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Build these wide range electrostatic loudspeakers For many years, electrostatic loudspeakers have been very highly regarded but beyond the reach of the do-it-yourself con­structor. Now it is possible, using new materials & a new method. This wide range design is capable of very satisfying performance, equal to the very best of loudspeakers. By ROB McKINLAY This project germinated some years ago after listening to some expensive electrostatic loudspeakers at an upmarket hifi store. The sound quality was superb but the price tag made them unobtainable for most people. The lasting impression of their sound 56  Silicon Chip quality made listening to all but the best box type loud­speakers tedious. My own hifi set up was of very good quality but obviously lacked the clarity and insight of panel speakers. After reading the limited information that was available on ESL design and building some small test panels, I decided that it would be possible to design and build a quality set of ESLs at a much lower price than the commercially available items. As this was to be a completely new design I had the freedom and flexibil­ity to try to eliminate a number of the more tedious construc­tional tasks. The design had to retain the sonic benefits of panel loud­ speakers. It would be a full range design with good bass re­sponse; always a tricky area with ESL panels. They had to be easily made with handyman tools. No special tools or jigs were desired. They had to be made with readily available materials and at an economical price. The ESL III Electrostatic Loudspeaker is a three-panel, full-range Fig.1: the schematic of the ESL III loudspeaker. Each channel of the audio amplifier is coupled to a step-up transformer with a turns ratio of 100:1 & this drives the fixed steel plates which are perforated to allow the sound from the moving diagram to radiate from both sides. The moving diaphragm is a very thin plastic coated with a resistive doping material & this is biased at about 3kV. Note that there are three panels but only one is shown in this diagram. This view shows the various connections to the three panels & the terminal panel. Two wires connect to the audio amplifier while the third connects to a 9V DC plugpack supply. This powers the high frequency DC-DC inverter. The rear view of the ESL III loudspeaker without grille cloth fitted. This clearly shows the three panels, treble in the centre and midrange/bass on either side. Note that the wiring runs at high voltage and would normally be covered by the grille cloth. design, consisting of two bass/mid­ range panels and one upper mid/ treble panel. The three panels of one loud­speaker are arranged in a curvilinear array, ie, the bass panels placed on either side of the treble panel face slightly outward from the speaker centreline. This reduces beaming effects and improves the off-axis stereo image. The design uses a mechanical cross­ over in that, as the frequency roll-off of the bass panel occurs, the treble panel rolls on. This happens over a wide frequency range, resulting in a seamless integration of the bass and treble panels. The advantages of this approach are the elimination of phase anomalies caused by crossover components, greater reliabil­ity, reduction in complexity, better use of available power and most importantly, a reduction in cost. Facing page (top): installed in their custom made enclosures, the ESL III electrostatic speakers are very impressive to look at. They stand 1470mm high, 640mm wide and 150mm deep, with a 700 x 300mm foot­print. You will need a fairly large listening room if they are perform at their best. Design features The overall dimensions of the three panels required to make one loudspeaker are 600 x 1205 x 27mm (W x H x D). At this point it is appropriate to explain briefly how electrostatic loudspeak­ers work. Essentially the speaker is a sand- wich comprised of a move­ able diaphragm suspended between two perforated metal plates. The surface of the diaphragm is made conductive by the application of a highly resistive coating. A negative bias of several thousand volts DC is applied to this coating to provide a polarising force. When operating, one of the plates will become positive in relation to the diaphragm and the other will be negative. The positive plate will attract the negatively charged diaphragm while the negative plate repels the diaphragm. Hence, as the audio signal fluctuates on the two plates, the diaphragm reacts in the manner described above, mimicking the signal. Due to the extremely light weight of the diaphragm and the damping created by the air load, the reproduced signal is faithful to the input signal, with little February 1995  57 a ring of nuts and bolts or bonded together with adhe­sive. Both of these approaches work fine unless you want to open the panel up to make a change to something. The nuts and bolts method is tedious but non-destructive to the diaphragm and air gap spacers. The bonded method can cause damage to the diaphragm or spacers when the joint is broken. I opted for a system that simply clips the two panel halves together. One is able to disassemble the panel in a matter of seconds without damage and reassemble in the same time. This allows access to the diaphragm, which is still held at full tension on one half panel, for service, or to allow experimenta­tion with the node points. Diaphragm tensioning The bottom compartment of the enclosure houses the audio step-up transformer & the DC-DC inverter board. This includes a Cockroft-Walton multiplier, hence the array of high voltage ceramic ca­pacitors. or no distortion as created by conventional cone type speakers. Due to electrostatic loudspeakers being a true dipole, ie, sound radiates equally from both front and rear, a certain amount of low frequency rolloff will take place. This is caused by the cancellation effect of the front and rear sound wave being 180 degrees out of phase with each other. Careful design of the enclosure will reduce these effects. The normal output voltage of a typical power amplifier is not high enough to create the electrostatic field required for normal sound pressure levels so an audio step-up transformer is required. The transformer used in this project has a turns ratio of 1:100; ie 1 volt in produces 100 volts out. This allows effec­tive plate voltages to be reached. Diaphragm nodes Each panel has a series of diaphragm nodes placed in the vertical centreline and scaled in such a way that individual sections of the diaphragm reproduce only the frequencies desired of them. This reduces the inter­ mod­ ulation distortion that would be created by one section of the diaphragm trying to reproduce, say, 20Hz and 20kHz at the same time. Diaphragm bias is provided by a fast recovery diode voltage multiplier driven by an 11kHz oscillator. 58  Silicon Chip The custom wound audio transformer is rated at 100 watts. This proved to be the most difficult item to source. There are no “off-the-shelf” transformers available with the necessary turns ratio and frequency response to drive these panels. The solution was to have transformers specially wound for the project. After testing about 20 designs, all of which had problems with their high frequency response, we were fortunate enough to obtain a transformer to fit the bill. Each panel consists of two half panels clipped together in a unique manner. One panel half has the diaphragm attached to it, the other half carries the EHT supply rail. When the two halves are clipped together the EHT bias is transferred from the rail to the diaphragm. Each panel can be disassembled in a matter of seconds to effect any repairs or service should it be necessary. Unlike most electrostatic loudspeakers, diaphragm installation or replacement does not require any specialised equipment and can be carried out by a competent handyperson. The design allows experimentation for the more adventurous homebuilder to change the frequency response of the panels. This can done by adding simple resistor networks in series with the panels to create low pass filters. Most conventional electrostatic loud­ speakers are held to­ gether by A satisfactory way of tensioning the diaphragm and attach­ing it to the panel had to be found. Most designs use a tension­ing frame to tighten the diaphragm prior to installing into the panel. Construction of this was likely to take almost as long as the speakers themselves. As an alternative, a diaphragm material was found which had a greater than usual heatshrink rate and was adhesive on one side when heated. This killed two birds with one stone as the adhesive would not creep under full tension and was compatible with the support structure. It simply had to be taped down over the support panel, a heat gun used to activate the adhesive and then tightened by heatshrinking the remainder of the diaphragm. Another area of concern was the conductive coating on the diaphragm. Most commercial designs use a vacuum deposited metallised coating on the diaphragm material. This is difficult to obtain and is very expensive to have made. An alternative to this is to forcefully impregnate the diaphragm with graphite. This works well but can result in areas of diaphragm that are too conductive or not conductive enough. Both of these conditions are detrimental to speaker performance. This approach is also very hard work as considerable time and energy goes into hand rubbing each of the six diaphragms. The design described in this article uses a conductive solution which is mopped onto the diaphragm surface A closer view of the rear of the enclosure, showing the plastic grid structure of the three panels which are held in place by cleats. and cures after a couple of hours. This approach allows the diaphragm to be made conductive only where it is desired, eliminating possible EHT leakage paths. The next aspect of the design was the node point positioning. The final spacing was determined partly by calculation, partly by building numerous small panels and meas­ur­­ing their response, and partly by listen­ ing to the completed full size panels. The positioning of the node points is quite critical to the performance of the speaker. The final design element involved the enclosure and it was found that this had a significant effect on the overall sound of the speaker. It was desirable to raise the panels off the ground so that the centre of the panel was at ear level when seated, due to the centreline symmetrical positioning of the node points. It then became necessary to provide an enclosure that was solid down to floor level to reduce bass cancellation effects. The side cheeks to the enclosure were also found to be critical to bass reproduction. After much experimentation, I decided on the design shown in the photographs. Results The end result was an electrostatic loudspeaker which per­ formed extremely well. So well in fact that some visitors that listened to them wanted to build a pair themselves. This set the plan in motion to provide kits that did not cost an arm and a leg but would still give a performance rivalling commercial models. The kit includes: (1) steel grids that are custom punched specifically for this project and insulated with a high dielec­tric strength powdercoat enam­el; (2) an EHT supply designed by Oatley Electronics and powered from a 9V DC plugpack; (3) custom wound audio transformers; (4) all support panels and air gap spacers cut to size; (5) easy to install diaphragms complete with a spare; and (6) all components required to make a working set of panels. The enclosure is not included but can be purchased ready built. Enclosure drawings are available, at modest cost, for those wishing to make their own. The finished product seriously competes with commercial designs costing much more. They possess clarity and transparency, with very credible bass performance. Soundstaging is excellent with pinpoint centre stage imaging and believable depth. The speakers are available in kit form at $1199 for a pair plus an extra $499 for the two ready-built enclosures. Freight, packaging and insurance charges will vary from state to state. For further information, contact Rob McKinlay, E. R. Audio, 119 Brook­ ton Highway, Roley­stone, WA 6111. Phone (09) 397 6212; fax (09) 496 1546. Next month, we will continue with SC the construction details. February 1995  59