text: quad electrostatic loudspeaker the quad electrostatic loudspeaker ( esl ) is the world 's first production full - range electrostatic loudspeaker , launched in 1957 by quad , then known as the acoustical manufacturing co . ltd . the speaker is shaped somewhat like a home electric radiator curved slightly on the vertical axis . they are widely admired for their clarity and precision , but known to be difficult speakers to run and maintain . the original esl , in production between 1957 and 1985 , has been hailed in sound & vision as one of the most important speakers of the 20th century . it was succeeded in 1981 by the esl - 63 , which remained in production until 1999 . as of 2013 , quad maintains four electrostatic speakers in its range . history acoustical manufacturing company had launched a ribbon - hybrid , horn - loaded " corner ribbon loudspeaker " in 1949 , but this was not commercially successful . the electrostatics were designed by david williamson , and the company 's founder , peter walker , from a principle elaborated by edward w. kellogg for general electric that was granted a patent in 1934 . however , more immediately , a book published in the us in 1954 extolling the scientific merits of the electrostatic [ esl ] transducer inspired walker to develop an electrostatic loudspeaker of his own ; this was technologically enabled by the mylar film developed by dupont that came to market in 1949 . walker demonstrated a prototype electrostatic panel speaker ( with exceptional clarity in the midrange ) , and a full - frequency infinite baffle speaker ( with ample bass response ) at the british sound recording association annual exhibition in may 1955 . after more research , the large panel speaker with better response over the whole frequency range was demonstrated to the physical society in march 1956 . the esl entered commercial production in 1957 , and was discontinued only in 1985 . development of its successor began in 1963 , giving rise to the product name " esl - 63 " , launched in 1981 . it attempted to address both the deficiency in bass response of the original esl and its extreme directionality at high frequencies . the latter goal is achieved by splitting the stators into eight concentric rings , each fed with a slight time delay compared to the ring immediately inwards , thereby attempting to emulate a point source . the last esl - 63 left the factory in 1999 , when it was superseded by the esl - 988 and esl - 989 . speaker stereophile described the speaker as having been designed for " serious - music ( call that ' classical ' ) listeners who play records more for musical enjoyment than for the sound " . quad launched the speaker in 1957 , priced at £ 52 , believing that an audio system should be able to extract everything of value that is on a recording , while de - emphasising the irritations of mediocre sound recordings . it consists of a minimalistic , slightly curved , flat panel . the " quad electrostatic loudspeaker " or " esl " , as the designers intended to call it , is variously referred to as the quad 57 and the quad 55 . its descendants are still often simply referred to as quad electrostatic loudspeaker as part of the legacy . walker called the concept behind the esl - 63 , being the acronym for " full - range electrostatic " . design an electrostatic speaker has only a frame to support the charged diaphragm . it uses ultra - thin stretched mylar ( - oriented polyethylene terephthalate ) film as the main component of its drive units . there are two perforated plates with a potential difference of up to 10 , 000 volts , between which a very thin film , coated with a mildly electrically conductive substance , is sandwiched . sound is produced when the audio signal – amplified by a step - up transformer – sets the film diaphragm into motion . the different charge between the two plates allows the diaphragm to be alternately attracted and repelled by each plate . in the original quad , two bass panels flank one treble panel to provide full range sound . its treble panel diaphragm is further subdivided into midrange and tweeter sections , achieved by clamping it on each side of its centre . both midrange and tweeter sections are driven separately , and the entire treble unit operates at substantially lower voltages – operating voltages are said to be for the bass and for the treble . the speaker needs to be plugged into the mains and must be charged prior to use . this is because the diaphragm operates in a " constant charge " mode , and it takes time for the charge to build up . in practice , the speaker is usable after charging for less than half a minute , but the sound gradually improves as the charge builds up . best performance is achieved when the speaker has charged fully , which can take up to an hour . the electricity is only to charge the diaphragm , so consumption is negligible . the electrostatic design does away with two major weaknesses of boxed speakers : enclosure colourations and signal degradation due to the crossover . the crossover in the esl consists merely of resistors which work with the inherent capacitance of the speaker elements to obtain a first - order response . because it is bipolar , both sides of the elements are not enclosed , but radiate directly into the air . thus , colourations due to crossover components and box - induced reflections are minimised . fast transient response and low distortion are both characteristic of electrostatics . the driver , being only 0 . - inch thick and weighing only 3 mg , is thus capable of stopping and starting seemingly instantaneously with no damping necessary to the drive units . however , the design also limits bandwidth to between 40 hz and 10 khz . kohli of tnt noted that upper register starts to taper above 10 khz and then falls off severely beyond 13 khz , and is 15 - down at 20 khz . the speaker 's design makes them extremely directional , and the general practice is to toe in the speakers to point directly to the listener . although a slight deviation did not seem to make a difference , it is in practice not possible for more than one person to enjoy a reasonably balanced sound stage with the esl . contrary to the flat appearance of the loudspeaker , all three panels are slightly curved along the vertical axis to improve quality . the panels themselves are flat , but obtain their curvature by being slightly bent to a curved backing . with the bipolar nature of the speaker emitting sound from both front and back surfaces as it does , the application of a felt sheet to the rear of the panels damps and absorbs high frequencies radiating from the back . a crossover network , set at 500 hz , remains necessary to adjust for back - to - front interference causing loss in output at low frequencies . in addition , the speaker should stand in free space to achieve optimum sound . the instructions book contains advice against placement of the speaker in corners or positions close to or parallel with walls , and further explains that a corner placement will adversely affect bass response and be detrimental to the midrange because of standing waves generated . its impedance is 1 . 8 ohms at 20 khz , to over 60 ohms at 150 hz , and its load is highly capacitive . it does not consume large amounts of power so much as it feeds it back to the amplifier in opposition at some points during each cycle . this is very demanding on amplifiers ' stability . although it is designed to be used with the quad ii , 303 , or 405 power amplifiers with limiters , a power amplifier capable of delivering 15 watts per channel suffices – excessive voltage input is known to cause arcing within the speaker panels . in fact , the instructions book states that any properly - regulated amplifier delivering no more than is unlikely to cause damage to the speaker . electrostatic speakers are more accurately rated by voltage , not power . in the esl 's case , at the rated nominal impedance of 16 ohms , the limit of 33 volts would be reached when the amplifier 's power output reaches 15 watts ( if it were driving a conventional load ) . the most common failure modes were loss of sensitivity caused by dust infiltrating the speaker panels , and internal arcing of the panels caused by excessive power to the step - up transformer ; use at high altitudes may also cause the same effect . also , because of its novel electrical characteristics , the speaker could cause some amplifiers to become unstable and could result in damage to either or both . late in the speaker 's life , many owners found that the highly - stable 15 - watt naim nait launched in 1983 worked well with the esl . construction the esl measures 31 by 34 . 5 by 10 . 5 inches ( 79 cm × 88 cm × 27 cm ) and weighs 35 pounds ( 16 kg ) . in terms of appearance , the original esl has an expanded metal grille , and three little wooden feet . the earlier version has golden - bronze grilles and classic teak end caps . later production versions have black grilles , and end caps have a rosewood finish ; other minor variants exist . the speaker 's electronics components , consisting mainly of an audio transformer , eht power supply , crossover and mains transformer , are fixed to the base of the cabinet . the original esl - 57 suffered from reliability issues , frequently caused by use of inappropriate amplification or being driven too hard . in the main , the diaphragm would arc from being overdriven , or the audio transformer would seize up from being exposed to excessive voltages . the design 's lack of structural integrity is seen as another significant weakness of the quad esl - 63 since its inception , and lived on in its various successors . other shortcomings include very limited sound pressure levels , meaning that it won 't play loud , and its bass response was lacking . in addition , one reviewer experienced problems with shut - downs playing music with high intensity bass drums . product revisions quad have now brought out five generations of the classic electrostatic design . originally designed for monaural sound in a domestic context , the popularisation of stereophonic sound prompted the manufacturer to modify the electrostatic panels to improve their stereo dispersion . these speakers featured larger panels and a revolutionary stator design , made up of eight concentric rings fed from the centre outwards through analogue delay lines , so that the audio signal gives the illusion of radiating from a point source one foot behind the panel . quad addressed the limitations of the original design to some degree with the esl - 63 . development of the newer design was initiated in 1963 and the product was released at the ces of 1981 . because of a changed orientation , the esl - 63 measures 36 by 26 by 6 inches ( 91 cm × 66 cm × 15 cm ) , the speaker seemed smaller than before . new " clamping " protection circuit was incorporated to prevent arcing . another " crowbar action " protection circuit could detect the high - frequency noise that accompanies the ionisation of air when the speaker arcs , and shuts down the power when that occurs . power rating is increased to . furthermore , because the panels in the original esl were prone to attracting dust , esl - 63 was made dust - proof . in 1989 , a new marketing position was needed for the us market . as the rising pound sterling made the product much more expensive for us customers , it was decided to improve the specification of the esl - 63 and target the $ 4 , 000 price point with a " us monitor " version . it was modelled on the more rugged " professional " model that possessed metal grilles and a steel frame ( instead of aluminium ) which had been created for the recording division of philips . reception its launch was eagerly awaited : even before it was launched , several articles were written about it in wireless world in 1955 ; it appeared on the cover of the 1956 hi - fi yearbook . generally well - received from its launch , the speaker was dubbed " walker 's wonder " very early on . the loudspeaker was used as studio monitors for controlling the sound quality of broadcasts . such professional users included philips and the bbc . the original esl , in production between 1957 and 1985 , sold 54 , 000 pairs during its life . despite some major weaknesses , it is universally hailed as one of the most important speakers of the 20th century . home theater review said it was " the most cherished hi - fi product ever " , and that walker had been elevated to god - like status in the audio world . what hi - fi notes the fanatical loyalty from owners of the speaker or fascination from people who yearn to own them . demand for second - hand versions of the original 1957 or the later remains buoyant many years after their being discontinued . demand from customers resulted in manufacture of the " esl - 57 " continuing in the uk until 1995 . quad 's machine tooling for their manufacture was then bought by quad 's representative in germany , quad who continue to manufacture complete speakers and spare parts , and overhaul customers ' speakers . there are small companies all around the world dedicated to servicing this speaker . commenting on the esl - 63 , j. gordon holt of stereophile considered it was a major failure of the speaker for it to shut down during orchestral climaxes . he said that " regardless of the sonic merits it possesses [ the speaker ] simply did not have the power - handling capability " for more recent program material on cd . signature sound the esl is renowned for sonic transparency and very low distortion . its midrange performance is usually described in superlative terms , common to other electrostatic designs . its sound is tonally neutral , exceptionally spacious , and transient response very fast . a properly configured stereo pair would generate a very small ' sweet spot ' where the sound stage snaps into focus . this led to criticisms of the speaker 's directionality which in practice was rarely a problem and had the benefit of reducing the effects of room resonances . the experience of listening to the els is often described as non - fatiguing or listenable . sam tellig wrote in stereophile in june 1989 of the neutral sound emanating from the esl - 63 : " there is no discontinuity from bass to midrange to treble — it 's all so natural . transient response is excellent . there are no boxy colorations because there are no boxes " . its superior sonic characteristics were offset by its moderate power handling , and the need for a relatively large space in which to " breathe " . its usable frequency response is from 40 hz to well past 20 khz , and is occasionally criticised for being bass light . its moderate bass extension is considered adequate for classical music fans , although later incarnations are said to deliver better bass performance .