=09 =09 =20

Automatic Loudness Control

Simple=20 add-on module, Switchable "Control-flat" option

In order = to obtain=20 a good audio reproduction at different listening levels, a different=20 tone-controls setting should be necessary to suit the well known = behavior of the=20 human ear. In fact, the human ear sensitivity varies in a non-linear = manner=20 through the entire audible frequency band, as shown by Fletcher-Munson=20 curves.

A simple approach to this problem can be done inserting a = circuit=20 in the preamplifier stage, capable of varying automatically the = frequency=20 response of the entire audio chain in respect to the position of the = control=20 knob, in order to keep ideal listening conditions under different = listening=20 levels.

Fortunately, the human ear is not too critical, so a = rather=20 simple circuit can provide a satisfactory performance through a 40dB = range. The=20 circuit is shown with SW1 in the "Control-flat" position, i.e. without = the=20 Automatic Loudness Control. In this position the circuit acts as a = linear=20 preamplifier stage, with the voltage gain set by means of Trimmer=20 R7.

Switching SW1 in the opposite position the circuit becomes an = Automatic Loudness Control and its frequency response varies in respect = to the=20 position of the control knob by the amount shown in the table below. C1 = boosts=20 the low frequencies and C4 boosts the higher ones. Maximum boost at low=20 frequencies is limited by R2; R5 do the same at high = frequencies.

Circuit = diagram:

Automatic Loudness Controller Circuit=20 Diagram

Parts:

P1_________________10K=20 Linear Potentiometer (Dual-gang for stereo)
R1,R6,R8__________100K = 1/4W=20 Resistors
R2_________________27K 1/4W = Resistor
R3,R5_______________1K 1/4W=20 Resistors
R4__________________1M 1/4W = Resistor
R7_________________20K 1/2W=20 Trimmer Cermet
C1________________100nF 63V Polyester=20 Capacitor
C2_________________47nF 63V Polyester=20 Capacitor
C3________________470nF 63V Polyester=20 Capacitor
C4_________________15nF 63V Polyester=20 Capacitor
C5,C9_______________1=C2=B5F 63V Electrolytic or Polyester=20 Capacitors
C6,C8______________47=C2=B5F 63V Electrolytic=20 Capacitors
C7________________100pF 63V Ceramic=20 Capacitor
IC1_______________TL072 Dual BIFET=20 Op-Amp
SW1________________DPDT Switch (four poles for = stereo)

Technical=20 data:

Frequency response = referred to=20 1KHz and different control knob positions:

Total harmonic = distortion at=20 all frequencies and 1V RMS output: <0.01%

Notes:

SW1 is shown in "Control flat" position.
Schematic shows left channel only, therefore for stereo operation = all=20 parts must be doubled except IC1, C6 and C8.
Numbers in parentheses show IC1 right channel pin = connections.
R7 should be set to obtain maximum undistorted output power from = the=20 amplifier with a standard music program source and P1 rotated fully=20 clockwise.

Source: Red Free Circuit=20 Design

Learning Electronics

Automatic Loudness Control