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. Four elements are placed above each other.  The top and bottom elements are for bass. In between are two elements with 7 circular concentric conductors, which are fed by an allpass network that acts as an electrical delay line. This elegant  construction gives the ESL the character of a point source and gives a smooth transition from high to mid frequencies. Beside that, the lossy character of the delay line 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 surpressed both by the cloth, and the cloth that is glued to the inside of rear stator.
When an ESL 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 speaker will not be damaged. Inside the pedestal is an antenna that detects when the air gets inonished, just before sparking. The attached circuit short cuts (!) the input of the speaker, a 1.5 Ohm load is left for the amplifier. Beside that a soft clipper is implemented that starts distorting the signal at -3dB as a warning. A characteristic of a dipole loudspeaker is that below the cancellation frequency, determined by the size of the speaker, the output will decrease with 6dB/octave. The series resistor of 1.5 Ohm compensates this effect that by increasing the Q factor. In parallel is a 220uF capacitor for bypassing the mid/high frequency's . 

The distortion produced by the ESL-63 is extremely low. The ability to produce a square wave that actually looks like one is quite unique. Average distortion is  0.03%, only rising to 1% at 40Hz -90% 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 micron 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 two new loudspeakers, the ESL988 and ESL989, based on the same principles. Both are essentially completely redesigned ESL63's, with up to date electronic parts. The newest 2805 and 2905 are a further enhancement with improved stifness. A quit remarkeble version seems to be the ESL-63 Qa from Quad Musikwiedergabe !

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