User Manual

Redback® A 4275B/85B 125/250W Mixer Amplier
www.redbackaudio.com.au Redback® Proudly Made In Australia 7
Redback® A 4275C/85C 125/250W Mixer Amplier
Overview
7 Input connectors
There are 6 sets of inputs which can be either balanced mic or line inputs. The line inputs are dual RCA
connectors which are internally mixed to produce a mono input signal.
8 DIP SW 1-3
Inputs 1-6 include an individual 4 way dip switch enabling adjustable input sensitivity (100mV or 1V) and Priority/
VOX muting for inputs 1-3.
Priority/VOX muting is not available on inputs 4-6.
Note, when the Priority/VOX muting function is selected (inputs 1-3) it will operate with the input set for either
microphone or line operation. (see section 2.3 for DIP Switch settings)
9 Pre - amp send and return
This allows external effects units, ie Graphic EQ’S , companders or signal processors to be connected to the
amplier. When not using this feature the send and return outputs must be linked externally (factory tted). The
amplier will not work without this link. Only when an external effects unit is used can it be removed.
(see section 2.8 for more details)
10 Tape Out
Dual RCA’s provided for recording purposes. This level is post master volume. (see section 2.8 for more details)
11 DIP SW 5
These DIP switches set the 24VDC switched output status, determine the Preamp output as pre master volume
or post master volume and also set the phantom power to the balanced inputs. (see section 2.3 settings)
12 DIP SW 4
Not currently used.
13 RJ45 Interface
This is for future expansion of product.
14 Battery Charging
The amplier is provided with a trickle charge circuit to maintain the batteries in a charged condition. The charge
rate is approximately 300mA. Simply link the terminals marked “link to charge backup battery” on the rear of
the amplier. A at battery will generally take overnight to charge, or even longer depending on the battery
capacity. (see section 2.9 for more details)
15 24VDC IN
Battery Backup: Provision has been provided to run the amplier from a suitably rated 24V battery system in the
event of a mains failure. Using appropriately rated cable, connect the battery to the “24V DC In” terminals.
Observe correct polarity when connecting. (see section 2.9 for more details)
16 DC Resettable fuse
This fuse protects the internal power supply. If the fuse is tripped it is easily reset by pressing the small button on
the fuse.
17 Output Connections
Speakers with total impedance of 4 to 16 ohms, or speakers tted with a 70V/100V line transformer may be
connected. Always ensure that the total load of the speakers does not exceed the rated output of the amplier ie
either 4Ω minimum for the 4-16Ω terminals or 80Ω minimum at 100V for the A 4275C (125W) and 40Ω
minimum at 100V for the A 4285C (250W). Otherwise either the DC or mains fuse could blow or the fault led
activate and the amp will shut down. (See the table below.) Always be careful to avoid short circuits and
connection to the wrong terminals.
18 240V AC power socket (Australian standard)
Connects to 240V AC mains power with the included IEC lead.
i
Wiring speakers in parallel for 70/100V line: Where several speakers are to
be used at one time, on one circuit, it becomes necessary to use speakers fitted
with line-matching transformers. This is to overcome the effects of connecting
speakers in parallel and cable losses. The amplifier generally has an output
voltage of 100 volts (70 volts is typically used in North America, however
operation is similar). In this configuration the total wattage load on the amplifier
is derived from adding all the line transformer primary tap ratings together. For
example, 70 one watt speakers will have a total speaker load of 70 watts. Or
alternatively, it is conceivable to connect 100 one watt speakers to a 100 watt,
100 volt line amplifier.
Measuring 70/100V Line Speaker Impedance: To measure amplifier system
load, you must use an impedance meter in order to measure the ac resistance
of the connected speaker network. Impedance cannot be measured with a
standard multimeter, as this measures the dc resistance.
About 70V & 100V Line Speaker Systems
Load 70V 100V
0.5W 9.4kΩ 20kΩ
0.66W 7.12kΩ 15kΩ
1W 4.7kΩ 10kΩ
1.25W 3.76kΩ 8kΩ
2W 2.35kΩ 5kΩ
2.5W 1.88kΩ 4kΩ
3W 1.56kΩ 3.3kΩ
5W 940Ω 2kΩ
7.5W 626Ω 1.3kΩ
10W 470Ω 1kΩ
15W 313Ω 666Ω
20W 235Ω 500Ω
30W 156Ω 333Ω
40W 117Ω 250Ω
60W 78Ω 166Ω
100W 47Ω 100Ω
125W 37Ω 80Ω
250W 19Ω 40Ω
500W 9.4Ω 20Ω
Redback Q 2004