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Automatic Loudness Control
Simple
add-on module, Switchable "Control-flat" option

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 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 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.

Circuit diagram:

Automatic Loudness Controller 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)

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 program source and P1 rotated fully
clockwise.

Source: Red Free Circuit
Design

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Electronics is published under the Creative Commons Attribution License
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