June 2012 - This page
is currently under construction,
I'll gradually add more to it...
These speakers were donated to me
by an audio collector and engineer. They are unique
because not only did this person
know Alex Shackman, he also designed
and built the speakers himself back
in the day. Also, the design came
from the idea of using an
electrostatic and a cone
loudspeaker together in one box in
order to use the advantages of both,
altogether creating quite a unique
piece of audio gear.
Why ESL?
Electrostatics have always
attracted a cult following among
some audiophiles - Quad's research
which led to their ESL series of
speakers created what are still
regarded as some of the finest
loudspeakers in the world. The
advantage of an electrostatic is
that it has a virtually mass-less
diaphragm, something which makes it
able to react very fast to transient
details in sound, something which a
traditional cone loudspeaker could
only hope to aspire to. The
electrostatic sound is often
referred to as very open and
incredibly detailed, something quite
unlike the sound from a cone
loudspeaker. Another advantage is
that a single ESL panel can range
most, if not all, of the audio
spectrum, which means there is no
need for crossovers and the phase
distortions which they bring.
The negative aspects are that
electrostatics are more difficult
for an amplifier to drive and also
tend to have a low sensitivity AND
low power handling (overdriving an
ESL could potentially cause the
diaphragm to strike one of the
charged plates, causing serious
damage to the driver). Room
placement is critical for good
imaging (and for adequate bass
response if the speaker is
full-range).
Another disadvantage of the ESL
comes from the fact that although
the diaphragm tends to have a large
surface area, its maximum excursion
is much smaller than a cone
loudspeaker is capable of and hence it can't move
the same amount of air at lower
frequencies, which gives the
electrostatic its reputation for
being bass-light; this is the very
reason that the hybrid
electrostatic/cone design evolved.
Why the hybrid design?
As you've already seen, one of
the main weaknesses of the ESL is
its lack of low end response, this
happens to be the very thing that a
traditional cone woofer excels at -
this is where the hybrid idea begins to
make sense.
If you create an ESL which can
work down to a couple of hundred
hertz
and then have the bass woofer take
over below that point, you not only
remove the frequency portion that
the ESL has difficulty with, but
also the woofer never works above
that same crossover frequency, so
the higher frequencies are handled
by the ESL alone and so don't get
coloured or mudded by the relatively
sluggish response of the cone driver - the
agility of the ESL reigns supreme
here.
If designed well enough, you
wouldn't even need any crossover
components - the ESL can be made to
have a natural response which ends
at several hundred hertz, the bass enclosure and bass driver can be
tuned in the same way.
How an ESL works
The electrostatic principle is
simple; two electrically charged
plates with a diaphragm suspended in
the middle (typically with a
distance of only a few millimetres).
The plates are charged to a high
voltage with the charge of the
diaphragm balanced between.
 |
|
ESL
principle (image
courtesy of Wikipedia) |
In practice, the HT supply is
commonly anywhere from a few hundred
to a few thousand volts, and because
this only needs to be a polarising voltage
and the required currents are very
low, something like a voltage
multiplier circuit would work fine.
The original Shackman drivers used a
relatively low HT voltage of +/-
500v, fed from said voltage
multiplier.
The audio input is coupled
through a step-up transformer, not
unlike a valve output transformer
connected in reverse. As the audio
signal alters the potential of the
diaphragm in relation to the
potential of the plates, the
diaphragm is attracted to the plates
in turn and so vibrates and produces
sound.
About Shackman
Alex Shackman was an ex-BBC
engineer who started his own company
to manufacture electrostatic
loudspeaker driver units. I'm
not quite sure what happened to Shackman, but he would've been
around 70 years old during the late
1970's and all trace of the company
seems to disappear around
1990.
About Dalesford
Dalesford was a small loudspeaker
manufacturer based in the north of
England; most of their work was OEM
- drivers built for other companies; they did quite a lot of production
work for KEF, among others. The
company was started in 1976 by John Sugden (of Sugden amplifiers fame)
but only survived for a short time,
going out of business in 1983. There
is little information around about
Dalesford, most of what I found is
from the
Falcon Acoustics Archive.
The speakers
When the speakers came to me,
they were missing the electrostatic drivers
(these had been damaged and removed
years ago) but were otherwise intact and in
good condition. The
speaker cabinets themselves were mid
sized floor standing boxes, divided
into two chambers - a ported lower
chamber for the bass driver, and a
separate upper chamber for the ESL
driver. The cabinets were built from
¾" MDF, with a double layer in
places such as the baffles, making a
total of 1½".
The bass drivers used in these
speakers were Dalesford D100/250, a
10" 8 ohm bass driver with 100 watts
power handling and an 88dB
sensitivity.
 |
|
Rear view of Dalesford
bass driver |
As the ESLs were missing by the
time they came to me, I had no idea
what should've been in the space,
except for knowing they were
certainly made by Shackman. The
MHT-85 was from that era and the
dimensions seemed quite close.
More to come
soon...
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