Hi everyone,
I am working on a small microphone preamp for my PC which can be powered either from a battery
or from a USB port. When it is powered from the battery everything is ok and the main source of noise
if the electret itself. As you can imagine the USB power line is not the cleanest... so I am trying to
filter it. I don't know the best way to do it, but I've used a Pi filter, a common mode choke and finally
a notch filter for a specific frequency.
Below are the results from each filter in succession. (The 5V are AC coupled with a 100nF capacitor
and fed to the mic in of the PC. On the PC no mic gain and 50% levels .)
5V USB, no filtering
5V USB and Pi filter
5V USB, Pi filter & Common Mode Choke
5V USB, Pi filter, Common Mode Choke and 12.7kHz Notch filter
At this point the 5V rail is much better and you can barely hear anything
even if you stick your ear to the HiFi speaker.
Now when I use an LDO to lower and regulate the voltage to 3V,
its output adds approximately 15db of white noise! LDO is the LP29050ACZ
and it is bypassed with a 100nF capacitor. Here is the result :
You can hear it as a slight quiet hissing sound.
https://soundcloud.com/hgg2k/5v-usb-noise-pi-filter2-3v-ldoIts not loud but this will get amplified through the mic preamp.
Is this a usual behaviour of an LDO?
Is there a better way to clean the 5V USB?
Thanks!
George.