Automatic Loudness Control Circuit

This is a simple design for automatic loudness control in audio. 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. This is a figure of the circuit.


The circuit is shown with SW1 in the "Control-flat" position, i.e. without the Automatic Loudness Control. In this position the circuit acts as a linear preamplifier stage, with the voltage gain 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.

This is a list component that must using for built the circuit.
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
C2 47nF/63V
C3 470nF/63V
C4 15nF/63V
C5, C9 1µF/63V
C6, C8 47µF/63V
C7 100pF/63V

IC1 TL072 Dual BIFET Op Amp

SW1 DPDT Switch (four poles for stereo)