Make a high-quality DAC board

1. Circuit Introduction

The schematic diagram is shown below. The whole circuit consists of three parts. The first part is digital filtering. The core device is NPC's SM5843AP. It is an 8x oversampling digital filter chip that can generate 20-bit requantized digital audio signals. The second part is digital-to-analog conversion. It uses Burr-Brown's high-performance digital-to-analog converter PCM63P-K, which is a classic R-2R resistor ladder structure D/A conversion chip with excellent low-level linearity, high accuracy and low distortion, and a dynamic range of about 108dB. The analog signal is output from pin 6 in the form of current. The third part is current/voltage conversion, analog active filtering and buffer output. The feedback resistor and capacitor of the current/voltage conversion circuit have been integrated in the D/A conversion chip. The analog filter uses a GIC filter with good phase characteristics. The system clock of the CD player board is taken from the 16.9344MHz signal generated by the CD player crystal oscillator, so that the DAC clock can be synchronized with the CD player, overcoming the distortion caused by the time base jitter between the two boards, and stably obtaining high sound quality.

2. Production and installation

The resistance value of the analog active filter is special and can be obtained by series connection. All the resistors and 6 capacitors of 1000pF in this part are measured and screened with a digital meter, and the error is controlled within 1%. The values ​​of the two components corresponding to the left and right channels should be basically the same to ensure the same frequency response of the two channels.

Electrolytic capacitors use 10-16V audio-specific electrolytic capacitors, and non-polar capacitors use 50V red WIMA or other small polypropylene capacitors. The positive and negative SVs of the motor board are powered by a transformer and a voltage regulator board (this part of the circuit is omitted due to space limitations). The positive and negative power supplies should be basically symmetrical, with a deviation of less than 0.2V.

After welding and checking, install it. The DSP of Dongda K999CD player is CXD1167Q, and the digital filter built into the chip is used. The filter should be turned off during modification. The specific method is to pick up the 56th and 57th pins of CXD1167Q and connect them to SV. The 6 wires BCLK, DATA, LRCK, CLOCK, MUTE, and GND on the mod board are connected to the 76th, 78th, 80th, 54th, and 19th pins of CXD1167Q and the ground wire respectively.

In addition, it should be noted that if there is no coupling capacitor at the input end of the amplifier, a 10μF capacitor should be connected in series at the output end of the board to cut off the DC output. The soldering iron housing should be reliably grounded during soldering to prevent possible leakage from damaging the integrated circuit.

3. Listening Evaluation

After everything was installed, I turned on the machine and preheated it for about half an hour. After it stabilized, I tried playing several familiar songs. On the premise that the power amplifier and speakers remained unchanged, I felt that the sound field became very wide, the singers (performers) could be seen at a glance, the sound field was reproduced lifelike, and the sound was more delicate and vivid.