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 both the 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.
When a non protected electrostatic loudspeaker is driven up to its maximum sound pressure, the danger of sparking exists and holes can be burned in the membrane. Several measures are taken to insure that the ESL-63 will not be damaged. Inside the pedestal is an antenna that detects when the air gets ionized, just before sparking. The attached circuit short cuts (!) the input of the ESL membranes, only a 3.3 Ohm load is left for the amplifier.  A characteristic of a dipole loudspeaker is that below the cancellation frequency -that is determined by the size of the speaker-, the output will decrease with 6dB/octave. An increased Q factor extends bass reproduction.
In parallel to the 1.5 Ohm input resistance is a 220uF capacitor for bypassing the mid/high frequency's . 

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.
A quite remarkable version seems to be the ESL-63 Qa from Quad Musikwiedergabe ! Again, the original ESL-63 panels and electronics in a sturdy frame.
The speakers we own are from 1988 and are in an original state. They still are performing well. We prefer them over our ESL-2805's. But it must be said, we use the ESL-63's  in combination with the dipole subwoofers. The ESL-63 has a certain smoothness without loosing detail and is somewhat forgiving on less optimal recordings.

Links

• Quad world. Information and links, good!
• Pol Berchet provided ESL 63 graphs and lots of vintage quad information.

Pictures

• Quad ESL-63 , Full Range Electrostatic Doublet

• Without cloth, reveals the concentric structure of FRED.

• Bottom view, shows electronics in the pedestal

• Mid unit, with concentric structure (picture by Stephane Tsacas)

• Lowest unit, folded open with Mylar on the half on the floor. The conducting layer is black. Behind a glimpse of the mid unit (picture by Stephane Tsacas)

Production pictures

• Coating of the diaphragm

• Stretching the diaphragm

• Shrinking the dust cover, which can resonate!

Technical

• Scheme, first series

• Scheme, first revision

• Scheme, last series

• Directional pattern of the front lobe, the back lobe is symmetrical. Measured frequency's :125, 500, 2k, 8k Hertz.

• Plots , distortion, sine wave response, 1/3 octave pink noise response, impedance characteristic and text

• Service data manual for the ESL63.

• Service data manual for the ESL2905 (SDS Sheldon Stokes).

• Sound (in press reviews)
  Cutting edge in terms of midrange finesse
  High detail levels
  Tonal smoothness
  Cohesive sound
  Excellent stereo imaging