( Simple add-on module Switchable "Control-flat" option ) In order to obtain a good audio reproduction at different listening levels, a different tone-controls setting should be necessary to suit the well known behaviour of the human ear. In fact, the human ear sensitivity varies in a non-linear manner through the entire audible frequency band, as shown by Fletcher-Munson curves. A simple approach to this problem can be done inserting a circuit in the preamplifier stage, capable of varying automatically the frequency response of the entire audio chain in respect to the position of the control knob, in order to keep ideal listening conditions under different listening levels. Fortunately, the human ear is not too critical, so a rather simple circuit can provide a satisfactory performance through a 40dB range. Circuit diagram : Simple Automatic Loudness Control Circuit Diagram The circuit is shown with SW1 in the "Control-flat" position, i.e. without the Automa...