Make a sound
card is no more a complex issue. If you use great IC PCM2702 from BURR
BROWN / Texas Instruments you can create a fully functional USB sound
card. This sound card can be powered from USB port and has one stereo
output. You donít need to install any driver for Windows XP and Vista,
because they are already inside. This is really plug and play.
Few months ago
I have seen USB sound card called Alien DAC. The construction on the
project web page inspired me to build this thing also.
The core of this
construction is 16-Bit Stereo Digital-To-Analog
Convertor with USB interface PCM2702.
only few additional parts to work. The schematic is not complex. Sound
card can be powered directly from USB port (jumper W1) or from external
power supply (jumper W3). PCM2702 needs two power supply 3.3V (3V-3.6V)
and 5V (4.5V-5.5V). I used fixed output voltage LDO TPS76733Q for 3.3V
(IO2) and adjustable output voltage LDO TPS76701Q for 5V (IO3). Both LDO
are produced by TI, I used this because I had it in my drawer. Any
similar LDO can be used. Output voltage of IO3 should be set to little
bit lower than input voltage to allow LDO good stabilization, in my case
output voltage is set to 4.8V. Output voltage can be set by adjustable
resistor R33. In case of low power supply, IO3 can be shorted by jumper
W3. LED D3 signalizes power on.
beads are placed before all power pins of PCM2702 and in Vbus and GND of
USB. These small beads reduce high frequency hum. I had a problem find
this small SMD ferrite beads in local stores but finally I acquire few
of them from old hard drive. They are not absolutely necessary, you can
use zero ohm resistors instead of them.
filter is placed in output signal path to reduce sampling frequency. An
OPA2353UA dual op amp is configured as a stereo 2nd-order low-pass
filter. Led diode D1 is illuminated when PCM2702 plays audio data
received from the USB bus. Led diode D2 is illuminated when USB bus
suspends audio data transmission to the PCM2702.
Schematic of sound card with
PCB assembly diagram
PCB - Download PCB in [EPS
format] or [PDF format]
Bottom side of PCB (single
side PCB, made by standard etching method)
Assembled bottom side
This circuit works very well. I only shorted crystal
during soldering so the circuit didnít work, but
after removing the short the sound card started to
work. I have tested in Windows 2000, XP and Vista.
It works in all mentioned systems. Drivers are
present in operation system so the sound card is
ready in few seconds after you connect it.
During writing this article I have found that
PCM2702 is now not recommended for new design, but
TI offer even better solution. PCM2704, PCM2705 have
same functionality as PCM2702, but they include
output filter. They are able to drive directly
headphones. Volume and Mute can be controlled
through SPI bus in PCM2705 or with pushbuttons in
case of PCM2704. PCM2704 and PCM2705 are in TSSOP28
package. PCM2706 is similar to PCM2704 and PCM2707
to PCM2705 but in addition they have I2S bus.
PCM2706 and PCM2707 are in TQFP32 package. I
recommend using these new chips for new design (look
at the TI web page).
PCM2702 Texas Instrument
PCM2702 Evaluation Board
this project in ZIP
diy4fun.blogspot.com / Miroslav Batek