Jan 20, 2013

Simpla USB Audio player


I want to build high quality preamplifier with built-in DAC from SPDIF or USB for my power amplifier Leachamp. I had available circuit PCM2902. I tried to design DAC from USB with this circuit on one-sided PCB and I was succesful.

Circuit description

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-. I used only DAC part. Other parts are not used. For high quality playback is needed to use external low-drop voltage stabiliser for DAC part. I used LP2951CM which was available at local store. 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 USB connector.

Schematics diagram


Placement procedure

After drilling holes for crystal, output pads and USB connector I placed integrated circuit PCM2902, because I want space around him for soldering. I placed circuit at right place, soldered one pin on the side a next goes over all pins on second side and next same on first side. Almost all pins are now shorted. Now I take up tin with a solder wick. There must be enough flux on place for good disconnecting of shorted pins. After visual and electrical check we can make identical procedure with voltage stabiliser. Here is not too critical, because there are more space between pins. Next I placed SMD resistors, ceramic capacitors and LED. These parts I shoulder, that after placing on right place hold with nail, screwdriver or tweezer. One pin I solder with small amount of tin. If part still sits on right place, I solder second end and next I finally solder first end. Next I place tantal capacitors and crystal because they are big. On the end I solder USB connector. Circuit must work immediately after first connection to system a advertise like standard USB soundcard. For normal operating systems like Linux or Windows XP are not needed special drivers.

Placement scheme


Printed circuit board

Circuit board is designed one-sided and everything beside crystal and USB connector are placed from the bottom side. Circuit board is available for free version of program Eagle, which can be free downloaded from their homepage. 



External Low Pass Filter

In 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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