This Pro-Ject Dac Box mod project started a couple of weeks ago. I did some research on the major audio chips used in the dac. I downloaded some data sheets of the chips and checked if the circuit design in the dac follows the recommendations of the manufacturer.
CS8416 of Cirrus Logic – There is a recommended circuit board layout of the analog power supply and the phase lock loop (PLL). The capacitors and resistor are to be laid right at the pins of the chip so as to reduce the unnecessary inductance of the circuit board trace. The picture of the chip showed Pro-Ject complies with the requirement.
It also requires a 0.1uF decoupling capacitor at the digital power supply, i.e. between pin 22 and 23. On the circuit board, there is no such capacitor. It is neither at the other side of the circuit board.
Action: Add a 0.1uF decoupling cap across pin 22 and 23.
For TI’s 320DAC23 and PCM2704, both are suggested to add decoupling capacitors at their digital power supply pins. Again, Pro-Ject has not added them.
Action: Decoupling capacitors (0.1uF) are to be added across pins 27/28 of 320DAC23 and pins 6/7 of PCM2704.
I will take the actions. If the mod is successful, I will show the modified DAC Box in the next post.