Crown Audio IQ-PIP-USP2_CN - Manual (Page 53)

IQ-PIP-USP2/CN
Page 53 IQ-PIP-USP2/CN Reference Manual
* These limits are somewhat arbitrary
but should be a good starting point.
optimizes the supervision
algorithm for voltage and current
levels versus the actual load.
Note: a higher nominal setting will
require higher output levels.
5. Set the high limit at twice average
and the low limit at one-fourth
nominal.*
6. Let the system run for extended
periods using any and all typical
program material.
7. Adjust the high/low limits, if
necessary, to account for any
variance in average measured
impedance.
8. Enable error reporting, if desired.
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 distribution
lines may require signal level near
20 dB below rated output.
The “Nominal Load Impedance” con-
trol is used to optimize the system
for the most accurate calculation of
load impedance. It should be set to
the expected nominal (or rated) im-
pedance 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 = 100 dB for
1W @ 1 meter
0.007W/1W = –20 dB
Required SPL for supervision test is
100 dB – 20 dB = 80 dB SPL @ 1
meter
10.6.1 Typical Load
Characteristics 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-PIP-USP2/CN
supervision algorithm in most sys-
tems. However, there may be some
extreme situations for very narrow
bandwidth (i.e. single-note) signals
and/or very widely varying loads