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Cable choice consists mainly of selecting the correct cross sectional area in relation to
the cable length and the load impedance. A small cross sectional area would increase
the cables’ series resistance, inducing power loss and response variations (damping
factor).
Connectors should be wired with a minimum of 2.5 mm
2
(13 gauge) cable. This will be
perfectly satisfactory under normal conditions. In the case of very long cable runs the
wire size should exceed this, refer to the following table for guidance: -
CABLE
RUN (m)
C.S.A. OF
EACH
CONDUCTOR
(mm
2
)
AWG
CABLE
RESISTANCE
Ω
% POWER
LOSS
INTO 4Ω
LOAD
dB LOSS
INTO 4Ω
LOAD
% POWER
LOSS
INTO 8Ω
LOAD
dB LOSS
INTO 8Ω
LOAD
10 2.5
4
6
13
11
9
0.13
0.08
0.05
3.9
2.5
1.6
0.17
0.11
0.07
2.0
1.3
0.8
0.09
0.06
0.04
25 2.5
4
6
13
11
9
0.33
0.21
0.13
9.3
6.1
3.9
0.42
0.27
0.17
4.9
3.1
2.0
0.22
0.14
0.09
50 2.5
4
6
13
11
9
0.66
0.41
0.26
17.0
11.4
7.5
0.81
0.53
0.34
9.3
6.1
3.9
0.42
0.27
0.17
100 2.5
4
6
13
11
9
1.31
0.83
0.52
29.1
20.5
14.0
1.49
1.00
0.65
17.0
11.4
7.5
0.81
0.53
0.34
5. Voltage and Power Selection (Distributed Line Operation)
When using 70.7V or 100V distributed-line systems, the transformer can be tapped at
60W, 30W and 15W, with an extra 7.5W tapping for 70.7V line systems. The taps are
selected via the rotary switch shown in Figure 4a.
When the relevant voltage tappings have been selected, add the individual power in
Watts at all speakers and select and amplifier with a rating equal to or exceeding the
total power in Watts. All of the transformer primaries should be connected in parallel to
the output of this amplifier. If for example, you select the 7.5-watt transformer tap, it
means that at full rated amplifier output the speaker will receive the full 7.5 watts. If the
amplifier gain is reduced, each speaker will receive a proportional amount of power,
maintaining the overall system balance.
It is recommended that a generous power safety margin (3dB of headroom) be left so
that the system does not have to operate continuously at its full rated output power
In order to comply with relevant fire safety regulations (i.e. BS 5839:1998), it is required
that in the event of fire, that failure of the circuit to which the loudspeaker is connected
does not occur before evacuation of the building is complete. Suitable measures include:
a) use of terminal blocks (for connection to primary) with a melting point of not less
than 650°C, for example constructed from ceramic materials;
c) use of terminal blocks of a lower melting point but protected with thermal insulation;
d) use of terminal blocks such that, on melting, an open-circuit or a short-circuit does
not occur.
6