[sdiy] Switching regulator and microphone noise

[Bert Schiettecatte]

Hi all,

I'm trying to build a good electret microphone amplifier here. I got
some cheap electret microphones from Digikey (horn industrial brand) and
some LM833D's here. I built a simple circuit first shown here:

http://www.geocities.com/ferocious_1999/md/micpreamp2.html

Instead of letting one opamp handle the gain, I've used the two opamps
in the LM833 with 14K, 47K and 2.2K resistors to create 4.4x and 7.4x
gain stages, so in total 32.6x gain. The signal coming out of the LM833
goes into an ADC (0 to 3.3V range). 

When I connect the circuit to a big lab power supply and run it at 3.3V,
It seems to work fine. When I connect it to a switching regulator, I get
a lot of noise in the background. The microphone signal is still there
and audible, but the noise is a lot compared to the lab power supply.
The switcher's frequency is between 1.2 and 1.8 MHz according to the
datasheet. 

There will be a CPU in the design as well so I'd like to isolate the
audio circuitry from all the digital stuff. How do I get rid of all the
noise and interference? Decoupling caps seem to filter the noise a bit
but don't remove all of it. 

By the way, is there a difference in quality or noise between LM833D
(On-Semi) and LM833M (National) ?

Thanks a lot for all the help!
Bert