Marko Kannisto, Maxim Integrated
Application Note 5147
Pretend that the weather is warm and it is a nice time to be outdoors chatting with friends. You want some background music, but where will it come from?
When people want portable music, they usually rely on battery-powered audio devices. Of course you can buy a sound system for outdoor use, but those can be quite expensive and, as we shall show, unnecessary. With just a bit of engineering blood (or curiosity) running in your veins, it is not that difficult to build a wireless Bluetooth® stereo audio system that can be controlled with any device that has a Bluetooth connection and a music player.
This application note describes how to build a simple and fairly low-cost wireless Bluetooth stereo audio system for outdoor use. The system has 20 WRMS output power. It can be controlled either with a handset that has a Bluetooth connection and a music player, or with an Apple® iPod® model. Three appendices discuss additional layout considerations, the differences between using a Class D and Class AB amplifier in the design, and the cost of the system.
Bluetooth Basics
Bluetooth technology was created on 1994 and is widely used to establish a short-range wireless connection between two handheld mobile devices. A Bluetooth setup also lets you transfer a stereo audio signal from a handheld device to a wireless headset. Today's Bluetooth headsets operate under the Bluetooth Class 2 specification with 2.5 mW (4 dBm) transmission power. The transmission range for a Bluetooth Class 2 module is approximately 10 meters.
Putting the Bluetooth Audio System Together
Bluetooth headsets are usually used with fairly small-sized, 32 Ω speakers. By connecting the audio signal from a Bluetooth headset to an external amplifier, it is possible to amplify the signal. The Bluetooth stereo headset then acts as a bridge, delivering the audio signal from the handheld device to a stereo audio amplifier and creating a wireless stereo audio system.
A block diagram of the system is described in Figure 1.
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| Figure 1. | Block diagram of wireless Bluetooth stereo audio system. |
This system is based on a stereo Class D audio amplifier which is driving a pair of 4 Ω speakers.
Determining the Output Power
The output power requirement is 20 WRMS (10 WRMS per channel). One should note that measuring RMS output power is not the optimal way to determine an amplifier's output power; RMS does not really reveal much about an amplifier's true performance. Therefore, in this article RMS output power actually refers to the average sine-wave power into a resistive load. RMS output power is used mainly for comparative purposes.
The 20 WRMS translates to 40 W peak output power. To achieve 10 WRMS per channel into 4 Ω speakers, the minimum supply voltage (VRMS) needs to be:
For peak output voltage, this translates to:
The above equations show that a 9 V supply voltage would be enough to achieve 10 WRMS output power per channel. However this design uses a 12 V supply voltage to allow some room for the power-supply unit's (PSU) output voltage to drop during a heavy load.
For an isolated PSU with a 12 V output voltage, a low-cost LED driver voltage supply is used. These supplies are readily available from any electronics store. Choose a PSU that matches the output power.
As an aside, it is important to realize that 20 WRMS total output power does not necessarily mean anything to most people. For comparison, think about low-cost car audio amplifiers or low-cost portable audio "boom-box" products. They are often advertised as providing 40 W total output power, but this is normally peak output power which translates to 20 WRMS output power.
You may ask, finally, whether 20 WRMS is really enough output power for this system to work adequately? Yes, it is. Low-cost car audio amplifiers do not normally have a step-up DC-DC converter, so they use 12 V from the car's battery to drive the speakers. (Actually, a car's battery voltage is approximately 13.8 V, but this is not important here.) Listen to these low-cost amplifiers – they produce plenty of sound. Admittedly, 20 WRMS output power is not enough to keep the whole neighborhood awake, but that is not a priority for our system anyway.
Selecting the Bluetooth Headset, On/Off Switch, and LEDs
You can use any headset with a 3.5 mm audio connector. A Nokia® BH-214 Bluetooth stereo headset is used in this design. It has the 3.5 mm audio connector, making it very easy to take audio signals from a headset's PCB and feed signals to the stereo audio amplifier.
An On/Off switch and indicator LEDs are also needed. These signals need to be taken from the Nokia headset.
Modifying the Nokia BH-214 Headset
The Nokia headset needs to be modified. Since some signals come from outside the headset's PCB, a few wires need to be soldered to that PCB to enable signal transmission. Figures 2 to 6 show how this is done.
With the front cover of the headset facing you, open it by gently squeezing with a small screwdriver at the right corners, top and bottom. Remove the plastic faceplate and remove the keyboard rubber. Gently pull the part containing the PCB from the bottom enclosure.
With the front cover open, make some extra room for wires by clipping off some plastic from the area circled in Figure 2. Also remove the light guide and the plastic enclosure for the On/Off switch.
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| Figure 2. | Headset front cover, inside view. |
Working from Figure 3, solder off the On/Off-switch (1), the bicolor LED (2), blue LED (3), and battery wires (4). Remove the battery from PCB.
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| Figure 3. | Close-up view of the PCB for the headset. |
Now solder wires to the On/Off switch pads (1), as shown by arrows in Figure 3. Then solder a wire to the bicolor LED common anode pad (2), as shown by those arrows. Solder a wire to the blue LED anode pad (3). Finally, solder wires to the battery pads (4).
Use the thinnest wires that you can find, and note the colors of wires for future reference.
There are three more wires (Figure 4) that need to be soldered to the headset.
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| Figure 4. | LED wires soldered to headset (right) with close-up view (left). | ||
Solder the green LED cathode (5), red LED cathode (6), and blue LED cathode (7) to resistors as shown in Figure 4. Now there should be wires for the following signals:
- Two wires for the Bluetooth headset's On/Off switch
- Three wires for the green and red bicolor LEDs (i.e., the common anode, green cathode, and red cathode)
- Two wires for the blue LED (anode and cathode)
- Two wires for the battery terminals (for 3.6 V from the LDO)
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| Figure 5. | The modified headset with cover replaced. |
When all wires are soldered to the PCB, gently put the headset back together. Close the enclosure with caution, since it is very easy to detach a pad from the PCB by pulling the wire. After modifications, the headset will look like Figure 5.
Examining the Stereo Audio Amplifier Board
The stereo audio amplifier in this design is the MAX98400A Class D amplifier. (See Appendix 2 for a discussion of why a Class D amplifier is better suited for this application than a Class AB linear amplifier.) This amplifier has a high 107 dB signal-to-noise ratio (SNR). Speakers can be connected directly to the amplifier's outputs without an output filter and without big, bulky DC-blocking capacitors. This latter point is very important. Traditional Class D amplifiers need an output filter to recover the audio signal from amplifier output. The MAX98400A relies on the inherent inductance of speaker coil to recover the audio component of the square-wave output. Eliminating the output filter and DC-blocking capacitors saves considerable space on the PCB.
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| Figure 6. | Stereo audio amplifier board schematic. |
A linear voltage regulator (MAX16910) is integrated in the stereo audio amplifier. The regulator converts a 12 V supply to 3.6 V. The LDO output is connected to the headset's battery terminals. The LDO's output voltage can be between 3.4 V and 4.2 V. With its 30 V (max) input voltage and built-in thermal and overload protection, the MAX16910 is ideal for this application. In fact, with the R1 and R2 values shown in Figure 6, the LDO's output voltage will be approximately 3.8 V, which is ideal here.
From Figure 6 it is apparent that the MAX98400A amplifier needs few external components for its function. The amplifier's gain is selected with SV1 and SV2; maximum gain is 32.9 dB, which equals 44.2 V/V:
The maximum gain is selected when SV1 is open and SV2 pins 1 and 2 are connected together (Table 1).
| Table 1. | Gain Settings for the MAX98400A Stereo Audio Amplifier. |
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Therefore, with a 12 V supply voltage and 44.2 V/V gain, the maximum input signal that can be used without clipping and distortion to the output signal is:
The amplifier has an integrated clipping-limiter circuit that prevents output clipping distortion. That capability is very convenient, because with maximum gain and without the clipping limiter the headset's output signal would need to be adjusted to 270 mV (max) to ensure that the audio amplifier's output signal is not clipped and distorted when volume is set at maximum level. Alternatively, the MAX98400A's gain can be lowered by choosing different settings for SV1 and SV2.
Materials on the topic
