PMC TLE 1S Active Subwoofer

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                   PMC TLE 1S
                   Active Subwoofer
                   HUGH ROBJOHNS MIBS reviews PMC’s unusual and
                   technically intriguing active subwoofer, designed to
                   complement the company’s smaller monitors.

                           he Professional Monitor Company has reputation        form to integrate
                           for monitoring systems that combine high              in the
                           quality reproduction with unusually extended          monitoring
                   bandwidths for the size of each model’s cabinet –             environment.
                   thanks to highly sophisticated Transmission Line              Market research
                   techniques. However, the laws of physics still mean           led PMC to the
                   that the full range, all-in-one loudspeaker requires a        conclusion that
                   large and imposing cabinet, and this can be difficult to      a tall, slim
                   integrate in the increasingly compact designs of modern       cabinet would
                   sound control rooms. Consequently, smaller ‘satellite’        be more practical and acceptable, both professionally
                   monitors are often used in combination with one or            and domestically – so the width of the front baffle was
                   more subwoofers to fill in the bottom octaves.                chosen to match that of the TB2 and FB1. This
                   Furthermore, a subwoofer is required to accommodate           restricted the maximum drive unit size to 6.5 inches,
                   the LFE channel (as well as handling the low bass from        but a pair of 6.5-inch speakers working together
                   the satellites in many systems) in 5.1 surround systems.      provides a similar surface area to a single 10-inch driver
                   The same trends are apparent in the domestic market           – and two smaller cones are inherently stiffer than one
                   too – an important consideration for PMC these days –         big one, therefore producing less distortion.
                   where sat-sub combinations achieve far greater                     As with all of PMC’s transmission line designs,
                   domestic harmony.                                             internal damping is provided by a bespoke foam
                       Although PMC already produces several large               material. The density, thickness, profile and location is
                   subwoofers for its professional monitoring systems, the       carefully specified and controlled to act as an acoustic
                   company recognised the need to develop a compact              crossover, absorbing the upper bass but supporting the
                   active subwoofer to complement its small two-way              lower bass. To reproduce frequencies down to 22Hz
                   systems: the LB1, TB2, FB1, and DB1, as well as the           (-3dB) a line length of three metres was required and
                   latest three-way model, the OB1, when used in 5.1             this fixed the final dimensions for the cabinet at 200 x
                   installations. The resulting design is called the TLE1, and   550 x 545mm (whd). The line is folded three times to
                   required over 30 months of R&D effort.                        exit at the rear, which enables the amount of very low
                                                                                 bass to be adjusted slightly relative to the higher bass
                   Design Philosophy                                             frequencies by altering the distance between the
                   Most subwoofers – and certainly many of those aimed           cabinet and rear wall.
                   at the consumer market – are optimised for use in 5.1              For the record, the unit weighs 17.5kg and four
                   cinema systems where the LFE channel carries little           adjustable spikes are included to secure and level the
                   more than big bangs and crashes. Consequently, these          unit on the floor. A removable cloth grille covers the
                   subs tend to be designed for volume and impact, rather        front drive units and matches the styling of other PMC
                   than the low distortion levels essential for accurate         speakers. The standard cost-effective ‘studio’ model has
                   music reproduction. In contrast, the TLE1 was designed        a black painted finish, but optional Black Ash, Cherry,
                   from the outset as a very low distortion system               and Oak veneers are also available.
                   specifically intended to extend the bandwidth of the
                   company’s smaller speakers, without compromising              Activation
                   their monitoring quality or resolution in any way.            The TLE1’s built-in amplification is essentially a Bryston
                        A significant advantage of a low distortion              2BSST modified slightly to provide 150 Watts (RMS)
                   subwoofer is that it really is impossible to locate its       into the three-ohm load presented by the two drivers.
                   position aurally. Many subs generate mid and high             There is no protection circuitry as such, but the power
                   frequency components purely through distortion in the         supply is designed so that it runs out of steam before
                   speaker itself, even though the input signal may be low-      the drivers can be damaged! The amplifier accepts
                   pass filtered, and these artefacts enable the ear to locate   inputs on both balanced XLR and unbalanced phono
                   the sub with ease.                                            sockets, with two of each for easier integration in
Price:                  Constructional simplicity has led most subwoofer         stereo systems – but there is no ‘filtered’ feed-through
£1124 +VAT         designers to use cuboid cabinets with a single 10 or 12-      facility.
                   inch driver. However, this is often an inconvenient

28 LINE UP Feb/Mar 2004
     The user controls are hidden behind a flap at the
top of the speaker cabinet, just like the AML1, with an
input selector button (balanced/unbalanced), level,
phase and turnover frequency controls. The low pass
filter can be adjusted between 40 and 155Hz, or
bypassed when the filtering is performed by a bass-
management system. The phase control is rather clever,
spanning a complete 360 degree phase shift from end
to end.
     The TLE1 is specified to cover 22 to 200Hz, but
with the filters bypassed the unit delivers a perfectly
respectable performance up to about 500Hz, and many
surround mix rooms take advantage of this capability to
reveal when inappropriate sounds have been routed
accidentally to the LFE channel.

In Use
I tested the TLE1 in two separate PMC speaker systems:
a compact stereo system using a pair of powered TB1S
speakers; and a 5.1 arrangement with IB1 main
monitors, an LB1C centre channel, and the TB1S
speakers at the rear. In the stereo system the TLE1’s
own crossover was employed, while in the surround
system I used the bass-management facilities in the
source equipment.
     In both cases integrating the subwoofer with the
rest of the system was surprisingly simple using a pink
noise source to match levels with a sound level meter,
followed with a little fine-tuning by ear to optimise the
level, crossover and phase settings. I switched the main
speakers off to check the TLE1’s output, and discovered
it to be satisfyingly rumbley with nothing audible in the
way of higher harmonics – confirming the low
distortion aspect of the design. Switching the sub on
and off had no obvious effect on the stereo imaging
either, but the extra weight and solidity of the sound
when the sub was working proved quite addictive! The
TB1S is a well extended speaker anyway, but adding
the TLE1 certainly filled out the bottom octave well,
adding weight and body to string basses, bass drums,
and organ pedals without detracting from the neutrality
and accuracy of the midrange.
     Both the use and the relative level of the LFE
channel seems to vary greatly in DVD-A and DTS-
encoded surround music sources, and this means that
the LFE level has to be adjusted with almost every
change of source material. However, when set
correctly the TLE1 adds a worthwhile extra weight and
solidity to the mix. On movie sound tracks, all the
bangs and crashes on the LFE channel were handled
effortlessly, the TLE1 generating frighteningly high SPLs
when required!
     The TLE1 seems able to add a fast and dynamic low
bass to the smaller PMC systems without destroying the
all-important midrange, and is more than capable of
handling the LFE channel in medium sized surround
systems. This technically impressive and unusual design
is the perfect partner for a wide range of PMC monitors
– especially the DB1 and TB2 – in both stereo and
surround applications.

Tel: +44 (0)870 444 1044


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