Quad ESL-63 (FRED)
The Quad ESL-63 is a full range electrostatic loudspeaker. It reproduces sound, soundstage and stereo image very precise and faithfully. Recording engineers and professional loudspeaker builders regard it as a reference loudspeaker for its liquid mid range.
The speaker is cleverly designed. I am not aware of tweaks that can
improve the sound quality, except for sturdiness. All parts are
carefully balanced. Four elements are
placed above each other. The top and bottom elements are for bass.
In between there are two elements with 7 circular concentric
conductors, which are fed by an all pass network that acts as an
electrical delay line. This elegant construction gives the ESL
character of a point source and an accurate time relation between
high and low frequencies. The ESL-63 has the quite unique ability
to produce a square wave that actually looks like one. The lossy
character of the delay line also corrects for the reflections at the borders
of the membrane. In between the elements are small spaces that suppress
resonance. The stators are made of a synthetic material with the
conducting layer on the outside. A highly resistive coating is applied
at one side of the membrane to hold a constant charge for minimum
distortion. Resonances are suppressed both by the cloth, and the cloth
that is glued to the inside of the rear stator.
The distortion produced by the ESL-63 is extremely low. Average distortion is 0.03%, only rising to 1% at 40Hz. Considerably lower than that of quality dynamic woofers- at a sound pressure level of 96dB. Maximum excursion of the membrane is ± 2.5mm. Thickness is 3 µ. Membrane Polarization voltage is 5.25kV. Maximum audio voltage is 10kV. The two high quality transformers give a 250 step-up rate. Total membrane surface is approximate 0.4 m2.
Quad has made
successors, the ESL-988 and ESL-989, based on the
same principles. The ESL-2805 and ESL-2905 are a further enhancement. Both
are mechanically redesigned ESL63's (and heavy), but with the same ESL panels
and electronics. The transformers though are of a new design. In full range mode bass
is substantially better than the bass of the ESL-63' s.