Circuit diagram: Parts P1 10K Linear Potentiometer (Dual-gang for stereo) R1,R6,R8 100K 1/4W Resistors R2 27K 1/4W Resistor R3,R5 1K 1/4W Resistors R4 1M 1/4W Resistor R7 20K 1/2W Trimmer Cermet C1 100nF 63V Polyester Capacitor C2 47nF 63V Polyester Capacitor C3 470nF 63V Polyester Capacitor C4 15nF 63V Polyester Capacitor C5,C9 1µF 63V Electrolytic or Polyester Capacitors C6,C8 47µF 63V Electrolytic Capacitors C7 100pF 63V Ceramic Capacitor IC1 TL072 Dual BIFET Op-Amp SW1 DPDT Switch (four poles for stereo) Comments: 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 respo...