I'm building an electret microphone pre-amp following this application note, the output of the preamp is connected to an LM386 and then to some headphones.
So my pre-amp circuit looks like this, where VCC is 3V, all other components are the same:
and the LM386 part:
And it works, I hear the sound being amplified but, it's buzzing quite a lot.
So here there are some details about the implementation and things that I've done to try reduce/identify the noise:
The preamp stage was implemented using a different op amp, the one that I had available, an OPA313 which doesn't seem to be the reason because it's a relatively low noise op amp, and most of it's characteristics are very similar to the one used in the AN.
My first guess was the LM386 circuit, so I grounded the input to check if the noise was still there, and it wasn't, so it's quite clear that the noise is coming from the preamp stage. Which makes sense because that stage has the highest gain.
The board was initially powered using a USB (with an RC filter at the input 100 ohms + 330uF). So to remove the possible 'unfiltered' noise I powered the board with a 9V battery and a 7805 and the buzzing was still there.
The wire that connects the microphone to the pre-amp is as short as it could get, to eliminate any induced noise there, so I basically connected the C3 cap pin to the op amp pin.
Since I don't have the electret capsule datasheet I used the component values from the AN, except for R1, because the pre-amp stage is powered from a 3V regulator.
The noise it's not at 50 Hz, it's a higher frequency, around 3 KHz.
I have tried with different electret capsules but the noise is still there. Am I missing something? The capsules I found are the generic ones, anyway I don't think that the amount of noise I'm hearing is due to that. I've been struggling with this for many days now and looks like the solution is really simple but I can't see it.