L-ACOUSTICS XT Manual V1.1 2/11/2003 19
3.3 GUIDELINES REGARDING SYSTEM PROTECTION
As standard, limit thresholds for XTA and BSS Processors are initially set at 3 dBu above the RMS
power handling for both low and high sections of the 112XT and 115XT as follows:
112XT low section : +6 dBu
112XT high section : +3 dBu
115XT low section : +6 dBu
115XT high section : +2 dBu
Amplifier input sensitivities (0 dB MLS switch setting) are as follows:
LA17a : +5.5 dBu
LA24a : +7.7 dBu
LA48a : +9.5 dBu
For the 112XT and 115XT low section, the +6 dBu limit threshold corresponds to 3 dBu above the
RMS power handling of the low section as well as the input sensitivity of the L-ACOUSTICS LA17a
(+5.5 dBu) and LA 24a (+7.7 dBu) so that system protection is performed by a combination of the
clip limiting circuits of the amplifier and the output channel limiters of the digital signal processor.
Note: L-ACOUSTICS recommends that LA power amplifier clip limiters be engaged at all times (CLIP switch
depressed in the “IN” position).
L-ACOUSTICS LA amplifier clip limiters are sonically transparent and function by monitoring the
output signal and comparing the distortion produced between the input and output of the amplifier. If
the distortion exceeds 1% THD for any reason (voltage or current clipping), the limiter reduces the
input signal proportionally (2 msec attack, 150 msec release). Under normal operation, clip limiting is
inaudible and L-ACOUSTICS recommends leaving the Channel A and B clip limiters switched ''on''
(rear panel button depressed) at all times.
As shown in Sections 2.1 and 2.2, additional protection can be obtained by using the following MLS
switch settings for power matching of the amplifier output to the power handling capacity of the XT
low and high sections:
MLS SWITCH SETTING
LA17a LA24a LA48a
112XT LO SECTION 0 dB -2 dB -2 dB
112XT HI SECTION 0 dB -2 dB -4 dB
MLS SWITCH SETTING
LA17a LA24a LA48a
115XT LO SECTION 0 dB -2 dB -4 dB
115XT HI SECTION 0 dB -4 dB -5 dB
For the 112XT and 115XT high section, the +3 dBu (112XT) or +2 dBu (115XT) limit thresholds
correspond to 3 dBu above the RMS power handling of the high section and system protection is
performed by the limiting circuitry of the digital signal processor.
Example: For the 112XT, the high section limit threshold is set to 3 dBu below the peak power
handling in order to provide secure operation, i.e, the RMS power handling is 135 W, peak power
handling is 540 W and assuming 32 dB amplifier gain, these power ratings correspond to limit
thresholds of 0 dBu (135 W) and +6 dBu (540 W). As standard, the 112XT HF limiter threshold is set
to +3 dBu (3 dB less than the peak power handling).
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