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  • 40W Class-D Audio Amplifier with Digital Master Volume Control

  • Created: Oct 10, 2017

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The OPA1688 device used in this design belongs to the SoundPlus family of Texas Instruments. It is a low-noise operational amplifier designed for audio applications. It can either operate using a single voltage supply or a split voltage supply. The MAX5486, which is also used in this project, is a dual 40kΩ digital logarithmic potentiometer that has a debounced pushbutton interface ideal to control the volume and balance the left and right channel of this project. It has 32 log-spaced tap points with a buffered wiper. The power amplifier in this design is the TPA3106D1, which is a 40W class-D audio power amplifier with 92% efficiency that eliminates the need for an external heat sink.

This reference design is a 40W class-D audio amplifier with digital master volume control. The design is composed of two channels, left and right, which is ideal for stereo audio source. The OPA1688 two internal op amps buffer the input signals at the stereo RCA jack. The left and right output signals from the two op amps are delivered to the left and right high terminal pins (HL and HR) of the MAX5486 potentiometer, respectively. The MAX5486 acts a volume control for the TPA3106D1 audio power amplifier. Instead of controlling the volume by using a mechanical potentiometer, the MAX5486 controls the volume by using pushbuttons. All of the MAX5486’s pushbutton inputs are debounced. The MUTE pushbutton allows changing between volume control and the -90dB mute setting. The MODE pushbutton toggles the mode between volume and balance control. In volume control mode, the DN/BALR pushbutton decreases the volume level while the UP/BALL pushbutton increases it. When switched to balance control mode, the DN/BALR pushbutton moves the balance to right while the UP/BALL pushbutton moves the balance to left. The MAX5486 also includes indicators to determine the mode and the volume level or balance settings. The LEDMODE LED indicates the mode while the LEDs LED0-LED4 indicate the volume level or balance settings. The left and right outputs (WL and WR) of the MAX5486 deliver the final audio signal to the TPA3106D1 audio amplifier. The TPA3106D1 delivers 40W continuous output power to the 8Ω speaker at 10% THD+N. Its gain can be changed by pulling the pins GAIN0 and GAIN1 to VREG or ground.

The OPA1688 and the MAX5486 in this circuit run on a 5V power supply. The TPA3106D1 operates by using a 24V power supply. The power supply for the power amplifiers must be able to supply the 80W power that the two TPA3106D1 amplifiers deliver to the two 8Ω speakers. The analog ground and the power ground must be connected with single point connection to avoid ground loops, hums, noise, or motorboating. This circuit is ideal for televisions, powered speakers, home-theater speakers, etc.