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Compact USB Audio Adapter


I was looking for a USB audio adapter for my Intel Galileo projects, and surprisingly I was not able to find a compact USB audio adapter which will offer line input or digital (optical) input and output. Most USB audio dongles available on the market are intended to be used with a headset and only offer monophonic microphone input. One exception was Behringer UCA202 which is bulky for my projects. And so I designed this adapter.


This USB Audio adapter can be used with small SBCs like Intel Galileo, Raspberry Pi, or BeagleBone Black to provide high quality audio input and output. Its size is only 2.1 x 1.6 inches (53.44 x 40.74 mm). This USB Audio adapter will also work with regular PCs, Android based smartphones and tablets, or almost everything else that supports USB audio. It is based on popular Texas Instruments (was Burr Brown) PCM2902C, and schematic mostly follows Texas Instrument's reference design.


Top side

Bottom side


As mentioned above the main component is Texas Instruments PCM2902 USB audio codec IC (U1). Additionally the circuit uses Texas Instruments REG103UA LDO IC for conditioning power for the analog circuitry (U2) and a pair of dual channel operational amplifier ICs that work as low pass filters for input and output (U5 and U6).

Schematic and PCB layout

See attachments section at the bottom of this page.

Design modifications and component substitution

There is a lot of flexibility in the design:
  • The USB audio codec IC (U1) can be PCM2902, PCM2906, PCM2900, and PCM2904.
    • PCM2902 and PCM2906 provide S/PDIF digital interface. PCM2900 and PCM2904 do not provide it.
    • The difference between PCM2902 and PCM2906 or between PCM2900 and PCM2904 is very subtle: PCM2900 and PCM2902 specify 100mA as Maxpower value in USB descriptor, while PCM2904 and PCM2906 specify 500mA. Operating systems should be using Maxpower values to estimate power consumption by the attached USB devices and disconnect USB devices if host is unable to supply enough power.
    • If using codec without S/PDIF support the components related to optical input and output should not be mounted.
    • I used the latest PCM290xC revision. Other, older revisions don't differ significantly and can be used as well.
  • The REG103 LDO (U2) is optional and can be removed to lower the cost at some audio quality degradation. It is also possible to replace it with cheaper LDO with minor schematic and PCB modifications.
  • The design uses high-speed operational amplifiers. Either one of these op amps or both can be bypassed, leaving just 100 ohm / 10 uF capacitors for audio coupling. It is also possible to use other compatible operational amplifiers (important characteristics: single power supply 3.3V, rail-to-rail)
  • Both input and output parts are optional. For example if only ADC part is needed, no need to assemble components related to audio output. Same applies for DAC only (audio output only) use. In this case I suggest grounding audio inputs of PCM2902 to avoid noise.
  • The PCB supports two options for connecting to USB:
    • Using Micro USB connector
    • Using 5-pin header (it is also possible to solder a USB cable directly to the board instead of this header).
  • Volume control buttons are optional

Assembly notes

Due to project's size and component availability the project mostly uses SMT components. I used through hole components for components that are subject to physical stress (connectors and buttons). It it is not too difficult to solder SMT components it with some practice. Here are some tips:

  • Have a well lighten workspace.
  • Use a low power (25W or so) soldering iron with thin tip, better yet a temperature controlled soldering station. Keep the tip of soldering iron clean.
  • Use thin solder with flux. Have some desoldering braid to remove accidental solder bridges or excess solder.
  • I found it convenient first to put some solder on a first pad of a component (or on two opposite pads in case of an IC. When put the component using tweezers and heat the pad at the same time. Next double check component orientation and solder remaining pads. Note that diodes and tantalum capacitors are polarized, so the orientation is important.
  • Solder ICs first, next solder micro USB connector.
  • After that solder all the resistors, inductor, and smaller (0603 and 0805) capacitors on the top side, going from ICs to sides of the board.
  • Solder Schottky diode, LED, crystal resonator, and tantalum capacitors. Make sure to check that they are oriented properly.
  • Turn the board up side down, and solder resistors and the capacitors on the bottom side.
  • Finally solder through hole components: connectors, optical modules, buttons.
  • Clean the flux remains with some alcohol. (Be careful not to get alcohol/flux inside optical modules, or maybe solder them after cleaning the board).
  • Search the Internet for SMT soldering tutorials.

Bill of materials

Link to the project on Mouser.com - View and order all components except of PCB.

Link to the project on OSHPark.com - View and order PCB.

 Component Type
 Reference Description Quantity Possible sources and notes
 Printed Circuit Board
  USB Audio PCB - V2
 1 Order at OSH Park or from any other PCB manufacturer. See attachments for Gerber files.
 Capacitor C1 - C3
 22pF C0G SMT 0603 capacitor
 3 Mouser 77-VJ0603A220JXACBC
 Note: C3 is not needed if not implementing optical S/PDIF input.
 Capacitor C4 - C7
 330pF C0G SMT 0805 capacitor
 4 Mouser 77-VJ0805A331KXACBC
 Capacitor C8 - C11
 1.8nF C0G SMT 0805 capacitor
 4 Mouser 81-GRM215C1H182JA01D
 Capacitor C12, C13
 100nF X7R SMT 0805 capacitor
 2 Mouser 80-C0805C104K3R7210
 Note: These capacitor are not needed if not implementing optical S/PDIF input and output.
 Capacitor C14 - C20
 1uF X7R SMT 0805 capacitor
 7 Mouser 80-C0805C105K4R
 C21 - C30
 10uF tantalum SMT capacitor
 10 Mouser 74-593D106X9016A2TE3
 Capacitor C31, C32
 220uF tantalum SMT capacitor
 2 Mouser 581-TLNT227M010R1300
 Note: Use the recommended value if driving low impedance loads (for example headphones). For high impedance loads (e.g. line output connected to an amplifier) cheaper 10uF capacitors can be used. See previous line for part number.
 Schottky Diode
 D1 Schottky diode with low voltage drop (350mV)
 1 Mouser 771-PMEG2010ER115
 LED D2 0603 LED
 1 Mouser 78-VLMTG1300-GS08
 Any color can be used :-) The suggested part is "pure" green. It is too bright to my taste, so if using it consider increasing value of R8. Or use "conventional" green and save a few cents.
 Inductor L1 47uH SMT 0805 inductor
 1 Mouser 963-LBR2012T470K
 Note: This inductor is not needed if not implementing optical S/PDIF input.
 Connector P1 Micro USB receptacle with through hole pegs
 1 Mouser 649-10118194-0001LF
 Note: Use either P1 or P2
 Connector P2 Pin Header 5x1, 2.54 mm lead spacing 1 Mouser 649-68000-105HLF
 Note: Use either P1 or P2
 P3 3.5 mm stereo audio jack, blue
 1 Mouser 806-STX-31203B284C
 P4 3.5 mm stereo audio jack, green 1 Mouser 806-STX-31203B577C
 Resistor R1 - R3
 2.2 ohm 1% SMT 0603 resistor
 3 Mouser 71-CRCW0603-2.2-E3
 Note: For R2 and R3 use the recommended value if driving low impedance loads. For high impedance load resistance can be increased to 100 ohm. See R6, R7 line for part number.
 Resistor R4, R5
 22 ohm 1% SMT 0603 resistor
 2 Mouser 71-CRCW0603-22-E3
 Resistor R6, R7
 100 ohm 1% SMT 0603 resistor
 2 Mouser 71-CRCW0603100RFKEB
 Resistor R8 470 ohm 1% SMT 0603 resistor 1 Mouser 71-CRCW0603470RFKEB
 Resistor R9 - R12
 1.5 Kohm 1% SMT 0603 resistor 4 Mouser 71-CRCW0603-1.5K-E3
 Resistor R13 - R16
 3.9 Kohm 1% SMT 0603 resistor 4 Mouser 71-CRCW0603-3.9K-E3
 Resistor R17 - R24
 12 Kohm 1% SMT 0603 resistor 8 Mouser 71-CRCW0603-12K-E3
 Resistor R25 13 Kohm 1% SMT 0603 resistor 1 Mouser 71-CRCW0603-13K-E3
 Resistor R26 27 Kohm 1% SMT 0603 resistor 1 Mouser 71-CRCW0603-27K-E3
 Resistor R27 1 Mohm 1% SMT 0603 resistor 1 Mouser 71-CRCW06031M00FKEB
 Switch SW1 - SW3
 6 mm x 6 mm tactile switch
 3 Mouser 653-B3F-1050
 Integrated Circuit
 U1 Texas Instruments PCM2902C
 1 Mouser 595-PCM2902CDB
 Integrated Circuit
 U2 Texas Instruments REG103UA-A 1 Mouser 595-REG103UA-A
 Optical Receiver Module
 U3 Everlight PLR135/T8 1 Mouser 638-PLR135T8
 Note: This module is not needed if not implementing optical S/PDIF input.
 Optical Transmitter Module
 U4 Everlight PLT133/T8 1 Mouser 638-PLT133T8
 Note: This module is not needed if not implementing optical S/PDIF output.
 Integrated Circuit
 U5 Texas Instruments OPA2353
 1 Mouser 595-OPA2353UA
 Integrated Circuit
 U6 Analog Devices AD8656
 1 Mouser 584-AD8656ARZ
 Crystal Resonator
 X1 12 MHz
 1 Mouser 774-445C25D12M00000

Sergey Kiselev,
Dec 11, 2013, 8:37 PM
USB Audio - KiCad - 2.0.zip
Sergey Kiselev,
Dec 11, 2013, 8:37 PM
Sergey Kiselev,
Dec 11, 2013, 8:37 PM
Sergey Kiselev,
Dec 11, 2013, 8:36 PM