radial pro di passive direct box radial engineering noise-free and very affordable! the radial prodi passive direct box is a high-quality, passive direct box that employs a proprietary transformer for both impedance conversion and isolation. linear from 20hz-18khz, with low distortion and virtually zero phase distortion makes it perfect for live or studio performances. because of the transformer isolation, problems such as hum and buzz caused by ground loops are eliminated, making it very easy and hassle-free to use. the radial prodi features 1/4" thru/input, -15db pad, and ground lift protected from damage by a protective front panel. heavy-duty construction, baked enamel finish, and 600-ohm balanced output. 2-1/2" x 4-1/4".