L-ACOUSTICS ARCS Manual V2.0
2/13/2003
20
3.3 GUIDELINES REGARDING SYSTEM PROTECTION
As supplied by L-ACOUSTICS, limit thresholds for XTA and BSS Processors are initially set at +9, +8
and +2 dBu for the SB218 subwoofer, ARCS low and ARCS high channels, respectively.
For the SB218, the LA48a is an excellent power match and the +9 dBu limit threshold is matched to
the input sensitivity of the L-ACOUSTICS LA 48a (+9.5 dBu) so that system protection is performed
by a combination of the limiting circuits of both amplifier and digital signal processor.
For the ARCS low section, the +8 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 LA 24a (+7.7 dBu) so
that system protection is performed by a combination of the limiting circuits of both amplifier and the
digital signal processor.
For the ARCS high section, the +2 dBu limit threshold corresponds to 3 dBu higher than the RMS
power handling of the high section and system protection is performed by the limiting circuitry of the
digital signal processor. As stated in Section 1.7, 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 ARCS HI section:
LA24a ARCS HI Section: MLS = -2dB
LA48a ARCS HI Section: MLS = -5 dB
Note: For ARCS, the high section limit threshold is set to 3 dB below the peak power handling in
order to provide secure operation, i.e, the RMS power handling is 100 W, peak power handling is 400
W and assuming 32 dB amplifier gain, these power ratings correspond to limit thresholds of -1 dBu
(100 W) and +5 dBu (400 W). As standard, the ARCS HF limiter threshold is set to +2 dBu (3 dB less
than the peak power handling). Standard thresholds should be suitable for most applications however
the following adjustments can be made depending on the program content:
1) classical music - lots of transients, low RMS signal content
increase standard HF limit threshold by 3 dBu to correspond to the peak power handling
(for example, ARCS HF limit threshold: +2 dBu -> +5 dBu)
2) rave/techno music - high RMS signal content, long duration
decrease standard HF limit threshold by 3 dBu to correspond to the RMS power handling
(for example, ARCS HF limit threshold: +2 dBu -> -1 dBu)
NOTE: Setting limit thresholds to the amplifier input sensitivity (or the rated power handling of the section
being driven) is important since this calibrates the output meter display of the crossover to correspond to
either the amplifier clip point or the rated power handling. This gives the system operator a direct visual
indication as to how hard the system is being operated.
The L-ACOUSTICS LA48a is an excellent power match for the SB218 subwoofer or the ARCS LOW
Section (3 enclosures in parallel) while the LA24a is an excellent power match for up to 2 ARCS in
parallel. For either LA48a or LA24a amplifiers, the clip limiter is sonically very transparent and works
by monitoring the output 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.
NOTE: The LA48a has a relatively low input sensitivity (9.5 dBu). This means that, in practice, it may
be necessary to equally scale up the individual crossover channel output gains in order to have
sufficient drive capability (for all OEM factory presets 3-way output gains are initially set at +3, 0, -8
for sub, low and high sections, respectively). It is better to use the output drive capability of the
processor DACs and analog output section rather than overdrive the input ADCs so do not be afraid
to increase the channel output gains in order to achieve a comfortable gain structure (for example:
+6, +3, -5 dB output gains for sub, low, high channels may be necessary if the LA48a is used).
Summary of Contents for ARCS
Page 1: ...Version 2 February 2003 L ACOUSTICS ARCS OPERATOR MANUAL ...
Page 9: ...L ACOUSTICS ARCS Manual V2 0 2 13 2003 8 Figure 3 ARCS System Block Diagram ...
Page 48: ...L ACOUSTICS ARCS Manual V2 0 13 02 03 47 Table 13 ARCS Rigging Reference Chart ...
Page 52: ...L ACOUSTICS ARCS Manual V2 0 13 02 03 51 Figure 26 ARCS line drawing ...
