Sonifex Redbox RB-UL4 Quad Stereo Unbalanced to Balanced Converter. Professionally Refurbished & Tested Products. IT Equipment at Affordable Prices. Sonifex Redbox RB-UL4 Quad Stereo Unbalanced to Balanced Converter – No Fuse. The RB-UL4 is a 1U rack-mount quad stereo unit for interfacing domestic or semi-professional unbalanced equipment to professional balanced line levels. All connections are on the rear panel. The eight RCA unbalanced inputs have an impedance of 10kO and. Are routed to eight balanced XLR-3 outputs with an output impedance of. Sign up to our newsletter for all our latest offers and new arrivals.