L-ACOUSTICS MTD Manual V1.2 11/30/2004 49
The subwoofer output for all LLC analog controllers has been calibrated to provide approximately 6
dB of low frequency contour when subwoofers are used with MTD108a, MTD112b or MTD115b
enclosures in a 1:1 MTD:subwoofer ratio. In order to compensate for sensitivity differences between
subwoofer models, the relative sub output channel level has been adjusted for individual subwoofer
settings (SB115, SB118 or SB218) so that the same 6 dB low frequency contour is obtained regardless
of the subwoofer model in use.
Note: For MTD108LLCa, LLC112b–st and LLC115b-st controllers, the 6 dB low frequency contour is only
obtained when both A and B channels are used (since the subwoofer output is derived as the mono sum of
both input channels). For these LLC models, if subwoofers are operated in stereo then two LLC controllers
are required and it is recommended that channel A inputs are linked to channel B in order to provide
increased subwoofer drive level. Please see Figures 9, 12 and 15 for examples.
Note: LLC controllers do not include sense return protection for the subwoofer feed. It is important that
power amplifier specifications are in agreement with the recommended values given in Table 17. For
additional protection, MLS switch load matching (and/or outboard compression/limiting) is recommended.
5.8.1 Combining MTD With Subwoofers
Two cases can be considered: ground stacked systems where MTD and subwoofer enclosures are
closely coupled physically or separate flown MTD loudspeakers with ground stacked subwoofers. For
some applications, subwoofers are used as an effect and are not driven with the same signal as the
main MTD system (separate auxiliary send from the console) while in other cases, the subwoofers are
used as a low frequency extension and are driven with the same signal as the MTD enclosure.
Note: An additional LLC controller will be required to drive subwoofers separately via auxiliary send.
In general:
X-OVER
mode for all LLC controllers provides a 100 Hz 24 dB/octave Linkwitz-Riley (LR24) low pass
filter for the subwoofers with a complimentary 100 Hz LR24 high pass filter for the MTD112b or
MTD115b low section. Complimentary 100 Hz crossover filtering optimizes the power bandwidth for
the subwoofer and the low frequency component of the MTD112b or MTD115b. For the MTD108a,
complimentary high pass / low pass filtering is at 125 Hz. X-OVER mode is recommended for all
stacked MTD/subwoofer applications where the enclosures are closely coupled physically.
FRONT or MONITOR
mode for all LLC controllers still provides a 100 Hz LR24 low pass filter for
the subwoofer feed but there is now an overlapping LR24 high pass filter for the MTD enclosure (50
Hz HPF for MTD112b, 45 Hz HPF for MTD115b). Due to the overlap in the operating bandwidths of
the sub and MTD enclosure, subwoofer polarity may have to be inverted due to the phase shift of
overlapping crossover filters. Subwoofer polarity will depend on how the subwoofers are driven, i.e.,
via AUX send or with the same signal as sent to the MTD system, and when using FRONT or
MONITOR settings for flown MTD / ground stacked subwoofer configurations, subwoofer polarity
should be experimented with as part of the time alignment and tuning process.
5.8.2 Ground Stacked Systems
For ground stacked systems, MTD and subwoofer enclosures are physically close to each other and
time alignment is simplified throughout the audience area since there are no arrival time (path length)
differences between the flown MTD system versus ground stacked subwoofers. Typically, LLC
controllers are used in X-OVER mode for ground stacked configurations in order to optimize power
bandwidth for the subwoofer and the MTD low frequency component.
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