MarkT:
You really need separate, linear, regulator for the mic preamp - the noise will be digital hash from the Arduino most likely, nothing to do with the audio amp. Switchmode supplies or sharing digital and analog power rails are two very good ways to get audible noise in such a preamp - microphone amps make microvolt level signals audible.The main difference between the Uno and Pro Mini that's likely to be relevant is the lack of large electrolytic capacitors on the tiny Pro Mini board. That would account for 10dB of difference in noise nicely.
Thanks for the response.
Are you saying I should power the mic preamp with separate power supply? Currently I am powering up all the circuits using laptop usb output. Actually currently I am packaging the components so that I can deploy for actual purpose, so the system would normally have a single 5v power supply probably a mobile charger adapter. You have any thought on this as well?
MarkT:
The main difference between the Uno and Pro Mini that's likely to be relevant is the lack of large electrolytic capacitors on the tiny Pro Mini board. That would account for 10dB of difference in noise nicely.
So should I put some 100uf on the pro mini power rail to remove some noises?
MarkT:
the noise will be digital hash from the Arduino most likely, nothing to do with the audio amp.
Also I do not understand what is digital hash from Arduino?
Thanks,
Debojit