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.
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.
List Component Of Loudness Aktif
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)
The circuit is shown with SW1 in the "Control-flat" position, ie without the Automatic Loudness Control. In this position the circuit acts as a linear Preamplifier stage, with the voltage gain is set by means of Trimmer R7. Switching SW1 in the opposite position the circuit becomes an Automatic Loudness Control and its frequency response varies in respect to the position of the control knob by the amount shown in the table below. C1 boosts the low frequencies and C4 boosts the higher ones. Maximum boost at low frequencies is limited by R2; R5 do the same at high frequencies.