This is the circuit quality preamplifier with built-in USB DAC for my Leachamp power amplifier. Scheme is PCM2902 datasheet. Circuit includes DAC and ADC, SPDIF input and output of HID and with 3 buttons + MUTE, VOL-and VOL.
For playback of high quality needed for external low-drop voltage stabilizer for the DAC. LP2951CM DAC is used, which was readily available in local stores. Output voltage is fixed at about 3.7 V with two resistors. Circuit board is designed with regard to the placement of good land, and the separation of digital and analog ground. These earth are connected in a single point in a USB connector.
The PCM2902 data sheet is recommended to connect a low pass filter the DAC output to filter high frequencies above audioband produced by the conversion of oversampling. Digital integrated circuits that includes LPF filter frequency above 100 kHz. In the Notes application filter on the pages of the manufacturer recommends first-order LPF (simple RC) or 2 nd order with amplifiers operating as a preamp that works well. I simple RC LPF with the recommended values R and C 1k 4N7. It is best to use the scroll-type ceramic capacitor in place. I did not hear the difference in sound between the connection filter or not, but with respect to other components in an audio chain is best used. For maximum cutoff frequency that can change the capacitor value of 3n3
