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Sonifex RB-MM1 - Mix-Minus Generator

by Sonifex
£449.94 inc. vat
£374.95 ex. vat
Please contact us for availability.

Sonifex RB-MM1

The Sonifex RB-MM1 is a unit for generating a suitable mix to send to a telephone hybrid or codec. A stereo output is taken from a mixer, together with a post fader output from the mono telephone fader on the mixer. The caller audio is removed from the station output so that it can be sent to the telephone line via the hybrid. 

Whenever programming originates from outside of the studio, or if listeners/viewers are calling up a phone-in using telephone hybrids or codecs, mix-minus feeds are required. Most telephone lines incur delays which prohibit off-air monitoring, because the caller, or remote talent, would hear their own voice in delay which is very disconcerting. The solution is to feed a mix back to the caller minus his or her own voice. Some mixing desks do not have a dedicated telco channel to generate a clean-feed, or mix minus, so the RB-MM1 can be used.

Analogue audio inputs and outputs are via Neutrik XLR connectors. The output level to the TBU can be adjusted using a rear panel pre-set potentiometer. To control the cancellation null (the amount of the telephone channel which is subtracted from the mixer output signal), 2 multi-turn potentiometers are provided, one for the LF null and the other for the full-band null. Additionally, a band pass filter can be switched in and out, via a rear panel switch, to condition the signal for the telephone hybrid. To use the RB-MM1 unit with full-band ISDN codecs, the band pass filter can be switched out. For stereo codecs, or conference calls, multiple RB-MM1 units can be used.

Sonifex RB-MM1 Specifications

Audio Specification

  • Maximum Input Level: +28dBu
  • Input Impedance: >20kΩ
  • Maximum Output Level: +28dBu
  • Output Impedance: <50Ω
  • Output Gain Range: Adjustable -15dB to +12dB, ref 0dBu gain via a multi-turn pot
  • Common Mode Rejection: >60dB
  • Band Pass Filter Range: 200Hz to 4kHz, 12dB/octave
  • LF Null Adjustment: Better than 40dB at 100Hz
  • Mix-Minus Null: Better than 40dB at 1kHz
  • Frequency Response: 20Hz - 22kHz ± 0.1dB
  • Distortion: 0.01% THD @ 1kHz, ref +8dBu output (C-Message weighted)
  • Noise: -90dBu unity gain +6dBu   


  • Analogue Inputs From Mixer Output: 2 x XLR 3 pin female (balanced) (L & R)
  • Analogue Input From Telephone Fader: 1 x XLR 3 pin female (balanced) (L & R)
  • Analogue Output To TBU: 1 x XLR 3 pin male (balanced) (L & R)
  • Mains Input: Filtered IEC, switchable 110-120V, or 220-240V, fused, 6W max.
  • Fuse Rating:
    • Anti-surge fuse 100mA 20 x 5mm (230VAC)
    • Anti-surge fuse 250mA 20 x 5mm (115VAC)

Physical Specification

  • Dimensions:
    • 28cm (W) x 10.8cm (D) x 4.3cm (H)
    • 11” (W) x 4.3” (D) x 1.7” (H)
  • Weight:
    • Nett: 1.1kg Gross: 1.5kg
    • Nett: 2.4lbs Gross: 3.3lbs