Oscillogram of how tone regulator works with square wave 

This is illustration to my web blog article about buildind and tuning audio bass and treble control. Based on well known buggy Burr-Brown/Texas Instrument application bulletin SBOA082. I am not using OPA134, but more common old NJM5532 op amp from New Japan Radio Co (JRC).
Audio input is fed by low frequency (~100Hz) square wave sygnal and output is displayed on oscilloscope.
Main idea, that in middle pot position audio is not distorted at all, meanwhile tuning bass and treble pot change oscilloscope image.