Automatic = Loudness=20 Control - RED

Switchable "Control-flat" option

=20

Circuit diagram:

Parts:

P1_________________10K   Linear Potentiometer (Dual-gang for stereo)=0A=
=0A=
R1,R6,R8__________100K   1/4W Resistors=0A=
R2_________________27K   1/4W Resistor=0A=
R3,R5_______________1K   1/4W Resistors=0A=
R4__________________1M   1/4W Resistor=0A=
R7_________________20K   1/2W Trimmer Cermet=0A=
=0A=
C1________________100nF   63V Polyester Capacitor=0A=
C2_________________47nF   63V Polyester Capacitor=0A=
C3________________470nF   63V Polyester Capacitor=0A=
C4_________________15nF   63V Polyester Capacitor=0A=
C5,C9_______________1=B5F   63V Electrolytic or Polyester Capacitors=0A=
C6,C8______________47=B5F   63V Electrolytic Capacitors=0A=
C7________________100pF   63V Ceramic Capacitor=0A=
=0A=
IC1_______________TL072 Dual BIFET Op-Amp=0A=
=0A=
SW1________________DPDT Switch (four poles for stereo)=0A=
=0A=

=20

Comments:

In order to obtain a good audio reproduction at different listening = levels, a=20 different tone-controls setting should be necessary to suit the well = known=20 behaviour of the human ear. In fact, the human ear sensitivity varies in = a=20 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=20 the frequency response of the entire audio chain in respect to the = position of=20 the control knob, in order to keep ideal listening conditions under = different=20 listening 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 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 = R7.
Switching SW1 in the opposite position the circuit becomes an Automatic = Loudness=20 Control and its frequency response varies in respect to the position of = the=20 control knob by the amount shown in the table below.
C1 boosts the = low=20 frequencies and C4 boosts the higher ones. Maximum boost at low = frequencies is=20 limited by R2; R5 do the same at high frequencies.

Technical data:

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

Total harmonic distortion at all frequencies and 1V = RMS=20 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 programme source and P1 rotated fully=20 clockwise.

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Automatic Loudness Control