This is the fiftieth anniversary of a hi-fi legend. It was 1957 when Quad stunned the hi-fi world by introducing the first full-range speaker based an electrostatic (as distinct from the usual electro-dynamic) principle of operation.

That original Electrostatic (for that was its formal name) stayed in production for more than two decades. Its successor, codenamed the ESL-63 (because work started in 1963), didn't actually appear until late 1981, and remained perpetually back-ordered until the mid-1990s, when it became impossible to build within modern safety standards.

Come the millennium, with the brand stabilised under IAG control, the electrostatic speakers had gone back into production, with two models rather than one. The ESL-988 and ESL-989 were based closely on the ESL-63, while incorporating a number of changes, partly to satisfy safety legislation, and to strengthen the speaker mechanically.

Further development work since then has served up two new versions - the ESL 2805, plus its bigger '2905 brother. Both models now have substantial steel and alloy frames, extended fabricated steel bases fitted with 8kg mass-loading billets, and a hefty strut that braces the top of the frame against the back of the base, and there are further improvements to the baffle plates and their mountings.

While it's exceptionally slim when viewed from the side, like any panel speaker it's also very bulky from the front, standing just over a metre tall and more than two thirds of a metre wide. Apart from the silver alloy extrusions that form the sides, and a curved gloss-black top, most of this is black grille, creating a sombre monolithic effect, but a gilt edge/brown grille classic version is also available. Decoration is limited to an illuminated Quad logo near the base on the front, which can be turned off.

Electrostatics operate on a totally different principle from conventional moving-coil drivers, generating their sound output from a series of large panels of ultra-light plastic film - six in the 2905, four here - stretched between high voltage charged plates and all working as one big panel. This distinct approach has numerous consequences, the most significant being that the panel radiates sound equally forwards and backwards, but the radiation of each is out of phase with the other, so where they mix (at the sides), they cancel each other out.

It's known as dipole or doublet operation. This front-to-back cancellation also means that the absolute bass extension is actually determined by the width of the panel. Dipole operation results in a figure-eight sound radiation pattern, rather than the substantially spherical pattern created by a conventional box loudspeaker, and this has several implications. The sound is delivered forward and back, but much less goes up, down or to the sides, resulting in a high direct-to-reflected sound ratio. It also allows the speakers to be placed close to side walls.

This latest ESL features improved materials for greater mechanical integrity and sonic transparency, but at its heart is still Quad's extremely clever 'delay line' approach, originally introduced in the ESL-63. The problem with any large-area diaphragm is that it will radiate higher frequencies in an increasingly narrow beam, whereas the ideal would be to maintain constant directivity at all frequencies.

This is a major reason why most speaker systems have two or more drive units, the addition of a tweeter providing wide directivity at high frequencies. The Quad design is different: it may sport two outer panels flanking a pair of smaller central panels, but these elements are fed the same frequencies and there are no 'treble' and 'bass' units... and no need for a crossover.