Inspired by the Federal AM-864/U compressor-limiter originally built for US military use in the 1940's, the Fed+delivers all the classic sonic signature of the original. It also enjoys some updates required of todays' Audio Engineer:- Full bandwidth (20hz-20khz)- Attack and Release controls- XLR in and out- Front panel Ratio and Threshold- Compression can be dialed out and used as a preAlso, this unit does away with the noisy and increasingly rare octal tubes, employing miniatures: (2) 6BD6's, a 6AV6 and a 12AU7, all easily available and affordable. The Schmidlin Fed+ All Tube Audio Compressor In keeping with the purity of the original, all knobs when set to zero on the Fed+ are stock (calibrated) settings for the Federal AM-864/U except for the Input control, of course.As for the sonic characteristics, we'll let others describe that on the Reviews page,or check out some before-and-after Clips