Quad ESL-63 loudspeaker Martin Colloms
The Quad ESL-63 uses an 0.00137" diaphragm, with 0.001" plastic film for the dust-cover membranes. (The ESL-63's diaphragm weighs just 0.003gm, or 3 milligrams, not too far removed from the tip mass of a pickup cartridge!) The diaphragm diameter is subdivided no less than eight times, providing a controlled reduction of element size with frequency, and thus maintaining a good radiation angle.
Electrostatics generally have a finite sound-level limit, due to considerations of peak voltage and electrical flashover, or saturation limiting in the cores of the step-up transformer. Other problems include aging—changes in the diaphragm tension—and dust build-up due to electrostatic attraction. The Quad ESL-63, however, is dust-proofed.
For the record, the Quad ESL-63, while not able to play rock bass to the same level as the MartinLogan CLS, Apogee Duetta,or the Magneplanar MG3.5 speakers, was, in fact, the best as regards tonal balance and low-frequency uniformity. It was also consistently neutral to a wide range of sources.—Martin Colloms