ACTIVE LOUDNESS CIRCUIT SCHEMATIC DIAGRAM
To get good audio reproduction at different listening levels, a different tone-setting controls should be necessary to suit the well known behavior 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 automatically varying 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.
