I designed this external sound card two years ago after I bought a Dell D610 laptop. It was a really good model with SXGA+ matrix, Pentium M processor on-board and… dramatic quality audio output.
The PCM2702 chip from Texas Instrument contains two DAC converters with an USB 1.0 compilant interface controller and offers quite impressive performance: 100dB dynamic range, 105dB signal to noise ratio and 0.002% THD+N distortion (measured for 1KHz signal frequency).
POWER and PLAY LEDs inform that card is ready. Five integrated voltage regulators provide the necessary power supply (two for the digital and three for analog stage). There is also space for crystal oscillator or an external generator running at 12MHz.
The PCM2702 system application note says that signal applied to XTI input must be 3.3V logic level (this input is not 5V tolerant). After increasing the value of R3 resistor to 300 ohm and changing U1 to LM2937ET-5V generator running on 5V can be also used.
The signal from the audio output of the PCM2702 goes to the low pass filter (U7/U8) witch removes “noise” generated by oversampling DAC circuit. C20 and C21 should be a good quality capacitors (I used Black Gate N series).
R2, R8, and R3 are 0 ohm resistors (jumpers). There is no need to mount U1, C3, GEN and R3 when Q1 is used.
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