Automatic Loudness Control

Simple add-on module

Switchable "Control-flat" option

Circuit diagram:

Loudness Control


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,R9_______________1M   1/4W Resistors
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)


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.
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.

Technical data:

Frequency response referred to 1KHz and different control knob positions:

Knob position table

Total harmonic distortion at all frequencies and 1V RMS output: <0.01%