Automatic Loudness Control

Simple add-on module

Switchable "Control-flat" option

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 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:

Total harmonic distortion at 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
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
amplifier with a standard music programme source and P1 rotated fully
clockwise.

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