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Loudness Automatic Control circuit

Loudness Automatic Control circuitLoudness Automatic Control circuit

In adjustment to access a acceptable audio reproduction at altered alert levels, a altered tone-controls ambience should be all-important to clothing the able-bodied accepted behaviour of the animal ear. In fact, the animal ear acuteness varies in a non-linear address through the absolute aural abundance band, as apparent by Fletcher-Munson curves.

A simple access to this botheration can be done inserting a ambit in the preamplifier stage, able of capricious automatically the abundance acknowledgment of the absolute audio alternation in account to the position of the ascendancy knob, in adjustment to accumulate ideal alert altitude beneath altered alert levels.

Fortunately, the animal ear is not too critical, so a rather simple ambit can accommodate a satisfactory achievement through a 40dB range.

The ambit is apparent with SW1 in the "Control-flat" position, i.e. after the Automatic Loudness Control. In this position the ambit acts as a beeline preamplifier stage, with the voltage accretion set by agency of Trimmer R7.

Switching SW1 in the added position the ambit becomes an Automatic Loudness Ascendancy and its abundance acknowledgment varies in account to the position of the ascendancy bulge by the bulk apparent in the table below.

C1 boosts the low frequencies and C4 boosts the college ones. Maximum addition at low frequencies is bound by R2; R5 do the aforementioned at aerial frequencies.

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)


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.