ART Pro-VLA II Two Channel Vactrol-based Compressor VCA-Less Vactrol Opto-Compression Design, Mastering Quality Audio Signal Path with Tube Gain Stage, Variable Threshold, Ratio & Output Controls Enhanced Link Mode - While in stereo link mode, channel 1 level control acts as a master output level and channel 2 becomes a balance control between them, - optimizing the Pro-VLA II for use as a mastering tool LED Backlit VU Input/Output Level Metering Indicates Both Average Level And Peak Hold Accurate Ten Segment LED Gain Reduction Metering. +4dBu/-10dBu Mode Switch Optimizes Pro-VLA II For Interface with Professional OR Consumer Grade Components Active Balanced XLR And -Inch Inputs & Outputs. Toroidal Transformer Reduces Mechanical & Electrical Hum