4) ESL Driver Construction

by Mark rehorst

This page will explain how you can make your own ESL drivers.

Soon to be updated!

previous version date: 6/7/96


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 require bias of up to 5000 VDC and use AC voltages up to 5000 V.  The DC bias is sometimes 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 an audio output transformer. The voltages produced by the audio 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.

Making the speakers

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.  It would be foolish to trade your vision for the pursuit of audio ecstasy!

Making the drivers:

Step 1. Decide how big to make your drivers.
It is generally easier to make small drivers than to make big ones, but with small drivers you may need several of them so it takes more effort to mount and wire them. 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 sqft                 2kV             1/16"
full range      >4 sqft             3-5kV             1/8"

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.

For midrange/tweeter drivers to be used in a hybrid system, there is considerable flexibility in the insulator thicknesses and spacing, bias voltage, and driving voltages. 1/16" PC board material is extremely common and low cost so it is almost ideal (except for the difficulty in cutting it) for this application. * 1/16" is easily enough room for the diaphragm to produce ear splitting volumes at frequencies down to 300 Hz or so, using a single transformer to drive each speaker.

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. 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.


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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