Automatic Loudness Control Circuit Diagram – Build Your Ideal Sound

Circuit diagram:

Parts:

Comments:

To achieve optimal audio reproduction across a range of listening volumes, adjusting the tone-control settings is necessary to align with the inherent, non-linear sensitivity of the human ear. The human auditory system exhibits variable sensitivity throughout the entire audible frequency spectrum, as depicted by the Fletcher-Munson curves. A straightforward solution involves incorporating a circuit into the preamplifier stage that automatically modulates the frequency response of the entire audio chain in relation to the control knob position, ensuring ideal listening conditions at differing volume levels. Fortunately, the human ear isn't overly critical, and a relatively simple circuit can furnish satisfactory performance within a 40dB range. The depicted circuit is presented with SW1 in the "Control-flat" position, meaning without Automatic Loudness Control. In this configuration, the circuit functions as a linear preamplifier stage, with the voltage gain determined via Trimmer R7. Shifting SW1 to the alternative position transforms the circuit into an Automatic Loudness Control, and its frequency response varies relative to the control knob position, as detailed in the table below. C1 strengthens the low frequencies, while C4 amplifies the higher frequencies. The maximum boost at low frequencies is constrained by R2, and R5 performs a similar function at high frequencies.

Technical data:

Frequency response referred to 1KHz and various control knob positions:

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

Notes:

IC1 TL072 Dual BIFET Op-Amp

The TL072 is a dual operational amplifier IC offered by Texas Instruments. It's a low-noise, rail-to-rail input operational amplifier that is commonly used in audio applications due to its low input bias current and high open-loop gain. It's a versatile component suitable for a wide variety of amplification and filtering circuits. Its JFET input transistors provide high input impedance, minimizing loading effects on the signal source. The device is supplied with a fixed supply voltage and has a wide operating temperature range.