Abstract: An apparatus for calibrating the gain in a tape drive read system, so that with a calibrated signal source, the output of the read system will be a clean, square wave, digitized signal. Means are provided for taking the output of a read head, reading signals from a tape, applying a controlled attenuation to the signals, such that the output of the read system will be of a correct amplitude output. A sensor assembly including a comparator, is used to monitor the output voltage from the read system, and to provide appropriate signals to the attenuator until the analog output voltage of the read signal is correct.