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.

5 thoughts on “Tuning a Cheap USB Audio Interface

  1. Hello, I know your post was made 5 years ago, I hope you will respond: “You can substitute the voltage regulator by any other low drop type – the enable input isn’t imperative”. Does that mean just a regulator between USB VBus and pin 3 of the PCM2902?
    Thanks!

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    • Hello Hans,
      Yes, any low-drop type is sufficient. Connect the input to Vbus and the output via the decoupling schottky diode D1 to pin 10 of the PCM… as in figure 37 of the data sheet (just ignore the two resistors and pin 4 and 5 of IC1). The regulator will draw some quiescent current from the USB Vbus line in standby, this violates the USB specification, but is tolerated by all systems.
      Martin

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      • When I tried to buy ‘any low drop’ I asked for a 5Volt low drop. The salesperson then explained to me that a 5 volt regulator connected to USB 5 volt, can not regulate. And a 3.3 volt regulator does not meet the 3.6 volt requirement of pin 10 / Vccci. Can you comment on that?
        Thanks again!

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  2. Or, maybe I can use a separate power source that is not connected to the ‘dirty’ USB 5 Volt? The Schottky diode is not required in that setup?

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    • You can lift the GND pin of the regulator with a normal diode (+0.7V), rising the output to 4.0V, the voltage drop of the schottky diode gives the 3.7..3.8V at pin 10.
      Why don’t you want to use the proposed adjustable REG103, it’s available as sample from TI for free if you register with a company or university mail address.
      Or use any other adjustable low-drop regulator, e.g. this one: https://www.segor.de/#Q=LM1086IS-ADJ&M=1

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