52
ENGLISH
GUIDANCE ON THE MEASUREMENT
OF THE SPEAKER LINE IMPEDANCE
In the DXT 9000 system, monitoring of the integrity of speaker lines is made through
impedance measurement at subsonic frequency (20 Hz).
This method was chosen because of its good stability and accuracy of calculated values,
which avoids false line fault reports that often occur with impedance measurements
faster at supersonic frequencies, but more prone to interferences and errors.
To use the DXT 9000 line monitoring properly, so that it fully complies with the European
Safety Standard EN 54-16, it is strictly necessary to consider the functional limitations of
all devices.
The two main limitations are:
-
Measurable impedance range.
-
The minimum / maximum tolerance compared to the calibration value of the
measurement for the line fault detection.
Let us now analyse in detail these two limitations.
MEASURABLE IMPEDANCE RANGE
There are three amplifier models that differ in their power, so each has its own
measurable impedance range:
-
Single channel amplifier (max. power: 500 W).
-
Dual channel amplifier (max. power: 250 W per channel).
-
Four-channel amplifier (max. power: 125 W per channel).
In a 100 V (or 70 V) constant voltage line, the minimum measurable impedance
Zmin
can be estimated (in all the three cases) considering a load having double the maximum
output power
Pmax
on a single line (and ignoring the efficiency of speakers and
approximating the impedance at 20 Hz with the real one; the phasor impedance at 20 Hz
is only 9 degrees out of phase with the real axis ).
MEASURABLE
IMPEDANCE RANGE
The results are:
1 x 500 W
→
Zmin
= 10 Ω (at 100 V),
Zmin
= 5 Ω (at 70 V)
2 x 250 W
→
Zmin
= 20 Ω (at 100 V),
Zmin
= 10 Ω (at 70 V)
4 x 125 W
→
Zmin
= 40 Ω (at 100 V),
Zmin
= 20 Ω (at 70 V)
The maximum measurable impedance
Zmax
can be estimated considering lines loaded
to a quarter of the maximum output power
Pmax
.
The results are:
1 x 500W
→
Zmax
= 80 Ω (at 100 V),
Zmax
= 40 Ω (at 70 V)
2 x 250W
→
Zmax
= 160 Ω (at 100 V),
Zmax
= 80 Ω (at 70 V)
4 x 125W
→
Zmax
= 320 Ω (at 100 V),
Zmax
= 160 Ω (at 70 V)
Zmin
=
100V
(100 V)
2
2 Pmax
Zmin
=
100V
(100 V)
2
0.25 Pmax
Zmin
=
70V
(70.7 V)
2
2 Pmax
Zmin
=
70V
(70.7 V)
2
0.25 Pmax
or
or
Summary of Contents for MX 9502
Page 2: ......
