This Pro-Ject Dac Box has been with me for some time. It is my Apple TV’s partner to stream music from iTunes and Airplay. It is a handy kit with only one button at the front panel.
Now, I am planning to modify my Pro-ject DAC Box USB. The main connection is optical though there are USB and coaxial inputs. It supports up to 96k/24bit. It is neat and cool audio gear, especially space is limited.
There are six screws on the chassis – two at the front, two at the rear and two at the bottom. Unscrewed the two at the bottom and the case can be slid open. The PCB occupied the rear half the the kit. It leaves enough space for modifications. Almost all the components are surface mounted.
There are three DAC (digital to audio converter) chips on the circuit board. I believe they are handling the digital to analog conversion of optical, usb and coaxial respectively.
First, the PCM2704 is an obsolete model from Texas Instruments. It is the USB chip. The chip is no longer recommended for new products. The new replacement chip is PCM2704C.
The one next to it is Cirrus Logic’s 8416-CZZ. It is the coaxial and fiber decoder chip. Though it supports up to three channels of S/PDIF input, the Pro-ject DAC Box uses two of them for the copper and light decoding.
The 8416-CZZ has no analog output but a I2S serial digital output. It needs another chip to translate the Os and 1s to music.
The last DAC chip is Texas Instruments’ 320DAC23. It takes the data from the 8426-CZZ and convert them to analog signal.
During the digital-to-analog conversion, there are rapid switching of transistors (the basic element of all integrated circuits) inside the DAC chips. A transistor consumes minimal power when it is at static ON (the voltage between the output and the ground is almost zero, though the current may be high, the product of voltage x current is almost zero) or OFF states (there is a gate-to-ground voltage but the current is zero, so the power is also zero). It takes most power while it is switching from ON to OFF or from OFF to ON. That’s why the power transistors of an amplifier need heat sinks as the transistors are always at the state between ON and OFF.
The rapid switching transistors in the three DAC chips may cause voltage drop at their Vcc (the positive power supply point of the chips). Some chips may mark the Vcc as Vdd or Vd. The chips with analog output are most sensitive to the slightest fluctuation at their Vcc.
In order to stabilize the Vcc, a localized capacitor can help. The capacitor must be with very low internal resistance and very low charge up time. The capacitor must be localized to the chip such that the leads of the capacitor can be as short as possible and the resistance can be kept at its lowest value.
A polyester film capacitor is an option.
Electrolytic capacitors have high capacitance but they are more leaky and their discharge rate is slow. They are more suitable to serve as the reservoir at the power supply end.
0.1uF capacitors are to be added right across the digital Vcc and ground of three DAC chips.
Ok, it is time to shop for the three 0.1uF caps. Will show the modified DAC in a few days. 🙂
Let me know if you have done similar modifications before. Thanks for reading.
<2013 Feb 02> A new post of the mod is added.