Automatic Loudness Control Circuit: Design & Implementation

Description

To achieve optimal audio reproduction across a range of listening volumes, adjusting the tone-control settings is necessary to align with the non-linear sensitivity patterns of the human ear. Specifically, the auditory system exhibits varying sensitivity throughout the audible frequency spectrum, as depicted by the Fletcher-Munson curves.

A straightforward approach to address this issue involves incorporating a circuit within the preamplifier stage, designed to automatically modify the overall audio chain’s frequency response based on the control knob’s position, maintaining ideal listening conditions at differing volumes.

Fortunately, the human ear is not overly demanding, and a relatively simple circuit can provide a satisfactory performance within a 40dB range. The circuit is illustrated with SW1 in the “Control-flat” position, meaning without the Automatic Loudness Control. In this setting, the circuit functions as a linear preamplifier stage, with the voltage gain determined by the Trimmer R7.

When switching SW1 to the opposite position, the circuit transforms into an Automatic Loudness Control, and its frequency response adapts to the control knob’s position as shown in the table below. C1 emphasizes lower frequencies, while C4 accentuates higher frequencies. Maximum low-frequency boost is constrained by R2, and R5 similarly limits high-frequency boosts.

Circuit diagram:

Parts:

Technical data:

Frequency response referenced to 1KHz and varying control knob positions:

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

Notes:

**TL072 Dual BIFET Op-Amp Details:** The TL072 is a low-power, rail-to-rail op-amp commonly used in audio circuits due to its low noise, high input impedance, and good performance across a range of frequencies. It's a versatile component suitable for various amplifier stages.