"The Quad
ESL-63 sets new standards in three important areas of loudspeaker technology,
low distortion, low colouration and controlled directivity, resulting in a a
level of performance limited solely by the quality of programme source.
With Quad Esl-63's
and a CD player less than 0,1% THD
intrudes between the listener and the studio master tape, reproducing exactly
what was recorded, nothing added, nothing taken away and bringing the listener
that much closer to the original sound."
From Quad Advert
An electrostatic
loudspeaker consists of light thin membrane on which is placed a charge,
suspended between two acoustically transparent, (i.e. with holes), electrodes.
When asignal is fed to the electrodes a field is generated between them and the
charged diaphragm complled to move by an electrostatic force. Reversing the
polarity of the signal on the electrodes causes the diaphragm to move in the
opposite direction and thus sound pressure is generated.
The electrostatic
loudspeaker has several inherent advantages. Since the diaphragm is driven over
its whole surface it does not ned to be stiff and can be made extremely light
and thus have negligible stored energy. Because of the good impedance match with
the air it is not necessary to load the loudspeaker into a box and the problems
associated with cabinet resonance disappear. Designing and manufacturing
electrostatic loudspeakers is by no means easy but the potential is there for a
higher degree of performance than with conventional loudspeakers.
An ideal loudspeaker
would consist of a single point source reproducing all frequencies. The Quad
ESL-63 uses the electrostatic principle to achieve just this.
Imagine a
theoretically ideal point source loudspeaker radiating sound pressure waves and
a plane in the air a short distance from the source at right angles to the
direction of propagation. Were we able to make the air at the plane visible we
should see concentric waves radiating out from the centre, as if we had just
thrown a stone into a still pool.
If we replace the
plane with a very light membrane, induce it to reproduce the air particle
motion already observed and suppress the original source, the results to a
listener on the far side are absolutely indistinguishable from those of the
ideal source.
The Quad ESL-63 does
exactly this.
It consist of a
diaphragm suspended between two sets of concentric annular electrodes. Signal
is fed to the electrodes via sequential delay lines and the diaphragm motion
produces a sound pressure pattern which is an exact replica of that from an
ideal source placed some 30 cms behind the plane of the diaphragm. The ESL-63
is a totally homogeneous sound source, phase true and very aperiodic with a
frequency response both on and off axis quiete free from the irregularities
which are inevitable with any multiway loudspeaker system.
The designer has
complete control over the directivity of the loudspeaker and by increasing the
directivity index smoothly with increasing frequency, optimum stereo
performance is achieved.
The Quad ESL-63 is a
dipole which has significant benefits in terms of room placement and stereo
perception. With a sound dispersion pattern which resembles a figure of eight,
a dipole radiates no energy in the plane of its diaphragm and does not excite room
modes whose axes lie in this plane.
The nett result is a
loudspeaker of unsurpassed accuracy which given the right programme material
will produce a more realistic and satisfactory illusion of a live musical
performance than has previously been possible.
Development
of the Quad ESL-63 has taken many years. Much of the time has been spent in
investgating and experimenting with new materials and production techniques,
the result of which has been that variations due to manufacturing tolerances
have been virtually eliminated, sample variation being to within less than ±½ dB and stability of the materials used is
such that the performance will be maintained for the loudspeaker's useful life
without the need for regular maintenance.
The
impedance characteristic of the lodspeaker is nominally 8Ω and largely resistive, presenting no problems at all to the amplifier,
but amplifiers which are not short circuit protrcted must not be used with this
loudspeaker. The amplifier should have
an output capabilty of 40 V peak (100 watts into 8Ω). Amplifiers with output capability of up to 55V peak (190 watts into
8Ω) can be used, but to no advantage.
The loudspeaker is
fitted with two protection circuits, one which limits the maximum input voltage
fed to the loudspeaker and the other which detects fault conditions and
instantaneously shorts the signal being fed to the loudspeaker. It is
impossible to damage the loudspeaker elements, but the input voltage limiter
has limited thermal capacity and persistent overdrive will overhear it.
Specification
Impedance: 8 ohms nominal
Sensitivity: 1.5 µbars per volt referred to 1m. (i.e.
86dB/2.83V rms)
Maximum
Input:
Continuous
input voltage : 10 V rms;
Programme
peak for undistorted output : 40 V;
Permitted
peak input : 55 V
Maximum Output: 2 N/m² at 2m on Axis
Directivity
Index:
125 Hz :
5.0 dB
500 Hz :
6.4 dB
2 kHz :
7.2 dB
8 kHz :
10.6 dB
Axis
Band limits: -6 dB at 3 Hz 3rd order
(Low
Level) : -6dB > 20kHz
Power
Supply: AC 240/200V 120/100V, 50/60Hz, 5
VA
Dimensions:
Height :
925 mm
Width :
660 mm
Depth :
270 mm including 150 mm base
Weight
Net: 18.7 kgs
Gross: 23 kgs