Specifications

TI 323 (6.0E)
9 - 36
3.1. Line signal connection
Line level signal wiring needs much more care than most
people believe. In total there are three major factors to
consider:
− The output impedance and maximum output
current capability of the driving device.
− The length of all connecting cables.
− The total capacitance between the signal
conductors (between pins 2 and 3 of
corresponding XLR connectors).
The number of controller input channels that can be
connected in parallel is limited by their total impedance
and the output capabilities of the drive unit. Standard
balanced outputs of most signal processing equipment
are rated for load impedances of > 600 ohms while
each channel of a P1200A mainframe has an input
impedance of 44 kΩ.
Where multiple units of equal impedance are connected
in parallel, the aquivalent total load impedance is halved
with each doubling of the number of units connected. In
the case of the P1200A mainframes this would allow a
theoretical maximum of approximately 64 input channels
(32 dual channels of P1200A) to be connected to a
standard balanced output. To maintain full available
headroom, a standard 600 ohms rated output should not
be loaded with less than 2 kΩ (corresponding to 22
channels or 11 dual channels of P1200A). With most
output stages, anything less immediately results in a
drastic reduction of available headroom.
Extra care must be taken when low impedances are
driven via long multicore cables: the small capacitive
load of the cable is effectively paralleled to the receiving
inputs. Capacitive loads dramatically decrease drive
capabilities of most output stages due to additional phase
shifts caused by the low pass filter formed by the output
impedance and the capacitive load. As a rule of thumb,
not more than 12 channels (6 dual channels of P1200A)
should be connected in parallel to one 600 ohms rated
drive line via a multicore.
Where a large number of controllers have to be driven
with the same signal, especially via long cables, suitable
line drivers should be used and/or the system must be
subdivided using multiple outputs and signal lines.
The C4 OUT circuit of each B2 controller offers a very
low impedance line driving capability (actual output
impedance < 20 ohms), and can deliver full headroom
into virtually any load.
Tips:
Use the C4 OUT provided on an A1/B2 controller to
drive P1200A mainframes for C4 systems. The C4 OUT
provides a filtered and buffered signal to the C4
controllers and effectively limits the lower frequency
range of the C4-SUBs to 60 Hz. The frequencies below
60 Hz are efficiently covered by the B2s, while the C4-
SUBs can benefit from the extended headroom available.
Using the C4 OUT, with its low impedance line driving
capability, is also a useful way to avoid drive line
impedance problems in larger systems. Where failure of
an A1/B2 controller occurs, including loss of mains
power, the input signal is automatically switched by a
relay to provide a hardwired connection to the C4 OUT.
Do not use the MONO OUT of a mainframe to
overcome the problem of drive line impedance. The
MONO OUT provides a signal which is a –3 dB sum of
channel A and channel B. If you apply the same signal to
both channels their sum is +6 dB, the MONO OUT
signal in this case would be +3 dB. The resulting gain
structure within the system would be compromised with
controllers driven in this manner running at a higher level.
In large set ups where multiple B2 systems are fed from
separate auxiliary or matrix sends, independent from the
master send, C4 systems can be fed from one or more of
the A1/B2 controllers per side leaving the majority of B2
systems under independent control.
Be careful with digital equalizers: digital equalizers
always have a nominal delay (latency) of around 1 to 3
ms, even when their delay setting is on zero.
When using different drive lines for a system it is essential
to make sure that every channel used has the same delay
time, including taking into account the conversion delays
of any digital equalisers or other processing units.
Different delay times can have disasterous effects on
signal coherence; at its best this results in a loss of
acoustic energy, at its worst frequency dependant lobes
are created in various directions.
When using a ratio of more than one B2 to four
C4-SUBs, we suggest a reduction in the input level to the
B2s to maintain an even frequency response of the
overall system, while increasing the low frequency
headroom. If a lack of punch is observed, very often the
relative B2 level is simply too high. The "punchy" part of
the bass is in the frequency range handled by the C4-
SUB from around 70 to 150 Hz and therefore too much
B2 energy masks all direct percussive output signals.
3.2. Speaker wiring
d&b recommends the use of high quality loudspeaker
cable of sufficient wire diameter. Do not be fooled by
tables presenting huge numbers in Watts of power loss
for a specific combination of wire diameter, load
impedance and cable length. A loss of 200 Watts or 20
% of the power, from 1000 Watts sounds a lot, but keep
in mind that this is approximately 1 dB in terms of level.
A 2.5 mm
2
(10 AWG) cable is recommended in order to
keep the losses below 0.5 dB when connecting a 4 ohm
load over a distance of up to 30 metres (98.4 ft). If the
load is 8 ohm, the length can be doubled. If the distance
is greater, the wire diameter has to be increased.