This is high quality Preamplifier circuit with built-in USB DAC for my power amplifier Leachamp. Schematics is from datasheet of PCM2902. Circuit includes DAC and ADC, SPDIF output and input and HID part with 3 buttons for MUTE, VOL + and VOL-.
For high quality playback is needed to use an external low-drop voltage stabilizer for the DAC part. DAC is used LP2951CM which was easily available at local stores. Output voltage is set to about 3.7V with two resistors. Circuit board is designed regarding to good ground placement and separating of analog and digital ground. These ground are connected in one point at a USB connector.
Skema Rangkaian USB Audio InterfaceIn datasheet of PCM2902 is recommended to connect Low Pass Filter to output of DAC for filtering high frequencies above audioband which are produced by oversampling conversion. Integrated circuit includes digital LPF which filters frequency above 100kHz. In application notes for filter on the manufacturer pages are recommended 1st-Order LPF (simple RC) or 2nd-Order with operation amplifiers which works like preamplifier too. I used simple RC LPF with recommended values R 1k and C 4n7. It's better to use roll-type capacitor instead ceramic. I didn't hear difference in a sound between connection with filter or without it, but with respect to other components in a audio chain it is better to use it. For a higher cut-off frequency we can change value of capacitor to 3n3.
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