias supply that can be varied from about 500V to 4000V. Adjustable bias can be useful if you find that you didn't quite put enough tension on the diaphragm of your speakers or if you have problems with corona discharge (I mean the whining sound that can sometimes come from the speakers, not excessive urination after drinking too many Mexican beers).

The adjustable regulator is made from common components including an LM317T 3 pin adjustable voltage regulator IC. The complete schematic of the supply appears below, along with a PC board layout. The dimensions of the board are 1.75" x 4".

You still need a HIGH VOLTAGE RESISTOR at the output of the supply to ensure constant charge (thus low distortion) operation of the speakers. High voltage resistors are not the same as your standard, run of the mill carbon or metal film resistors. Standard resistors will slowly self-destruct (open) if you apply too high a voltage to them (anything over 500V should be considered too high). High voltage resistors are available from Caddock (try MX-431-20M-10%; go to www.caddock.com) and Micro Ohm (try RG-3H-U-20M-M; go to www.micro-ohm.com). Surplus dealers may occasionally have them also.

Figure 8. ESL bias supply schematic (click to enlarge)

The 1 uF capacitors are tantalum electrolytic type. The 10 uF is an aluminum electrolytic. Watch the polarities when you install them. The negative side connects to ground.

The 590 Ohm resistor is a 1% metal film unit, but feel free to substitute a lower tolerance carbon resistor. 590 Ohms is about what is needed for 12V maximum output. If you use a little bigger resistor, the maximum output voltage will go down slightly. The approximate maximum output voltage can be calculated from the following formula: Vout = 1.25 x (1 + R2 / R1) + 0.00005 x R2 R1 is the resistor connected to the regulator output pin. R2 is the potentiometer. When you wire the pot, be sure to short one end of the pot to the wiper contact. If the contact fails, you will still have 5k in the circuit and everything will still work.

You can apply any voltage from about +20 to +10V to the adjustable regulator and get some output from the bias supply. The 15V shown on the schematic is a nominal value and will allow you to swing the bias supply output voltage over a range of about 300 to the maximum output for the module you are using. The regulator won't spit out more than about 12V so it will not damage the DC to DC converter module no matter what voltage you apply to the regulator input. Don't apply more than about 20V to the regulator input unless you provide a heatsink for the regulator chip.

Figure 9. ESL bias supply board layout (click to enlarge)

Figure 10. ESL bias supply parts placement (click to enlarge)

I have set the board up so that you can use two high voltage resistors so you can run two speakers from one bias supply. You can also just build one supply for each speaker if you don't want to run high voltage wires all over the room.

Questions and Answers on DIY ESL's

Question I read your article on how to make electrostatic loudspeakers and I had a couple of general questions. I found the article really fascinating! From your experience, I was hoping you could estimate a couple of things. First, to make a speaker that stands about 4 feet tall, approximately how much money should I set aside for the project of making 2 speakers?

Answer The biggest cost is the audio transformers used to drive the speakers. Expect to pay about $70-100 each if you buy them new. You will need either two or four - four provides higher speaker sensitivity and generally works better... If you use perforated aluminum you may run into a total cost of about $60-90 depending on where you buy and what you buy. Steel will cost about 1/3-1/2 of what aluminum costs. Most of the rest is pretty cheap stuff you can get at home depot...

Question For a first attempt at making these speakers, what would you say the success rate would be (i.e. 75%)?

Answer This is entirely dependent on your personality. If you take your time, think things through, plan, test, and learn from errors, you'll be 100% successful at your first attempt. Even if you're sloppy and impatient, your first attempt will produce some sound which will probably inspire you to try again.

Question Approximately how much time should I set aside? For example, did it take you a couple of days or a couple of weeks (assuming I have all of the necessary materials). Again, I know that you can't tell me exactly what to expect. But, I would appreciate any information that you would have based on your own experience.

Answer It depends on many factors. Preparing all the materials is what takes the most time. You need to cut insulators accurately and "dry-fit" them to make sure they will fit your perforated panels. You need to gather adhesives and plastics, plan and build some sort of frame to hold the drivers. It could take months, or only a week or so. Actually assembling the electrostatic panel is the quickest part of the whole project. I have found that I can assemble a complete driver (almost any size) in about 30-60 minutes, provided I have all the materials cut and clean and ready to go. I have a few more recommendations...

First, make your speakers from multiple smaller panels. It is difficult to put sufficient tension on the diaphragm of large panels. I would recommend that if your drivers will be 4 feet high and say 2-3 feet wide, you split them into 4 panels each. This will allow you to make a pneumatic stretcher table so you can put very high tension on the film (MUCH higher than you can achieve with a heat gun). I recently bought some Quad ESL63s and opened them up. Their high sensitivity is achieved by putting very high tension on the diaphragm (almost to the breaking point) and using very high bias voltage (5.75kV). The ESL 63s have 4 panels in each speaker, measuring about 24" x 9" (I don't have exact dimensions with me).

Regarding the bias supply...

I recently had a nearly tragic accident while working with a transformer to make a bias supply. I have decided that it really is best to use something safer. I bought a DC-DC converter module from Emco High Voltage that is perfect for biasing ESLs. The output voltage is proportional to the DC input, so you can easily make an adjustable bias supply- very nice if you discover that you didn't put quite enough tension on the diaphragm, of if you get corona discharge from sharp points on you perforated metal- you can just reduce the bias voltage a bit. The module I bought puts out a maximum of 4000V at very low current when 12V, 100 mA is input. I built an adjustable voltage regulator so I can vary the input voltage from about 3V to 12V which varies the output from the converter from about 500v to 4 kV. Now I use a 15VDC wall-wart to power the bias supply. One of these days I'll update the ESL article with the variable bias supply design. The DC-DC module is about 1.5"x1.5"x.75" and costs about $60, but if it saves you from the experience I had, it worth 100x that price. The product line includes modules that will squirt out up to 6kV for a little more money. You still need a large series resistor to force the speaker to work in constant charge mode.

Question Thank you so much for responsing so quickly to my note. I appreciated all of the information that you gave me. This really is an exciting process, but a little frustrating trying to find all of the necessary pieces. For example, when trying to find the mylar, they specify it in mils. The thinnest that I can find is 1 mil. In your article, you specified a thickness of 5-6 microns. I was told that 1 mil = 25 microns, or 5 microns would be 1/5 of a mil. Where were you able to find such thin mylar? Is 5-6 microns correct?

Answer Microns is correct. The stuff is almost lighter than air. The film I use was manufactured by Toray, a Japanese plastics company that makes the stuff for capacitor manufacturers. I tried to get a sample out of them originally, but their distributor couldn't do it so they sold me the minimum order- 1200m

Question We were also having trouble finding the perforated aluminum. Do you have any suggestions about where to find this? We could special order it from a sheet metal place, but the cost is going to be astronomical. We thought about just getting aluminum and drilling the holes ourselves. I know this would take time, but it would be a lot cheaper. Again, though, if you have another suggestion as to where we could get already-perforated aluminum, we would appreciate it.

Answer Look up perforators in the yellow pages. They usually have a scrap pile you can raid for pretty low cost. Otherwise, just order some standard stock from them. They can usually sheer it to size for you (within 1/4"- they aren't usually very precise). Steel is much cheaper than aluminum, but also much heavier.

Question As a side note, if you have extra raw materials that you wouldn't mind selling (i.e. mylar or perforated aluminum, etc.), we would be willing to either drive to your location or pay for shipping. In your article, you sounded like you had enough mylar to last a lifetime. If you don't think you will use it all and you wouldn't mind selling a little of it, we would be interested in purchasing some. If not, we understand. It's just an idea. We live in Columbus, IN and would even be willing to drive a day for anything like that.

Answer Unfortunately, I don't have an easy way to handle the stuff. It's on a roll and the best way to ship would be to transfer it to another roll and put it in a shipping tube. I don't have a good way to transfer it to another roll. It generates huge static charges when you unroll it. The static picks up every bit of dust in the air. I could just unroll it and fold it into an envelope, but it is hard to fold because it traps a lot of air... Let me see what I can do. I have seen ads offering mylar film for sale in the back of speaker builder magazine.

Question My friend and I had some questions around safety of the completed speaker. You say never to touch the stator when the speaker is on. Is this still an issue after plastic-coating the stator?

Answer You can touch one stator, just don't touch two at a time. The transformer output voltage can be as high a 5000V. Though the source impedance is high, it is not so high that it won't burn your skin and hurt like hell. I found out about this the hard way when I was working on my first prototype. Martin-Logan plastic coats the stators and it supposedly makes it safe to touch the speakers, but I still wouldn't recommend it. If the coating is thin, or there is a pin hole, or the dielectric strength isn't what you expect, you'll get zapped. Martin Logan may put resistors between the transformer and stator to reduce the current available for shocking people.

Question If not, are there any other materials that we could paint onto them to make it safer? He has a dog and we don't want to have to worry about guests touching it, etc.

Answer It's unlikely the dog will get hurt by the speakers. More likely that the dog will piss on them, or his hair will get between the stator and diaphragm and cause problems. You could always put some wire screen or similar material on the outside of the frame that holds the drivers. That will keep fingers out. Quad uses plastic dust covers made from the diaphragm material and adds peforated steel to keep fingers, dust, and dog piss off the drivers. Quad even connects one side of the bias supply to the perforated metal covers to get dust to stick to them instead of the diaphragm.

Question Many thanks for your prompt and helpful reply, and for refering me to your www.glmt.com/esl website. I was very impressed. Unfortunately, however, none of the search engines I used ever referred me to this site, and I'm sure there are many others like me who are searching, but not finding. In my case it may be because I used the key-word "Quad" along with the other key-words "electrostatic loudspeaker repair."

Answer My article can also be found on Bill B's science hobbyist page at http://www.eskimo.com/~billb/index.html. Bill has lot's of other interesting stuff there as well. I believe my article can also be found at a couple sites in Europe.

Question Thanks also for referring me to the John Borwick book, which I shall order. I know him from his job as the Audio editor for the "Gramophone." I already have the Roger Sanders textbook, for which Barry Waldron wrote the Introduction. I contacted Barry Waldron's website to order Mylar. He quoted me a price of $10US per 4' by 10' sheet. I ordered 4 sheets over a month ago, which he has yet to send me. Given that you bought an entire roll of Mylar for less than $100, his profit margin is quite healthy. I'll give Mr. Waldron another month or so of grace, and if he still doesn't deliver, I could cancel the order and buy from you, if you can bear to part with some of the 1,500 yards or so which you have lying around. I'll pay you the same price as I would have paid him, provided your stuff is the genuine Dupont Mylar. Barry Waldron has been extremely helpful, and I don't want to stiff him, but if after two months he can't deliver on a commercial order, it's his loss

Answer The film can be difficult to handle, and then there's the hassle of shipping it, etc. I don't think his profit margin is entirely out of line. The film I have was made by a Japanese plastics company called Toray. As for "genuine DuPont Mylar", I'd say polyester is polyester (so is Mylar). Mylar is DuPont's brand name for polyester. As long as you get thin material, the manufacturer doesn't matter. I don't really have a good way to handle and ship the film. I can unroll it, sort of fold it (light weight and static charge make this very difficult to do neatly) , and stuff it into an envelope. When you receive it, it will be wrinkled and have some dust and cat-hair stuck to it do to the triboelectric charge developed from handling. When you use it, the wrinkles won't matter because putting it under tension will pull them out. Dust and cat-hair can be cleaned off with a vacuum cleaner or wiped off with a soft, moist cloth.

Question As for attaching dustcovers, and even for fastening the Mylar diaphragm, I was planning on using the double-sided tape that comes with 3M Window seal kits. I live in an old house in Montreal, where the temperature goes down below -30 Fahrenheit, and stays there for days at a time. Each winter I cover my leaky windows with a window seal kit. Some windows in a rarely-used room are permanently taped. So long no air gaps develop between the tape and the items it glues together, it will hold an 8' by 6' plastic sheet under strong tension almost indefinitely. In fact, when I peel the tape off in the Spring, it usually peels off quite a bit of the window-ledge paint it's attached to. I presume that once the diaphragm is clamped between the two stator panels, the combined strength of the double-sided glue tape and the compression it is under will prevent any creepage. If I'm wrong I'll just recommence with something more exotic. One advantage tape has over epoxy cement is that the stator-diaphragm gap will be constant.

Answer You need a lot more tension for the speakers than you need for window sealing. I believe that you'll find the tension on the film will pull the adhesive from the tape into the speaker, while also reducing the tension on the film. This may take anywhere from a few hours to a year or so, so if you don't mind replacing diaphragms every year or so, it shouldn't be a problem. I have found the contact cement listed in the web article to be extremely reliable.

Question Why does the diaphragma need to make more excursion at lower frequencies in order to keep the sound pressure level constant ? Is this only to compensate for phase cancellation or are there more mechanism's involved ?

Answer In any speaker, reproducing loud low frequencies requires the movement of large volumes of air. This is why woofers are so much larger than tweeters.

Question As for a In Mr. Fikier's book and in Ronald Wagner book (in the article reprints, in the back), there are plots of the lowest frequency versus the required minimum diaphragma excursion. Do you know where these plot's come from or how to calculate them or do you have (or know) some literature that explains how these plot's are calculated ? ttachi

Answer I don't know how the curves were derived.

Question In my ignorant reasoning, I would expect that for a certain sound pressure level, a certain excursion is necessary and that this is independent of the involved frequency, as long as you ignore phase cancelation. Is this correct?

Answer No. For constant SPL, and constant diaphragm size, you need larger excursions for lower frequencies. Phase cancellation makes the situation worse, which is why ESLs are not very good at reproducing low frequencies.

Question In thinking about which kind of stator (wire or perforated metal sheets) I'm going to use, I stumble across the following reasoning: In order to maximise output (as the "cookbook" states), it is best to make the field strength as high as possible. I think that with perforated metal the effective amount of metal that creates the field is bigger then with wires (in general). It is even possible to etch a steel perforated stator, this (about) doubles the effective surface of the stator and thus also increases the field (and thus output). This goes of course at the expense of a higher capacity of the ESL, but this is only a matter of a suitable amplifier.

Answer Mechanical force on the diaphragm is most important. More force means more excursion means more dB spl. Force is the result of the electric field in the speaker. The only way to increase the force on the diaphragm is to increase the voltage (bias and/or AC drive), reduce the plate to diaphragm spacing (which limits peak excursion), or improve the open area % (the ratio of the total area of the holes to the area defined by the gross dimensions of the driver x 100).

Etching the surface of the stators increases the surface area of the stator on a microscopic level but this bears no relation to the output spl of the driver. The real question relates to % of open area and total mechanical force on the diaphragm. If we start with the ridiculous case of a single thin wire used as a stator (99.99% open) , we can easily imagine that we won't get much output from the speaker because very little mechanical force will be applied to the diaphragm. If we go to the opposite extreme of solid plates, we also won't get much output, but clearly the mechanical force will be at a maximum. So somewhere in between the 99.99% open area and 0% open area, there must be at least one point where maximum output from the speaker will be obtained.

I don't know if anyone has ever done detailed tests or calculations related to the % open area versus output spl. My speakers have always used a rule of thumb that says the open area should be maximum obtainable consistent with hole size no larger than the plate to plate spacing. The reality is that it probably doesn't matter too much, as long as you avoid the extremes. It is also a function of the materials you have readily available. If you only have perforated sheet that has 50% open area and holes that are 5 mm diameter, then use it. It will work. It may not be optimum, but you can compensate for reduced output level other ways (use higher bias voltage, for example).

I have a pair of Quad ESL63s. I took a look at the stators in those. The hole size is very small, only about 1.5 mm. The open area of the driver panels appears to be less than 50% (maybe only 30%). They are very sensitive speakers and only require a few watts to drive them to very high volume levels. They use stator to diaphragm spacing of about 3mm and 5.75kV bias.

Question But if the above reasoning is correct, why is not everybody making perforated metal stators. What am I forgetting ?

Answer You're forgetting that speakers have to look good in order to sell. A couple companies make ESLs using wire stators. They look different from perforated metal stators. Which is better? It probably doesn't matter. With wire stators you may be able to avoid corona discharge a little better, but in terms of sound from the speaker. Wire stators are probably more expensive to use because it costs more to weld a bunch of wires together than it does to punch holes in a piece of sheet metal.

Question Why does the diaphragma need to make more excursion at lower frequencies in order to keep the sound pressure level constant ? Is this only to compensate for phase cancellation or are there more mechanism's involved ?

Answer In any speaker, reproducing loud low frequencies requires the movement of large volumes of air. This is why woofers are so much larger than tweeters.

Question Yes, but what is the mechanism behind it ? Does it have to do with the wavelength that has to be reproduced, because at low frequencies the wavelength doesn't fit anymore to the woofer size and you try to compensate for this with more excursion ?

Answer I'm not sure. It may have to do with the definition of sound pressure level. I'll have to check into it.

Question Or is it because in music you usually have more power in the lower frequency band and thus need a driver that can handle more power and thus is bigger or needs to be able to make more excursion.

Answer No, the requirement for more a