Crown Audio IQ-MCA-IQ-MC8 - Manual (Page 22)

Page 22 Reference Manual
IQ-MC Modules
This procedure should work well for
most applications. However, some
applications can be a little more
difficult. Some very low-level and/or
low duty-cycle signals may not ad-
equately “test” the load. Lab and
situation testing have shown output
levels as small 40 dB below rated
amplifier output to be enough for
most low-impedance loads. Higher
impedance loads such as those
used in “lightly-loaded” 70V distri-
bution lines may require signal level
near 20 dB below rated output.
The “Nominal Load Impedance”
control is used to optimize the sys-
tem for the most accurate calcula-
tion of load impedance. It should be
set to the expected nominal (or rated)
impedance of the “normal” load. The
high limit should be set for at least 2
times the expected nominal or ac-
tual measured load, while the low
limit should be set to ½ the expected
nominal or actual measured load.
The following example calculates
the SPL necessary for supervision
of a typical 8-ohm system. While the
resulting 80-dB SPL @ 1 meter is
definitely above conversation level,
it is not uncomfortable.
An “8 ohm” example:
30 mA into 8 ohms = 0.007watts
8-ohm driver sensitivity = 100dB for
1W @ 1 meter
0.007W/1W = –20dB
Required SPL for supervision test is
100dB – 20dB = 80dB SPL @ 1 meter
5.5.1 Typical Load Character-
istics to Know and Understand
It is well known that the typical loud-
speaker impedance is not the same
for all frequencies. This variance is
due to the effect of electrical proper-
ties such as the expected increase
in impedance at high frequencies
due to driver voice-coil inductance,
or the peaks and valleys due to
passive crossovers. Testing of vari-
ous passive boxes has shown peaks
of 100 ohms or more! Low frequency
impedance variation can come from
the interaction of the driver compli-
ance with that of the box. The low
frequency variations are usually wide
bandwidth and may vary from 6 to
30 ohms on an 8-ohm driver.
These anomalies are easily aver-
aged out by the IQ-MC supervision
algorithm in most systems. How-
ever, there may be some extreme
situations for very narrow bandwidth
(i.e. single-note) signals and/or very
widely varying loads that the algo-
rithm simply cannot overcome. In
these cases, widening the high and
low limits will help decrease the “sen-
sitivity” of supervision and decrease
chance of “nuisance” error reports.