Tannoy I9 - Manual (Page 4)

When the number of speakers is large over long cable runs, it is recommended that a
distributed (70.7V/100V) system be used. For distributed line, the cable need not have a
large cross sectional area as for low impedance, resulting in savings in cable cost. All
speakers are placed in parallel with the output of the amplifier, allowing easy
installation. Please refer to section 5.2 for installation guidelines. The wattage tappings
can be individually adjusted, giving more flexibility within an installation.
4. Amplification and Power Selection (Low Impedance Mode)
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. Headroom of at
least 3 dB 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, 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.
When running low impedance loudspeakers in parallel, care must be taken not to allow
the impedance to drop too low, as damage may be sustained by the amplifier. Most low
impedance amplifiers will be able to handle loads of 2
Ω to 8Ω. This means that when
using a 4 Ω amplifier, 2 × i7 Contour may be connected in parallel per input, although
care should be taken as impedance varies with frequency and at some frequencies the
impedance will drop to below 8
Ω. For examples simple series/parallel combinations,
please see Figures 4b, 4c and 4d.
Figure 4c: Simple series
connection: 16
Ω.
Figure 4b: Simple parallel connection: 4 Ω.
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