TOA Electronics IQ 18B - Manual (Page 6)

6
CABLE RUN
(m) C.S.A. OF EACH
CONDUCTOR (mm) CABLE
RESISTANCE
Ω Ω
% POWER LOSS
INTO 8
Ω Ω LOAD
% POWER LOSS
INTO 4
Ω Ω LOAD
10 2.5
4.0
6.0 0.14
0.09
0.06 1.7
1.1
0.73
3.5
2.2
1.5
25 2.5
4.0
6.0 0.35
0.22
0.14 4.3
2.7
1.8 8.6
5.4
3.6
50 2.5
4.0
6.0 0.69
0.43
0.29 8.6
5.4
3.6 17.0
11.0
7.2
100 2.5
4.0
6.0 1.38
0.86
0.58 17.0
11.0
7.2
35.0
22.0
14.0
4. Polarity Checking
It is most important to check the polarity of the wiring. A simple method of doing this is
to use a pulse based polarity checker for LF units. Connect the speaker leg which you
believe to be connected to Pin 1 to the +
ve terminal of the amplifier and likewise Pin 2 to
the – ve terminal of the amplifier. Check the phase as instructed by the polarity checker
manufacturer. If you have wired it correctly, the polarity checker should indicate the LF
unit is wired in phase, indicating the wiring is correct. If however the polarity checker
indicates negative phase, the input connections must be inverted.
If problems are encountered, inspect the cable wiring in the first instance. It should also
be noted that different amplifier manufacturers utilise different pin configurations and
polarity conventions. If you are using amplifiers from more than one manufacturer,
check the polarity at the amplifiers as well as the loudspeakers.
5. Amplification & Power Handling
As with all professional loudspeaker systems, the power handling is a function of voice
coil thermal capacity. Care should be taken to avoid running the amplifier into clip
(clipping is the end result of overdriving any amplifier). Damage to the loudspeaker will
be sustained if the amplifier is driven into clip for any extended period of time.
Headroom of at least 3dB should be allowed. When evaluating an amplifier, it is
important to take into account its behaviour under low impedance load conditions. A
loudspeaker system is highly reactive and with transient signals it can require more
current than the nominal impedance would indicate.
Generally, a higher power amplifier running free of distortion will do less damage to the
loudspeaker than a lower power amplifier continually clipping. It is also worth
remembering that a high powered amplifier running at less than 90% of output power
generally sounds a lot better than a lower power amplifier running at 100%. An amplifier
with insufficient drive capability will not allow the full performance of the loudspeaker to
be realised.
It is important when using different manufacturers amplifiers in a single installation that
they have very closely matched gains, the variation should be less than ±0.5dB. This
precaution is important to the overall system balance when only a single
compressor/limiter or active crossover is being used with multiple cabinets; it is
therefore recommended that the same amplifiers be used throughout.
6. Operation