Whilst excellent results are possible from
passive systems, a multi-way loudspeaker
system will only realise its true potential if
implemented as an active design, due to the
advantages displayed top right.
The
SCM20ASL Limited Edition realises these
advantages via its on-board
ATC Amp-Pack,
a 2-way amplier module featuring balanced
input, precision 2-way active crossovers and
a pair of class
A/B power ampliers. The
location of the amplier module, in the rear
panel of the loudspeaker cabinet, results in a
very short connection to the loudspeaker
drivers and outstanding low frequency speed
and control.
Connection to the amplier is made via
XLR
and balanced input circuitry is used for high
noise rejection. To ensure the best match to
the partnering source/pre-amplier, the
amplier Input Sensitivity can be adjusted via
a combination of switch and precision
potentiometer. In addition, a low frequency
shelf cut/boost allows the user to tailor the
bass response to the room and/or taste.
A classic Linkwitz-Riley 2nd order active
crossover is employed to integrate the audio
to the two drive units. The crossover has the
added benet of an all-pass lter stage which
allows the Phase response to be optimised
through the crossover region, resulting
in superior tonal balance and imaging vs the
equivalent passive model.
The two power amps deliver 200W and 50W
to the bass/mid and high frequency drivers,
respectively. The Output stages are a class
A/B MOSFET design, and each amplier
is carefully optimised for its application.
As with all of
ATC’s loudspeaker driver
components, the amplier is designed and
manufactured at
ATC’s factory in the UK.
The magnetic characteristic of the steel
used in the magnet assembly of a loudspeaker
is inherently non-linear due to magnetic
hysteresis. The magnetic eld generated
around the voice coil modulates the
permanent magnetic eld of the motor and
the magnetic hysteresis of the steel results
in a delayed return to the permanently
magnetised state. This results in a distortion
of the input signal in the loudspeaker causing
audible colouration.
To counteract this problem, the bass driver
tted to the
SCM20ASL Limited Edition
employs
ATC’s Super Linear Magnetic System.
Rings of Super Linear Magnet Material are
added to the magnet assembly pole and front
plate, adjacent to and concentric with the
loudspeaker voice coil. The eect of the rings
reduces 3rd harmonic distortion by between
10-15dB across the 100Hz to 3KHz frequency
band. The addition of the
SLMM produces a
signicant improvement in sound quality,
reducing the level of distortion so
dramatically as to reveal another layer of
information to the listener. Ambient sounds
and low-level eects that were previously
masked are now clearly audible and create an
enhanced sense of realism.
The driver motor features a 75mm/3”
diameter voice coil, operating in
ATC’s
favoured short-coil-long-gap (underhung)
conguration. The large diameter increases
coil surface area, in-turn increasing power
handling, and reducing power compression.
A long magnetic gap delivers +/-10mm of
linear excursion, ensuring exceptional bass
reproduction.
On rst inspection, the high frequency
driver tted to the
SCM20ASL LE looks like
many other 25mm soft dome tweeters,
however, when we focus on the details, this
device is far from ordinary and employs some
unique features, inspired by the renowned
ATC 75mm/3” mid-range.
The rst is a dual suspension system whereby
instead of supporting the voice coil (and
dome) with a single suspension, the
SH25-76S
features dual-suspensions. One is level with
the dome and visible from the outside of the
part and the other is adjacent to the coil
and hidden within the assembly. The spaced
pair of suspensions oer superior stability to
the common single suspension solution and
ensure very precise control of the dome-coil
assembly, even at high drive levels. The
increased control allows for a much narrower
magnetic gap, increasing magnetic ux
density which increases eciency and high
frequency extension.
Secondly, to maximise eciency and reduce
harmonic distortion to an absolute minimum,
the
SH25-76S features a very large N48M
Neodymium magnet assembly,
FEA optimised
to achieve complete magnetic saturation of
the iron pole and front plate, in the regions
close to the voice coil.
These two special features in combination
with exceptional production engineering
and quality control result in a high frequency
driver that perfectly translates every
aspect of a recording, from a thunderous
crescendo, to the minute detail of a gently
plucked bow-string.
SCM20ASL LE
TECHNOLOGY
10
ACTIVE SYSTEM
AMPLIFIER
BASS DRIVER
HIGH FREQUENCY
DRIVER
7 ADVANTAGES OF AN
ACTIVE SYSTEM
1 MORE ACCURATE
CROSSOVERS DUE TO
FIXED FILTER ‘LOAD’
2 WIDER PRACTICAL
CHOICE OF CROSSOVER
TYPE/SHAPE DUE
TO ACTIVE LINE LEVEL
OPERATION
3 GREATER DYNAMIC
RANGE DUE TO
ELIMINATION OF PASSIVE
CROSSOVER LOSSES
4 IDEALLY MATCHED AMPS
AND MORE EFFICIENT USE
5 IMPROVED FREQUENCY
RESPONSE AND STEREO
MATCHING DUE TO
SPEAKER-AMP SYSTEM
CALIBRATION
6 BETTER DRIVER
CONTROL DUE TO A
MORE DIRECT
CONNECTION TO THE
DRIVERS WITH FAR LOWER
ELECTRICAL RESISTANCE
7 LOWER
INTER-MODULATION
DISTORTION DUE
TO AMPLIFIER
REDUCED OPERATING
BANDWIDTH
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