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USB soundcard with PCM2902




For testing my DACs I built a simple USB soundcard based on USB codes from Texas Instruments PCM2902. Card has stereo analog input and output, electrical S/PDIF output and electrically isolated input. There is too optical TOSLINK input and output.

Circuit description

Hearts of the circuit is PCM2902 which is complete USB audio codec. Maximum sample frequency is 48kHz. Integrated circuit includes USB controller for ADC and DAC, HID part for three buttons, output volume control and S/PDIF encoder and decoder.

Circuit is based on recommended circuit from datasheet. For better quality of analog part is used external voltage regulator with IC2. All voltages are blocked with 1uF capacitors. There is optical transmitter TOTX173 and receiver TORX173 connected to IC2. Electrical S/PDIF output is realized with RS422 transmitter IC4. Because for transferring of commercial S/PDIF is used unbalanced coaxial cable with 75R impedance, that I used only one output and with help of voltage divider R13/R14 I reduce output voltage and change output impedance. Usually is specified output voltage maximally to 0.6Vpp. On output of divider is a little greater voltage but I never see troubles with it. S/PDIF receiver includes RS422 transceiver too, but now connected inverse like convertor from RS422 to TTL voltage levels. Input is terminated with 75R resistor R7 which we can place before or after pulse transformer. I had better experience with resistor before transformer. Pulses was looks better on oscilloscope. Digital input is connected over small switch electrical/optical. PCM2902 automatically switch from analog input signal to digital, when it detects presence of S/PDIF signal. Analog output doesn't have additional amplifier, that we can't connect them directly to low impedance headphones. It should be connected to external amplifier.

There is not needed special drivers. I tried full functionality in a Linux, Windows XP and Windows 7. Drivers are included in operating system. There also works HID buttons, volume control and mute.

Schematics diagram

Schematics diagram schematics in Eagle 5 format


We assemble circuit board from smaller parts to bigger. First I solder IC1 circuit which is really too small. I fix the two pins on a side of circuit with small soldering iron. When I was sure, that circuit is on the right place, that I applied flux on all pins which helps with better diffusion of tin and I soldered first all pins on one side and next on the opposite side. A lot of pins was soldered together. Overflowing tin I easily drain with solder wick. Next I soldered other circuits and I continued with SMD resistors and capacitors. Next I soldered wire junctions and all components from the upper side and last connectors.

Component view

Component view

Printed circuit board

PCB is designed with only one side and two wire junctions. It allow his easy producing in amateur conditions. His dimension fit in plastic box U-KP35B. After assembling I discovered that input switch is reverse connected. Here is corrected PCB version. SMD parts have 1206 size and they are layouted with respect for easy soldering.

PCB PDF format Eagle5 format


Bottom view Top view With cat


Most components I found in a local store GM Electronic, USB connector in GES Electronic and integrated circuit PCM2902 for example in FK Technics. Pulse transformer I bought on EBay, concretely PE-65612. If we doesn't require electrically isolated input, we can replace transformer with two 100nF capacitors.

Part list
name value and type quantity
R1-R2 22R SMD1206 2x
R3 1M SMD1206 1x
R4,R9-R11 1k5 SMD1206 4x
R5 180k SMD1206 1x
R6 91k SMD1206 1x
R7 75R SMD1206 1x
R8 2R2 SMD1206 1x
R12 8k2 SMD1206 1x
R13 360R SMD1206 1x
R14 91R SMD1206 1x
R15 330R SMD1206 1x
C1-C8 1uF SMD1206 ceramic 8x
C9-C14 10uF/25V elektrolytic SMD size B 6x
C15-C16 22pF SMD1206 ceramic 2x
C17 10nF SMD1206 ceramic 1x
C18-C20 100nF SMD1206 ceramic 3x
IC1 PCM2902, PCM2902B 1x
IC2 LP2951CM SMD SO-08 1x
IC3-IC4 75176B SMD SO-08 2x
L1 47uH axial 1x
LED1 LED green 2mA 1x
OX1 TORX173 toshiba 1x
OX2 TOTX173 toshiba 1x
Q1 12MHz crystal mini 1x
S1-S3 microswitch 6x6mm high 3x
S4 Switch P-KNX125 1x
TR1 LL1572, nebo S22083, or PE-65612 1x
X1 miniUSB socket USB PCB MBW 1x
X2-X3 3.5" socket EBS35 2x
X4-X5 cinch socket to PCB TOBU3 2x
BOX1 Plastic box U-KP35B 23mm x 54mm x 104mm 1x



Introduced circuit works reliable. Measured values approximately match parameters in a datasheet. If we want to use only some inputs or outputs, it is not needed assemble all connectors. Except circuit PCM2902 are expensive optical transceivers and pulse transformer. We can use this circuit like second soundcard or S/PDIF output to STEREO amplifier with digital input. We can use S/PDIF input for connecting other devices with digital output to computer like DAT, miniDISC or CD Player. I measure parameters with RightMark Audio Analyzer (RMAA) and Baudline programs. With RMAA I was much worse results under Windows 7 than under Windows XP. There was some volume control troubles under Win7. I don't know why. Next I continued with testing only under Linux.