USB 5V noise filtering

Maybe - the noise may be coming from somewhere else.

First, I'd try a clean 5V supply. Maybe even 4.5V from batteries.

USB uses half-duplex differential signaling. One of the tricks you can try is to get a short USB cable, open up the +5 line (RED), put a 10 ohm resistor in series, and bypass caps from the load side of the resistor to the ground (BLK) line. I usually dig through the junk box and put in a surface-mount slug tantalum (Hobson's choice, usually 4 to

But how about ground loops? Open up BOTH the +5 and ground lines, and put 1 ohm resistors in series (1 to 10 ohms, again, hobson's choice depending on what you have and how much current the thing on the end of the USB cable wants), and bypass that.

A pair of these cables makes it easier to find/diagnose noisy (cheap) designs.

I build a small filter with 22 ohms with bypass caps 15uF/0.01uF on the supply line. I still have 4.3V at the output. This got rid of most noise by about 80% (subjectively...). But I still have some kind of steady high frequency wistle noise when recording with the mic. I know for sure the mic is good. Just to be sure, I tried 4a clean 4.5V supply from three "AA" batteries as suggested by Ian and the result is the same. At least the filter is good for filtering noise from the computer. I guess the remaining noise is "internal" to the adapter just before the microphone signal is sampled and converted into digital form.

Finally I reduced (drastically) the mic remaining noise by adding a bigger cap right at the supply input of the adapter. At least 220uF does the trick. I kept the resistors in the circuit.

Thanks for your suggestions guys! John