The ADAU1701 is a complete, single-chip audio system that integrates a 28/56-bit audio DSP, ADC, DAC, and a microcontroller-like control interface. It offers a wide range of signal processing capabilities, including balance, mixing, bass boost, multi-band dynamic processing, delay compensation, and stereo image expansion. These features help overcome real-world limitations in speakers, amplifiers, and listening environments, while dynamically enhancing the perceived audio quality.
The first circuit we will discuss below features an analog audio input, Class D output, using the ADAU1701 SigmaDSP codec, the low-power SSM2306 Class D amplifier, and the ADP3336 LDO regulator. This configuration provides a compact and efficient solution for audio applications.
The circuit shown in Figure 1 connects the ADAU1701 codec, which includes an integrated SigmaDSP® core, to the SSM2306 2W stereo Class D amplifier and the ADP3336 low dropout regulator. The ADAU1701 has two ADCs and four DACs built-in, allowing it to process one stereo audio signal and simultaneously output separately processed signals to both a line-level output and an amplified output. This flexibility enables different signal processing modes for each output, such as custom EQ, chip-level compression, or spatialization effects tailored to specific speaker configurations.
The ADP3336 provides a stable 3.3 V supply for the ADAU1701. The SSM2306 is an ultra-low idle current, high-efficiency Class D amplifier that doesn’t require bulky external inductors, making it ideal for space-constrained designs. Its power is drawn directly from a 5 V system supply, not the regulator. This setup is well-suited for low-power audio systems like radios, multimedia docking stations, or PC speakers.
The DAC output of the ADAU1701 is connected to the SSM2306 through resistors and capacitors at each amplifier input. A 0.10 μF capacitor in series with a 13.0 kΩ resistor forms a 28 Hz high-pass filter between the ADAU1701’s output and the SSM2306’s input. These components also set the amplifier gain to approximately 6 dB. With a full-scale output of 0.9 V RMS from the ADAU1701, the SSM2306 amplifies this to 1.8 V RMS (5.09 V PP) when operating at 5 V. This matches the amplifier’s clamp level precisely.
The circuit uses the multipurpose (MP) pin of the ADAU1701 to control the active-low shutdown pin of the SSM2306. Combined with a 10 kΩ pull-up resistor, this allows the SigmaDSP program to disable the Class D amplifier gracefully, avoiding any popping or humming sounds.
To ensure stability, a ferrite bead and a 1.0 nF capacitor are placed on each speaker pin. This helps maintain a clean and stable output from the SSM2306’s Class D amplifier.
The SSM2306 is powered directly from a 5 V supply, such as a battery, while the ADAU1701 requires a 3.3 V regulated supply, provided by the ADP3336. The output voltage of the ADP3336 is set to 3.3 V using a feedback network of 140 kΩ and 78.7 kΩ resistors. A 1.0 μF capacitor between the output and ground ensures stable regulation, while a similar capacitor at the input decouples stray inductance between the board and the 5 V supply. The regulator’s shutdown pin is simply tied to the input voltage, so it powers up automatically when the input is present.
In addition to the ADAU1701, this circuit can be adapted using other SigmaDSP processors with integrated DACs, such as the ADAU1761. The ADAU1702 can also be used as a replacement, differing mainly in the size of its SigmaDSP program and data memory. Other Class D amplifiers, such as the SSM2301, SSM2302, and SSM2304, have slightly different characteristics. For example, the SSM2301 is a mono amplifier, whereas the SSM2306 is stereo, and the latter does not require external resistors to set the gain.
This digital audio system solution based on the ADAU1701 and ARM7 demonstrates how powerful and flexible modern audio processing can be. With detailed schematics and practical implementation examples, it serves as a valuable reference for engineers designing audio systems with advanced signal processing capabilities.
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