When you search for a networked stand-alone audio player with a touch screen, most likely chances are you will only find big consumer grade amplifiers. Those with network support may not have a touch screen or else may be very expensive. Most disappointing will be those having an issue with space and mobility. The best way out of this dilemma is to build one with the famous single board computer, the Raspberry Pi (RBPi).
You must start with an application that works on the RBPi. You can already find good quality DACs on the market. The makers of the application Volumio have used PCM1794A, the DAC from Texas Instruments with good results. As this is a 24-bit device, it can handle sample rates up to 200 KHz, and offers an 8x oversampling filter built-in.
The PCM1794A requires two voltages for proper functioning. It needs the 3.3 V for its digital part and the 5 V for its analog part. Although it seems possible to use the two voltages available on the GPIO expansion connector, the noise present on these voltages precludes their use for the DAC. Another possibility would be use the power supply for the DAC to power the RBPi. However, that is also not advisable, as this would mean degrading the power supply of the DAC. Therefore, the two devices need two distinctly different DC adapters.
For the I/V converter, another voltage is necessary and this has to be a negative voltage. The designers derived the negative voltage using the LM27761 IC, a special switched capacitor low-noise regulated voltage inverter. The IC is extremely small, only 2 x 2 mm, and operates at 2 MHz, introducing very little noise into the circuit.
Both the 5 V and 3.3 V required by the DAC are generated by ultra-low-noise positive linear regulators of the typeTPA7A4700 and TPS7A4901. Voltage dividers made by two resistors fix the output voltage, one pair for the 5 V and the other for the 3.3 V. A Schottky diode protects the input to the power supply against reverse polarity—it drops only 0.3 V from the single power supply of 7-8 V.
The 3.5-inch display goes above the Audio DAC. If necessary, use two standard-size stacking headers to place the display higher to clear the components. This will place the 25-way socket of the display above the Audio DAC PCB.
Performance
Plotting the amplitude of the output as a function of frequency shows the cut-off frequency at about 63.5 KHz. The total harmonic distortion plus noise was measured as a function of frequency with sampling rates of 48, 96, and 192 KHz shows it to be far lower than the acceptable limits—at 0.0007%. Although the RBPi generates several spurious frequencies that are just visible, the level for the fundamental frequencies is very low at -120 dB (1 µV). Those for the second and third harmonics are barely visible.
Various FFT analysis of a 16-bit, 1 KHz full-scale sine wave at different sampling rates shows the harmonic distortion to be far below the acceptable levels— at 0.002%. All these measurements show this tiny board to offer a great audio experience.
