www.redbackaudio.com.au
Redback® Proudly Made In Australia
3
Redback® A 4235A/45A/55A
PA Mixer Amplifiers
SPEAKER CONNECTIONS
Speakers fitted with 100V line transformers may be connected to the output terminals on the rear of the amplifer. Always
ensure the total load of the fitted speakers does not exceed the rated output of the amplifier (ie 30 watts for the A 4235A,
60 watts for the A 4245A and 100 watts for the A 4255A amplifier) otherwise damage may result. When fitting speakers
with line transformers the impedance of the load cannot be measured using a standard multimeter. An impedance meter
is required. Fig 2 lists the impedance at certain loads of speakers fitted with 70V and 100V line transformers. So for a total
load of 60 watts using 100V line transformer fitted speakers, the impedance of the speaker load should be 166Ω.
Fig 2. 100V Speaker Line Impedances
Fig 3. Redback Q 2004 Impedance Meter
TROUBLE SHOOTING
If the REDBACK amplifier fails to deliver the rated performance, check the following:
No Power, No Lights
Make sure amplifier power switch is on. Make sure mains power switch is on at the wall. Check the mains and DC fuse.
Replace with only the correct type and rating. Over rated fuses with invalidate warranty.
Distorted Output
Check that the speaker type is correct for the output that you are using (ie. 100V line). Check for any short circuits on the
speaker line and check the speaker line impedance. Note you must use an impedance meter for this such as Redback
Q 2004 Impedance Meter.
Very Low Output Volume
Make sure that the input is the correct level (check for shorted connectors). Check for any short circuits on the speaker
line.
Check input DIP switches are set correctly.
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
