2.1-channel (satellite communication / bass) speaker system

Abstract: This application note describes the traditional 2.1-channel audio design for portable computers and the design of the solution for the different output power requirements of satellite communication systems and subwoofers. This article details Maxim's high-efficiency, low-cost 2.1-channel solution, which provides 2 × 2W and 1 × 9W output power when powered by a single 5V power supply.

Preface The audio designers of portable computers are committed to continuously improving the system's audio effects. In a space-constrained design, a better solution is to use a 2.1-channel configuration, that is, two stereo satellite speakers to handle mid-frequency and high-frequency (typically 150Hz and higher) and one woofer to handle low-frequency ( Typical values ​​are 150 Hz and lower frequencies). This application note proposes a 2.1-channel audio power amplifier system that uses Maxim's professional audio and power ICs to achieve 5V single power supply, with 2 × 2W and 1 × 9W output power.

The main problem faced by designers of traditional audio system solutions is that satellite speakers and subwoofers have different output power requirements. Typically, woofer speakers require 4 to 5 times more output power than satellite speakers to achieve proper sound balance. When using a single 5V power supply, there are many audio power amplifier solutions available, but all have defects. The most common solution is to use two sets of stereo amplifiers with the same output power. One group is used to drive satellite speakers, and the other is used to drive woofer speakers. The satellite speakers use 8Ω speakers, while the woofer speakers are 4Ω speakers. This constitutes a 2.1 channel solution for 2 × 1W satellites and 1 × 2W subwoofers. Although this solution is simple, it does not provide enough power for the woofer to produce a true bass effect. In addition, the use of 8Ω satellite speakers cannot maximize the sound pressure level (SPL) of the satellite speakers. Therefore, the overall acoustics of this solution are very limited. By replacing the speakers in the above scheme, using 4Ω satellite speakers and 2Ω subwoofers, a 2.1 scheme of 2 × 2W satellite speakers and 1 × 4W subwoofer can be formed. This scheme doubles the output power and improves the sound pressure level. However, it is very difficult and expensive to purchase 2Ω speakers and power amplifiers to drive such speakers. In addition, the power supply current demand will probably double, which will reduce the efficiency of the solution, especially in systems where circuit board space is limited, may cause heat dissipation problems. Better than the above two solutions is the use of 2 × 2W amplifier for satellite speakers, 1 × 9W amplifier for woofer speakers. In this configuration, the satellite speaker is 4Ω, and the 5V power supply voltage is fully utilized; at the same time, the subwoofer is 8Ω, which can produce sufficient bass effect at 9W power. However, the 9W subwoofer amplifier requires a 12V power supply, which increases the complexity of the solution. For a system that can only provide one 5V power supply, one 12V power supply needs to be generated. The advantage of using 2.1-channel speaker system in traditional scheme analysis is that it can produce "shock" sound effects from a small space. To achieve this, the power of the woofer amplifier is at least 4 to 5 times greater than the power of the satellite speaker amplifier. For a 2W satellite speaker amplifier, the output power of the subwoofer amplifier must be at least 8W to 10W.

The solutions # 1 and # 2 mentioned above are easy to implement because only a single 5V power supply is required. However, these two solutions cannot solve the problem because they lack sufficient power to drive the subwoofer.

Solution # 3 is ideal, but only if an additional 12V power supply can be easily obtained.

Figure 1. Maxim ’s complete solution for a 2.1-channel speaker system
Figure 1. Maxim ’s complete solution for a 2.1-channel speaker system

Maxim's solution and its advantages. Figure 1 shows Maxim's complete solution for a 2.1-channel speaker system. This solution uses a compatible Windows Vista®, a MAX 9789 2 × 2W stereo amplifier with stereo headphone driver, a MAX 9768 1 × 10W mono class-D amplifier, and a MAX8740 low-noise boost DC-DC converter.

The MAX9789 integrates a stereo 2W Class AB speaker amplifier and 100mW stereo DirectDrive ™ headphone amplifier driving a satellite speaker in a 2.1-channel system in a single device. The MAX9789 is designed for portable computer systems using the Windows Vista operating system and is fully compatible with the Windows Vista specification. The headphone amplifier has Maxim's patented DirectDrive structure, which can generate a ground-based output from a single power supply without the need for an output DC blocking capacitor. ¹ This DirectDrive structure saves costs, reduces board space, reduces device height, and eliminates the output DC blocking capacitors needed to eliminate clicks and pops. In addition, the MAX9789 also integrates an LDO with adjustable output from 1.21V to 4.75V to provide clean power for audio codec or other analog circuits.

The MAX9768 is a Class D amplifier that does not require a filtered output. It can provide 9W output power to an 8Ω load with 10% THD + N when powered by a 12V power supply. The Class D modulation method does not require an output filter, which can reduce costs and provide sufficient 9W power to the subwoofer in a 2.1-channel system. The MAX9768 has 87% efficiency and no heat sink is required. In addition, its spread-spectrum modulation mode allows the device to pass FCC EMI limits with only low-cost ferrite beads and capacitance on each output when using 0.5m cables. ² (Figure 2 below shows the FCC radiation scan diagram when the MAX9768 user evaluation board outputs pink noise and uses a 1m cable.) Please note: If the subwoofer in the system does not require large power, the MAX9768 output power can be reduced.

Figure 2. MAX9768 unfiltered EMI measurement with 1m speaker cable
Figure 2. MAX9768 unfiltered EMI measurement with 1m speaker cable

The MAX8740 is a fixed-frequency, pulse-width modulation (PWM) step-up DC-DC converter with a built-in n-channel MOSFET. The device is designed for various DC-DC conversion applications. For our solution, this device is used as a step-up DC-DC converter with a 5V input voltage to generate a 12V DC voltage to power the MAX9768. The MAX8740 is easy to use, requires only a few external components, and is available in a µMAX® package. The MAX8740 has a conversion efficiency of 90% and a shutdown current of less than 0.1µA, making it ideal for battery-powered portable system applications.

Conclusion The 2.1-channel overall solution makes full use of Maxim ’s expertise in audio and power ICs. The MAX9789 provides a fully integrated audio solution for portable computers running the Windows Vista operating system. The MAX9768 provides high efficiency and large output power, and can fully drive the woofer in a portable 2.1-channel system. The MAX8740 provides an easy way to convert a 5V power supply to 12V to complete the system and power the woofer amplifier with 90% efficiency.

¹For more information about Maxim ’s patented DirectDrive architecture, please refer to Application Note 3979: "DirectDrive Technology".
²For more information about the MAX9768 spread spectrum modulation mode, please refer to application note 3881: "Spread Spectrum Modulation Mode Minimizes Electromagnetic Interference for Class D Amplifiers."

µMAX is a registered trademark of Maxim Integrated Products, Inc.

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