For many people the Electrostatic loud speaker is still something mysterious. This is caused by the looks of the speaker that are very different from ordinary speakers. The electrostatic loudspeaker has no casing but is flat and open at the front and back. The high voltage on the Esl is seen as dangerous by many. But in fact it's not much different from the statics experienced in daily life, the tiny shock you feel when taking off your sweater or touching your car.
Facts and Ficton
An Esl can't reproduce low tones!. Bass But a fullrange ESL can do such perfectly; for example the one shown on the right ESL 65-8.1 can do such even as low as 16 Hz.
An ESL must be postioned quite precisely in your living room! But "ordinary"speakers must also be put correctly in your living room to give good performance. An ESL has a very low output.But very good "big"speakers usually also have a very low output aswell. An ESL only has a bundled sound Indeed many ELS's have a bundled sound but the ESL's shown below prove the contrary.
The way it works is quite simple. Between two acoustically-transparant plates or grids (the stators) a mylar-foil is mounted, the membrane. This membrane (mylar-foil) is kept precisely in the middle by distance pieces inbetween the stators. On the membrane is a constante highvoltage of several thousant Volts. This is the so-calles electro-static load. On the stators comes the signal from the amplifier, but multiplied fom some thousand Volts. (a audiotrafo with a ratio of 1:1000 builds the voltage from the amplifier from lets say 30 Volts to 3000 Volts). The whole thing is encased in a frame that makes it all very safe. The membrane is pulled back and forwards by the signal from the amplifier inbetween the stators and thus causing the sound.
The materials uses are extremely simple and consist of the following: ordinary pliable electrowire diam. 0,5 mm (the type you would use on your bicycle-light) metal carriers for the wires. Pertinax isolationstrips. Mylar (very thin foil 0,007 mm) Wood or MDF for the encasment. High-voltage powersupply. (HS) audiotrafo's (f.e 2 pieces 1:75)
This ESL is the standard design for the DIY builder as modeled in E. Fikier's book on the subject. This book elaborately describes how to build these. Unfortunately it's only available in dutch. The sound quality is considerd very good in comparison with comercially build ESL's like those build by Quad, Audiostatic, Final and all the rest.
The size of the membrane is 180x18cm.The wooden frame is built out of several layers of MDF. The statorstrips are made out of jellow brass 5x5mm instead of pvc-piping. The high-voltage powersupply according to the manual had to be 7500 Volts but actual voltage only came just up to 5000 Volts. The audiotransformers are in line 50:1 and 75:1. Primary paralel aligned with and LCR-circuit in serie to improve the connection to the amplifier. It makes the amplifier work better. The stators are divided medium high and low tones by a resistance of 175 kilo Ohm.
While testing the following showed: Low tones were apparent around 60 Herz strongly present.Later that seemed to be resonancefrequence and that was increased by the heavy wooden frame.
The amplifier of the ESL had a difficult job and gave lots of problems in the beginning. The LCR-circuit gave some relief. I tried lots of amplifiers but thing remained troublesome.
After studyng an ESL of Audiostatic I modified the construction of my frame to the one of Audiostatic: Only the Partinax 5mm used with rods of jellow brass 6x6mm for the whole construction. And that was one of my brighter ideas for it works fine, both for building the thing as soundwise: less resonance.
The partion of low tones and medium-high tones sounds resonablewell, but the need for separation of the membranes became obvious.
The construction orginates from the need for a lower resonance fequence and a separation of membranes.
By widening the baspart (low tones) to 36 cm the resonance fequence has lowered to 28 Herz. The surface of this part is 180x36cm. The width of 36 cm is my idea. This means that this total area is not restricted in movement unlike in other desings where they are partitioned. According to the book by Fikier this is not possible. The reason is that static stability is not garanteed.
The medium-high part is 180x6cm the width of 6 cm gives it his own resonance of 190 herz. The separation between the stators is done by means of a resistance of 360 Kilo Ohm which makes the baspart low in high tones. While stesting stability was indeed a problem but it was improved by lowering the high voltage. After a few weeks of playing that high voltage could be increased and the membrane was stabilized enough to work well. The troublesome resonance disapeared and the bass-sound is of astonishing quality such as I have never heard with any speaker. The medium-high is far bettr noticeable, the partition is a great improvement. The total sound image . It sounds very balanced without "holes"in the sound at the transition from bass to medium-high sounds. The build of the frame is equal to the ESL 18,the parts are divided by means of a strip pertinax 10mm wide. The membrane runs along underneath this partition.
This model is somewhat diferent from 36-6.1: The stators of meduim-high are made of thinner wire and are closer together. (thickness 0.15 mm instead of 0.5 mm). The membrane is replaced by mylar made by Final. From comparing tests between ESL 36-6.1 and 36-6.2 the sound of 36--6.2 was somewhat rawer. Apart from that there wasn't much difference.
ESL 65-8.1 test version
This model is totally different. The two parts Bass and Medium high are seperate units. This means that the high-voltage can be differentiated and the membrane-tension can be different. Positioning of both units can be in many different ways.
The bass-unit: sizes 65x85 cm width 65 cm that width is once again my own idea. Stability of the membrane is once again the same as with the ESL 36-6.1. Same way of building.
The medium-high unit; sizes 85x8 cm. The width of 8 cm has as resonance-fequence of 160 Herz. The bass-unit is linked to the stators by means of two audiotransformers of 50:1 and75:1 in series. The medium-high unit is powered by the audiotransformer of 50:1. The medium and high tones are therefore of very high quality because of the fact that the ratio of transforming is relatively low. The high-voltage is set at an maximum of 7500 Volts.The stators of the bass-unit have been improved: from tests it appeared that the connecting of all statorwires on one side is a great improvement in the bass-sounds. The bass-sound is deeper and tighter. This conecting means that the length of the wire is shortenend from 200m to 1 m for the signal. Apparently this improves it.
ESL 65-8.1, hinge
From the test it's proven that thisESL is of extreme good quality. A very good bass-sound that nears reality in true sound. The bass is very tight and direct without any loud bangs like the true sound of a plucked accoustic bass sounds in reality. The medium and high connect seamless and are of extreme precisian specially in muted passages details can be heard that you have never heard before. The finishing of the ESL 65-8.1 is like a painting on the wall. The whole is connected with a large hinge on the wall. (see one of the photo"s)
The front is covered in silk on which a painting is painted, in this case reproduction of a painting by Matisse called "the blue woman". Any other reproduction can be made to order.
glueig the membrane
The membrane is made out of Mylar with a layer of silveroxide that was ordered from Ab and At from Winterswijk, Netherlands. This mylar is tightend/stretched onto my self built frame and then extended by means of enlarging the frame. The frame is built from wood and I use bolts to widen the frame (see photo) The mylar is then stuck to it with packaging tape. Then tighten slowly on all sides about an centimter(2rds of an inch) After doing that the effective size of the membrane is taken and I etch away the guiding layer with hydrochloric acid. A new conductive layer with high resistance is then put on consisting of Metyl-cellulose. (apply with a paint-roller and the last layer has to be applied with a stencilling brush in a way that you would do when stencilling something onto a wall, use a thick brush.
The stators of the medium/high unit are built out of pertinax 5mm and flexible copperwire of 0,15mm The stators of the bass-unit are made out of pertinax 5 mm and steel square rods 8x8 mm and of flexible copperwire of 0,5mm. The whole thing is glued together with epoxy-glue.(two componant glue)
High voltage supply
schematics HV supply
The high-voltage on the membrane is obtained from a cascade-circuit of 7500 Volts. The medium/high unit has full power on the membrane and the bass-unit gets a few steps lower on the cascade.
The tension board is built out of a plate the same size as the stator. On both sides sliding parts are mounted that can be moved by means of a bolt. That makes the foil expand.
detail stretching jig
By stretching the bolts (unscrewing bolts) the foil is extended. By unscrewing the bolts evenly the foil will have the same tension all over.
The power of the ESL-unit can be done with a high-voltageamp (without audiotransformer) existing of High-voltage valves with a working voltage of 5000 Volts. With that you don't need a step-up transformer of 1:75 anymore.A well-known OTL-amp is the Acoustat. While testing the OTL the quality of the sound wasn't totally satisfying. Schematics for the Acoustat Monitor X OTL amplifier (Available on exchange basis)
The schematic outline of the OTL is by Acoustat. There were some mistakes in this like the crossing of wires and a forgotten capacitor.
The powervalves used are of a different type than those of Acoustat. The type used is a EL 519 by Philips. The large high-voltage capacitors were bought secondhand at a shop. The balance between the powervalves must be precise to have a good sound from them.
If the Acoustat-amp. would be used then you don't need the step-up transformer. But for an "ordinary"amplifyer you do need a step-up trafo to reach the high signal-amplification on the stators. The audiotransformers used are of the brandname Amplimo 1:50 in series with 1:75 on the secondary side.
Preparing the grill cloth
With the ESL 65-8.1 we used painted silk as cover. A friend made the painting on silk. Any wished painting can be made to order. We chose a design that we liked from a famous artist but of course it differs from the real thing. We chose this because to much detail is a lot of work and mistakes wouldn't be noticed anyway. We are both happy with the result, it looks nice.
Resonance frequency (click to enlarge)
How do these designs sound? All ESL's sound have an open sound and all colouring of sound is missing. The better the set to drive your speakers the more you can enjoy this unique sound.
The small types ESL 18 have the disadvantage that their resonance frequence is around 60 HZ and around that frequence their bas-sound is a bit overdoing it but as soon as it gets below 60 herz it goes down fast. That is not the biggest problem because most dynamic loudspeakers loose it at 60 herz.
The wider types ESL 36 and 65 have already a lower resonance of 28 herz and 16 herz. and below that level the bass sound falls back. The sound of this bass-sound is hard to describe: once you've heard it you start to notice how colored the dynamic types sound. Go visit an acoustic concert (where music is in no way amplified) and then you will know how uncoulored sounds sound and you will appreciate the ESL more and more.
Acoustics of the room
Both in a concerthall aswell as in your livingroom you will have to deal with acoustics. When using ESL's that is more important than with dynamic systems. The area where you listen to music must not sound dry (without reflection) it's the reflections that give the sound it's colour and overall surround. Modern houses have the problem of so-called tough walls (concrete) the sound reverberates in a very annoying way. A solution for this is the application of good absorbing tiles on the ceiling and the problem is solved for little money. It's the cheapest way of improving the sound quality of your livingroom or musicroom. (but you shouldn't look at your ceiling all the time because you probably don't like the view)