C. Wiring for Surround Channels
JBL recommends that a stereo amplifier be used for surround power, whether or not split
surrounds will be used. The reason is simply that this will generally result in better
loudspeaker matching as well as facilitating eventual split surround usage.
If there are twelve 4 ohm loudspeakers in the surround array, they can be series-parallel wired
in the booth to give a resulting impedance per side of 6 ohms, as shown in Figure 32A. Twelve 6 ohm
loudspeakers, such as the JBL 6340, can be series-parallel wired to give a resulting impedance per
side of 5.3 ohms. Both wiring arrangements provide equal feed to all loudspeakers.
Amplifiers
t
0
0
Wire nut
Barrier Strip
Figure
Wiring at
strip in the booth
For each 6340,100 to 150 watts should be allocated. Thus, for the 5.3 ohm per side
configuration a single model MPX1200 will be appropriate, with each side feeding a 5.3 ohm load and
delivering approximately 150 watts per loudspeaker.
For the 6 ohm per side configuration, we can specify a single MPA750 amplifier to deliver
about 100 watts per loudspeaker.
In general, determining series-parallel loading of surround loudspeakers is about as
complicated as cinema systems engineering will get in the field. The system designer must carefully
note manufacturer’s specifications regarding amplifier loading. Since most modem transistor amplifiers
carry a 4 ohm rating, the designer needs only to ensure:
1. That the amplifier will not be overdriven in normal operation, and
2. That the individual loudspeakers will receive a signal input within their power rating.
Figure 32A and B detail the series-parallel wiring for both the JBL 6330 and 6340 systems.
