Page 34
XO1 Operators Manual
Setting Accurate Limiter Thresholds
The limiters built into the XO1 are intended to be used for for loudspeaker driver
protection, as opposed to amplifier protection. All modern professional power
amplifiers designed for live sound use have their own limiters which are tailored to
protecting the amplifier from clipping.
The following section describes how to set up the XO1’s limiters to provide
exceptional protection against driver overheating, and cone over-excursion.
Most speaker systems are given a power rating in Watts RMS. This is the maximum
continuous power that the system will handle and often appears very conservative. In
reality, as music program is far from continuous in nature, the peak power of the
system is much higher – up to ten times the continuous figure. Any limiter, which is
to protect the driver from damage, must be able to fulfil the following tasks.
Have an attack time which is calculated to allow transients through but keep the RMS
level below the speaker manufacturers specification;
Have a release time which is sufficiently long to avoid the limiter itself modulating the
program ;
Be intelligent enough to adjust the envelope of the limiter according to the frequency
content of the program material.
The XO1’s limiters are capable of performing all these tasks. The only parameter
which the user must set manually is the threshold, and it is crucial that this is done
correctly. Consider the table below.
dB
Ratio
Vrms
Pwr 8
:
Pwr 4
:
Pwr 2
:
45 177.83 137.74 2371.71 4743.42 9486.83
44 158.49 122.77 1883.91 3767.83 7535.66
43 141.25 109.41 1496.45 2992.89 5985.79
42 125.89 97.52 1188.67 2377.34 4754.68
41 112.20 86.91 944.19 1888.39 3776.78
40 100.00 77.46 750.00 1500.00 3000.00
39 89.13 69.04 595.75 1191.49 2382.98
38 79.43 61.53 473.22 946.44 1892.87
37 70.79 54.84 375.89 751.78 1503.56
36 63.10 48.87 298.58 597.16 1194.32
35 56.23 43.56 237.17 474.34 948.68
34 50.12 38.82 188.39 376.78 753.57
33 44.67 34.60 149.64 299.29 598.58
32 39.81 30.84 118.87 237.73 475.47
31 35.48 27.48 94.42 188.84 377.68
30 31.62 24.49 75.00 150.00 300.00
Using this table it is a straightforward procedure to work out the required setting of
the limiter thresholds for the system. Follow the steps described overleaf.
