Electrostatic loudspeakers are about the lowest distortion audio drivers that can be produced. An electrostatic loudspeaker is a seemingly simple device at first glance .Simplicity of operation and stunning sound reproduction that has kept the electrostatic speaker at the very top of the Hi Fidelity loudspeaker pyramid. Electrostatic loudspeakers have been around for many decades but the invention of the legendary QUAD ESL-55 by British audio guru Peter Walker has put the Electrostatic Speaker at the top of the Hi Fidelity sector. The original QUAD was a truly amazing speaker capable of a level of naturalness that previously had never been thought possible. Many people still believe it has never been matched by any other design even though it has been nearly fifty years since it was invented and the original QUAD still remains the reference loudspeaker in many Hi end manufacturers R&D lab to this day.
My electrostatic speakers
Of course, my electrostatic loudspeakers are not QUAD. They are home made and is the next best thing to have (sounds nearly as good too!) The speakers have two bass panels and one mid-high frequency panel per channel. Each bass panel measures approximately 1m x 0.3m and the mid-high panel is approximately 1m x 0.1m . The bass panels have a diaphram to stator spacing of 4mm and the mid-high frequency panel has a diaphram to stator spacing of 2mm.
The bass panels have a DC bias voltage of 6kV and the mid-high panel is biased with 1.5kV DC. The DC bias voltages are derived from a step up transformer and a standard voltage multiplier line. The high frequency components are fed to a high quality audio step up transformer with a centre tapped high voltage winding. The turns-ratio for the mid-high frequency audio transformer is 1:50 and is a critical low distortion audio transformer which was obtained from Innersound (through Roger Sanders). The audio transformer for the bass panels had a turns-ratio of 1:100 and they too are centre tapped. The requirement for the low frequency transformer was less stringent and hence it was made locally in Colombo by the Silvertones Radio Company of Sri Lanka. The diaphrams of both bass and mid-high panels had 10um Mylar C coated with a very thin coating of graphite to give it high surface resistivity.
The Electrostatic Loudspeaker (Right channel) while being tested in my living room.
The Electrostatic Loudspeaker (ESL) is a dipole radiator (i.e. it emits sound from the front as well as the back in equal amounts). The radiation pattern is like that of an ideal dipole and there are two maximums in the front and back. The sides have nulls.
The dipole radiation pattern makes the positioning of the ESL a critical matter for correct operation. Ideally the ESL should be positioned at the centre of the listening area so that the back radiation gets minimum chance to reflect off walls and other objects and cause interference effects and coloration of the sound.
Fortunately my living room was ideal for high performance dipole loudspeakers. An entire wall of the room is completely open to the garden and open area thereafter. This causes all the back radiated sound to escape to the surrounding area and be absorbed. Therefore the issue of coloration from reflected sound is not a concern.
Pair of ESL's being tested. The greenery you see is the Sri Jayawardenapura Kotte Bird Sanctuary. This is a protected marsh home to many wetland birdlife.
Notice that the ESL panels are almost completely transparent and the background can be seen right through the panels. This is clearer in the left speaker due to lighting conditions at the time.
The electronics for one channel of the ESL system.
This unit contains the high voltage bias supply, the two audio transformers and a passive crossover filter that separates the low and high frequency components of the audio signal.