The Quad ESL-63 and Gradient SW-63 Combination
Reviewed by Mats Törnqvist, November 1999.
Now the time has come for me to review
my speaker combination. I think it could be interesting to read
my opinion about the tweaked ESL-63/SW-63 combination since there
is a significant difference in performance when the tweaks are implemented.
A difference from regular reviews is that I have not done A/B testing
with other speakers, I have lived with the loudspeaker for many
years playing all sorts of music. This is a test of time that reviewers
are not capable of and it takes some time, often months, to spot
the failures and added sound that HiFi equipment are responsible
for. The first reactions are sometimes wrong, and A/B testing certainly
favours equipment that add the little extra that makes it stand
out in front of the others. My ESL-63s have serial number 16185
Technical Description Quad ESL-63
The Quad ESL-63 is an electrostatic loudspeaker with a unique
feature in the concentric rings that are delayed in a manner that
makes it work like a point source positioned 30 cm behind the membrane.
The membrane in electrostatic loudspeakers are usually considered
to be lighter than air, meaning that all the resistance made to
the output signal when it moves the membrane comes from the air
itself. This is what makes the electrostatic speaker response so
exceptionally fast and controlled. The light membrane is also the
reason to why a single element can be used to reproduce the whole
frequency range, and this is where the concentric rings comes in
and makes the response free from beaming found in all other large
panel speakers, and the off-axis response is unusually even for
a dipole speaker. No filters are needed and this is a major advantage
since such are difficult to make perfect and there is no interaction
between filter components and the amplifiers output impedance (includes
the wires). The other underlying physical element that makes electrostatic
loudspeakers interesting is that the moving force is distributed
throughout the entire membrane, making it controlled and free from
break-up found in all cone speakers.
The delay line is made up of six concentric segments,
each delayed with 24us which is the same as if the segment was positioned
8mm further away than the previous. The speaker is equipped with
four elements, the outer two elements are not segmented and are
used as (delayed) bass reinforcement panels.
The delay principle really works which is obvious
if you look at the polar plot.
At 8 khz the response is within -3dB at angles ±
25 degrees and this is a lot better than the Quad ESL-57's 7 degrees.
The directivity is only 5dB up at 8 kHz which is remarkable for
such a large membrane. The -6dB points are at 35 Hz and above 20
kHz. Sensitivity is 86dB at 2.83V input, maximum undistorted input
is 40V, maximum output pressure is 2N/m² at 2m distance.
The polar plot above is an example which probably
is similar to the Quad ESL-57 bahaviour at 8 kHz. The old ESL has
a very narrow sweet spot. At the end of this page there are some
dispersion examples and a directivity curve for a 12" speaker.
Technical Description Gradient SW-63
SW-63 subwoofer is designed as an add-on unit for Quad ESL-63 and
the combination is actually better looking than the Quad ESL-63
on its own. The SW-63 is a dipole subwoofer with two 12" long-throw
(± 12mm) cone speakers pointing in opposite direction to
reduce distortion levels. The ESL-63 is placed under a lid on top
of the subwoofer with an opening for connections. I have placed
my ESL-63's on sorbothane feet and clamped them with the lid, making
the Quads very firmly held in place. The ESL-63 sounds quite a lot
better on its own when mounted in the Gradients because of the higher
position and better stability. The overall stability of the Gradient
subwoofer is pleasingly good, the subwoofers are not cheap but very
well made. The dipole action gives a 6dB/octave reduction of level
and this is corrected by the Gradient filter unit giving a flat
response with -3dB at 28Hz (I have changed this to -3dB at 22Hz).
The filter unit has two outputs, SW with a 3rd order filter at 115Hz
and ESL with a 2nd order filter at 115Hz. There is also a mono switch
and two attenuation settings giving a slight reduction in the low
midrange. The set-up requires a separate amplifier for the subwoofers
and the two 8 Ohm units in each SW-63 can be connected either in
parallel (recommended) giving a 4 Ohm load or in series giving a
16 Ohm load. The combination is able to deliver an SPL around 110dB
with good bass extension.
action (sound is emitted in anti-phase from the behind of the speaker)
actually makes the set-up quite easy, the only thing to remember
is not to place the speakers too close to the wall behind (at least
1 m distance) because if the reflection from the back wall is not
delayed enough it will alter the frequency response. If the distance
is large enough the brain can sort things out and practically ignores
the reflected sound (nice brain). It is the early reflections that
make up the alive feeling in good recordings and the more delayed
sounds are heard as echoes rather than ambience. In my living room
the distance to the rear wall is more than 3 meters. The reflections
from the side walls is of no major importance with dipole speakers
and neither is the ceiling reflex, the floor reflex is damped well
enough with a thick carpet. The standing waves are only excited
in the speaker direction dimension. Even if the reflexes and standing
waves are damped the positioning and angling of the speakers is
important, I have spent much time trying to find the optimum position
within the limits of the rest of the furniture and the overall looks
of the living room. Even if the positioning is important the ESL-63/SW-63
combination will never sound bad, you can put them in the corners
if you have to but it is not optimal. I use a tube amplifier (Jadis
DA-30) for the ESL's and a sand amp for the subs. I think that the
ESL-63 combined with a tube amplifier is one of those happy marriages
and it seems as if the new Quad II-forty combined with ESL-989 raised
the Quad performance quite a bit at the London HiFi show, possibly
the valve amplifier is a major part of that.
I am very sensitive to equipment with a sound of their own,
at first it may sound nice giving a more exciting sound than the
sound-free equipment but in the long run it is simply boring when
all records sound similar and the sound also masks the important
details that makes music listening such a great occupation. This
is why I like the ESL-63/SW-63 combination, the ESL-63 has absolutely
NO sound of its own and the SW-63 is the most sound-free bass units
I have heard.
The Quad ESL-63 sounds very good without
any tweaking, but with the tweaks the resolution and clarity of
sound is improved to an astonishing level. Take a look at the 'Quad
Tweaks' page. The tweaks I have performed are:
Replaced the input electrolytic capacitor
with two Solen polypropylene 100uF/250V capacitors in parallel
and replaced the 1R5 resistor with a higher rating (50W) resistor
of better quality.
Removed the metal grids.
Removed the dust covers.
Power line filters are applied.
The SW-63 is also an exceptional add-on without
tweaking but a lot is gained (more than the ESL-63 tweaks) by the
tweaks I have performed:
By-passed the high-pass section and use
a polypropylene capacitor on the input of the main amplifier
(DA-30) for a first order hi-pass.
Altered the frequency characteristics
of the low-pass section giving a flat response almost down to
Increased the input impedance to 500 kOhm,
making it a suitable load for my tube pre-amplifier.
If the Quad ESL-63 in original shape can be described
as a window to reality with some limits in the sight and slightly
grey (not coloured) and slightly distorted glass, the glass
is changed into an almost clear glass with no distortion (the electrolytic
capacitor distorts) when the tweaks are performed. The limits of
the sight is also reduced a little.
Now we add the original
Gradient set-up and the window is increased in size, the clarity
of the window itself is also better and more details are recognizable
in the soundscape, but there is also some filth appearing on the
window making it obvious that it is a window not reality itself.
Responsible for the filth is the Gradient high-pass filter which
is not quite good enough to be invisible in my set-up. The bass
performance is exceptionally good but there seem to be some difficulties
around the cut-off frequency and this problem is addressed in the
same way as the high-pass filter problem. By-pass of the high-pass
filter is the solution (read more on the 'Quad ESL' and 'Quad Tweaks'
page) and the problems reported above are no longer. The window
size is now even larger and the filth is gone, reality is well within
reach and the soundscape appears very detailed and in proportions
rarely found. The bass is much more even and controlled, tuneful
and heavy when it shall be, almost unrecognizable when it shall.
This is landmark bass performance, but with the alteration of the
low-cut frequency to -3dB at 22Hz instead of 28Hz the bass reaches
an even higher performance level, the window size is by now very
large almost surrounding you and the window is nearly invisible,
rewarding you with a palpable soundscape as realistic as it can
be, you can almost feel the presence. The last tweak is the increase
of the input impedance which left me gasping in surprise of the
increase in performance level, the reason is that the output impedance
of my pre-amplifier turned out to be several kOhms and loaded with
only 22 kOhm (original value) the loadline of the pre-amplifier
circuit is altered considerably, reducing the level of performance.
The window is still present, but you will only notice it when you
tap with your finger upon it or try to walk through it (a slight
exaggeration of course).
Given the performance raise of
the described tweaks, what does it sound like then?
does not sound at all, it reproduces the program material. The character
of the reproduction is similar to that of the original Quad ESL-63
on their own, but with more detail and improved clarity. The dynamic
capabilities are very much better as is the bass extension which
also makes up for a much larger and more lifelike soundstage where
you can feel the air moving, combined with the ease and freedom
of coloration often found in speakers with less bass (large boxes
add coloration). The sound is perhaps not as exciting at first as
some of the popular high-end speakers of today, but it is much more
musically rewarding. Most modern high-end equipment is peaked in
the treble giving some more detail and initial immediacy, but no
other speaker I have heard can reproduce the sound of real instruments
as convincingly as the tweaked ESL-63/SW-63 combination.
At demonstrations snap and slam is over-emphasized and the program
material used are often studio recordings with transient-clipping
(almost always present in modern close-mike recordings) and with
fast speakers such as ESL's the clipped transients are clipped,
but with ringing dome-tweeters some energy is added and it can actually
sound more lifelike at such a demonstration. I can not accept speakers
that sound better on clipped program material than on good recordings
(they will ring then), let us instead hope that the recording quality
is increased. With real stereo (two-mike recordings) the Quad/Gradient
combination sounds marvellous with all the treble extension and
energy you could wish for and when the microphones are used at some
distance the early reflexes which make up the 'alive' sound are
not damped, giving you a soundscape as real as reality.
The most impressing kind of recording on ESL-63/SW-63 is a LARGE
choir recorded in real stereo, you can actually spot the individuals
and the voice reproduction is exceptional as is the sense of the
room where it was recorded. Reproduction of voices is the most challenging
task for loudspeaker constructors because the brain is so good at
recognizing voices, you can actually recognize your friends voice
in an environment full of other voices and this is by no means an
easy task, you can also easily hear if this person holds the hands
before the mouth, try to make a measuring unit that can do that!
This means of course that the speakers that can handle the human
voice best are the ones with the lowest coloration. Bass and treble
is actually much easier to reproduce in a way that fools the brain
that it is reality, still this is the areas of most interest in
many reviewing articles. These areas are of course important, but
it shall never be on the expense of the voice reproduction capabilities.
Other areas in which these loudspeakers excel is the even response
with no emphasis on specific elements or frequency ranges, the treble
that does not make any
sound of its own and is just right and
incredibly 'short', the tuneful bass with its depth and low coloration
and of course the exceptional imaging qualities.
the sound is not perfect with all recordings, but that is due to
the recordings not to the speakers. Studio recordings with no room
information are reproduced in the same way, resulting in a lifeless
flat sound in between the loudspeakers. Recordings using equalisation
and compression will sound compressed and with a non-flat frequency
characteristic. There is not much to do about the fact that most
recordings are far from perfect, but one thing is certain, the ESL-63/SW-63
combination never sounds bad. An interesting aspect of high resolution,
low coloration reproduction is that added sounds like metallic noise
from microphones is separated from the voice, making it much more
easy to listen to bad sounding recordings. The loudspeakers shall
be used for music listening and I listen to the records with music
I like rather than the ones that sound good.
not mean that I would prefer other loudspeakers for the non-perfect
recordings. I think that the Quad/Gradient combination is equally
well fitted to play Led Zeppelin or Metallica as grand scale orchestral
pieces or small scale acoustical music. The music always comes through
in great detail without any fuzz and listener fatigue is an unknown
phenomena. The only limitation is the maximum SPL which with my
tweaks probably is around 107dB, but seriously how often do you
play that loud except when you are demonstrating the system to your
There are some measurements
on the 'Quad ESL' page that I have made on my loudspeakers, these
are amateur measurements but they show some of the qualities of
the reviewed loudspeaker combination, for instance the excellent
impulse response, the even waterfall plot or the indecently low
The Quad ESL-63
is a remarkable design that will hold the opposition away for many
years to come. The design is so brilliant that any major improvements
are unlikely. At the moment I am anxiously awaiting the new Quad
ESL-988 and ESL-989 and I seriously hope it will be a major upgrade,
but I doubt it. I am sure that they will blow away an un-tweaked
ESL-63, but a tweaked ESL-63/SW-63 combination is not easy to better.
When the new ESL series arrives I will come back to this.
It is no secret that I like the ESL-63/SW-63 combination, but there
is no such thing as a worlds best loudspeaker. Other individuals
may not agree with me and they are not wrong in doing this, music
reproduction is to a large degree down to personal taste and the
Quad/Gradient combination seems to attract the more mature music
lovers with a great interest in acoustical music. Young hip-hop
lovers or head-banging heavy-metal fans would probably not make
it their first choice, so it is really up to the reader to make
up his or her own mind what kind of sound is the most preferable.
The only thing for me to say is that I am satisfied and that I can
recommend the loudspeakers to those interested in low coloration,
high resolution equipment and that the performance in the end is
the result of pure physics and ingenious design. You will probably
never reach this level of performance with ordinary methods such
as dynamic elements and boxes.
More About Directivity
Below are some examples showing the directivity
behaviour of loudspeakers, a not well known phenomena.