Tuning a Cheap USB Audio Interface

Some years ago I got a cheap UCA202 audio interface from Behringer. Unfortunately the Burr-Brown (now TI) USB audio codec pcm2902 is broken by design. The internal voltage reference shows a terrible distortion because the 1 kHz usb clock isn’t decoupled properly. The data sheet recommends the use of an external voltage reference for a high-performance application (figure 37) – unfortunately Behringer uses the circuit for a simple application (like figure 39).
I tested my UCA202 with jaaa, a great tool written by Fons and got a “nice” peak at 1 kHz – it went away after adding a reg103-a to my UCA (you can get free samples from TI).
If you compare figure 37 and 39 of pcm2902 data sheet you’ll see the difference. I’ve soldered the two resistors 27k and 13k directly onto the IC1 (these devices are SMD parts – I had to use a magnifying glass an a very fine pitch soldering iron) and made a connection to the pcm2902.
You can substitute the voltage regulator by any other low drop type – the enable input isn’t imperative. The pcm2902 codec needs about 3.6..4.0 volts at pin 10 to override it’s internal voltage source.
Good luck.


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