June Birth Magoncia

user Electronics Engineer

city Innovuze Solutions, Inc.


  • 30W Stereo Audio Amplifier with Clipping Protection

  • Created: Mar 07, 2016

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Description

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Summary

This reference design features a 30W stereo audio amplifier that employs a motorized slide potentiometer to provide clipping protection. The PSM60-081A-103B2 is a half-watt, 10kΩ, 60mm, motorized slide potentiometer. It provides two built-in 10kΩ variable resistors labeled as line track and servo track. The line track variable resistor can be used for other purpose, while the servo track variable resistor can be used for tracking the position of the potentiometer lever. The built-in DC motor controls or adjusts the position of the potentiometer lever as well as the resistance of the line track and the servo track variable resistors.


This 30W stereo audio amplifier is based on TDA7391 IC. Each channel of this stereo audio amplifier can deliver 15W of power to a 4-ohm speaker at 1% THD when it is supplied by a 12V power source. The master volume of the TDA7391 audio power amplifier in this project is controlled by the line track variable resistor (RLineL/RLineR) of PSM60-081A-103B2. The amount of power that the TDA7391 IC delivers to the speaker is dependent on the PSM60-081A-103B2 line track variable resistor since the gain of TDA7391 is internally fixed at 30dB. Due to the fixed internal gain of TDA7391, there is a big chance that it will clip. At instances that the sound level (gain) of the songs being played differs from each other, the present song may not make the amplifier clip but the next song might clipped it assuming that the next song has a very much higher gain compared to the present song. In this case, the speaker will blow, or worse, both the amplifier and the speaker will be damaged. To avoid this situation, an ATmega328P microcontroller and two transistorized H-bridge DC motor drivers were added in the circuit design. The TDA7391 is equipped with an internal diagnostic circuitry able to detect the clipping in the output signal when THD is at 10%. This internal diagnostic circuitry saturates an internal transistor when THD of the output signal reaches 10%. The CD pin of TDA7391 is connected to the collector of the internal transistor. If the internal transistor saturates due to clipping (THD=10%), it pulls the CD pin to ground as well as the other devices connected to the CD pin. In this design, the CD pin of TDA7391 is used as an input signal to the microcontroller. Whenever a song makes the TDA7391 ICs output clipped signals to the speakers, the CD pins will pull the PD7 and PB0 pins of the ATmega328P microcontroller to ground. This will make the ATmega328P microcontroller adjusts the sound level at the input of TDA7391 by rotating the built-in motor of PSM60-081A-103B2 to a direction that minimizes the sound level of the input signal. The motors will keep on rotating until the PD7 and PB0 pins of ATmega328P sense a HIGH signal.


The ATmega328P microcontroller doesn’t directly drives the built-in motor of PSM60-081A-103B2. It uses transistorized H-bridge DC motor drivers, which really drive the motors. Also, the wiper pins of the servo track variable resistors are connected to the analog-to-digital converter pins (PC0 and PC1) of ATmega328P to monitor the position of the potentiometer lever. The ATmega328P runs on a 5V power supply, while the drivers operate at 9V. By using the ATmega328P microcontroller and the two transistorized H-bridge DC motor drivers to control the built-in motor of PSM60-081A-103B2, the TDA7391 is protected against clipping. It ensures that either of the TDA7391 or the speaker won’t get damaged. This reference design can be used as amplifier for home theaters, car sound system, computer speaker system, etc.