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.


PCB driver
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 (yellow) along the bottom edge of the driver. Be sure to extend the board beyond the stators (blue) to leave room for hardware to mount the driver on some sort of frame.  It sn't necessary for the copper traces to loop around the perimeter of the driver.  A single contact point is really all that's necessary.

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.  You may want to clean the insulators with alcohol and a very clean cloth before proceeding. 

Top side of two PCBs used for ESL driver

The image above is the top sides of two pieces of PCB material used to make an ESL.  The one on the left only needs copper (yellow) on one side, the piece on the right needs copper on both sides.

Bottom side of PCB used to make ESLs

The picture above is the bottom sides of the two PCB pieces used to make an ESL.

The electrical connection is made by physical contact between a metal strip and the conductive 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.  Cut the perforated stators to the size that fits your insulators.  You have to use a shear or maybe a saw- be careful not to bend the material- you want it to be flat!  Once the stators are cut to size, epoxy them to the insulators making sure they are aligned with the electrical contacts.  Now your driver is in two pieces, two insulators with stators attached.

Half driver assemblies
The image above is the two half-driver assemblies, ready to be glued to the diaphragm.  This is the outer side with the stators epoxied to the insulators.

Half driver assemblies, diaphragm side

The imge above is the half-driver assemblies viewed from the diaphragm side.  The diapgragm electrical contact, yellow, will be pressed against the conductive coated side of the diaphragm.

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 have read that it "relaxes" over time and loses tension.

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 (green in the images below) 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 (blue in the images below) 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! 

top side of stretcher table

underside of stretcher table
Figure 3. Top and bottom views of the 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 (green) attached to the inside of the table edge. Inflating the tube stretches the film tight. 


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? Cement the driver half without the diaphragm contact to the stretched filem using contact cement (4693H if available).  Put some contact cement on the driver half, and put some on the diaphragm where the driver half will touch it.  Follow the instructions for the contact cement and allow it to dry for a few minutes, then put the half-driver down in the film.  Be careful- you only get one chance to get it right.  Pile some weight on it or clamp it and leave it sit to allow the cement to harden.  Once the cement is completely dry, you can release some of the air pressure from the tube, then cut the diaphragm around the edges of the driver to release it from the stretcher table.  

When I first made ESLs I tried to use powdered graphite to make the diaphragm electrically conductive.  It was messy and usually made a diaphragm with relatively low resistance- not good.  Then I discovered Licron antistatic spray.  It's a little bit neater and produces a very high resistance coating, and takes only seconds to apply.  Turn the driver assembly over and with the diaphragm side up, mask the driver mounting holes with some post-it notes.  Mask off anywhere else you don't want the conductive coating, too.  Then follow directions on the can and spray a light coating of Licron onto the diaphragm.  Let it dry.  Remove the masks, and place the other half of the assembly down on the diaphragm and screw the assembly together.  You could contact cement the other half of the driver to the diaphragm, but if there's ever a problem and you need to replace the diaphragm, you may have a difficult time separating the two halves of the driver.  If you screw the pieces together you'll be able to separate them again, peel off the old diaphragm and install a new one.


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.

eslfig4.gif (5773 bytes)
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

Home