A 4275B • Phase5 Public Address Mixer Amplifier 125W 6 Input

The flagship model of the Redback PA amplifier range, the Phase5 series have been engineered for professional PA installations requiring flexibility for a wide scope of applications such as schools, shopping centres, workshops, mine sites and office buildings. This model conservatively delivers 125 Watts RMS output power for 70 & 100V line PA systems.

SKU: A 4275B Categories: , , ,

Designed in conjunction with Australia’s leading PA professionals to cater for the needs of multi-source installations. This redesigned unit offers the ability to monitor amplifier status via the colour LCD screen, plus custom labelling of inputs. The A 2711 optional USB & SD card module provides music playback when fitted.

 Voice/Vox muting, including three levels of input priority, bass and treble controls and phantom powering capability enable unparalleled flexibility for a wide scope of applications such as schools, shopping centres, workshops, mine sites and office buildings. Operates from 240V ac mains or 24V dc permitting battery backup operation during mains power failure. Output can be 100V line, 70V line or low impedance (4 – 16Ω) loads. The circuit includes thermal overload shutdown, over current and overload protection plus fuses on both ac and dc supply rails.

LCD: Indicates input and output levels, amplifier status; including heatsink temperature, overload and over current conditions. 240V ac or 24V dc operation. Also allows custom labelling of input sources via USB keyboard.

Input configuration: Each input can be configured for either balanced microphone or line use. A dip switch is supplied for each input providing the following features: Phantom power on/off, Mic or line, 100mV or 1V, Vox muting on/off (priority)

Three levels of priority: Priority selection is by a dip switch on the rear panel: Input 1 over 2-6, input 2 over 3-6 and input 3 over 4-6.

Overload protection: In the event of an overload, the output current is limited by the internal circuitry, thus reducing the output voltage at the speaker terminals.

Input Limiting: Limits the output to 110V regardless of input level. This prevents dangerously high voltages from damaging the loudspeaker.

A 4573 Alert/evac option board: Generates pre chime, bell, alert and evacuation tones. The alert tone can function continuously, or switch to evac tone after a preset time delay (adjustable 30 seconds to 4.5 minutes). Simply connects to the amplifier internally via the cable provided.

A 2280B Remote volume wall plate: Connects via figure 8 cable to the “remote volume” terminals of the amplifier. Enabling overall volume to be adjusted as required. Features a PCB jumper to reverse the volume control direction. Maximum cable length 100m.


Developed, designed & assembled in Australia.
Since 1976 Redback PA products have been manufactured in Perth, Western Australia by Altronics. With over 40 years experience in the commercial audio industry, we offer consultants, installers and end users reliable products of high build quality with local product support.

Backed by an industry leading 10 Year Manufacturer Warranty.
Our Redback products have a long tried and tested history of bulletproof reliability. Altronics offers a 10 Year Warranty on all Australian Made Redback products. Our local in-house service department has all the know how to answer technical queries and provide timely service.

Key Features
  • Very low noise and distortion
  • Thermal overload protection
  • Thermally cued forced fan cooling
  • Protect Mode
  • 4 levels of priority/Vox muting
  • LCD display of amplifier status
  • Minimum and Maximum Master Volume Over-ride
  • Minimum and Maximum Remote Volume Over-ride
  • USB keyboard interface for labelling of inputs and installer contact details. • 6 inputs configurable as balanced microphone or line
  • Music Input on front panel
  • Adjustable line level sensitivity
  • 70V, 100V and 4-16ohm outputs
  • 240V AC or 24V DC operation
  • 300mA Backup Battery charging
  • Phantom power (on microphone inputs)
  • Tape out (Dual RCA)
  • Pre-amp send and return connections (RCA)
  • Balanced line output (3 pin male XLR)
  • Remote Volume option
  • 24V switched output via PTT microphone (not supplied)
  • 19” Rack Mount (2 unit).
  • Optional Alert/Evac tones with voice-over message (via A 4573 module)
  • Optional MP3 input with remote control (via A 2711 module)
Power Output: 125 watts RMS
Distortion: < 0.5%, @ 1kHz
Output line: 70V, 100V or 4 – 16Ω
Line output: 600Ω balanced, 0dBV
Frequency Response
Mic inputs: 50Hz – 12kHz, -3dB
Line inputs: 50Hz – 15kHz, -3dB

Mic inputs: 3mV balanced
Line inputs: 100mV-1V
Signal To Noise Ratio (Limiting by-passed)
Mic inputs: > 75dB below rated output
Line inputs: > 81dB below rated output

Output Connectors
Speakers: Screw terminals
Line out: 3 pin XLR, Tape out RCA / send & return RCA

Input Connectors
Inputs: 3 pin XLR balanced or 2 x RCA
24V dc power: Screw terminals
240V ac power: IEC power connector
Muting: PTT via microphone switch contacts or VOX muting

Mic inputs: Volume
Line inputs: Volume
Bass: ±10dB @ 100Hz
Treble: ±10dB @ 10kHz

Indicators: Power, fault. All other indicators on LCD
Power Supply: 240V ac or 24V dc
Current Draw: 350mA Standby, 8A Full @ 24V dc
Protection: 5A ac , 10A dc
Dimensions: ≈483W x 380D x 88H (410D inc. handles)
Weight: ≈14.5kg
Colour: Black

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