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. 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 board was initially powered using a USB (with an RC filter at the input 100 ohms + 330uF). Which makes sense because that stage has the highest gain. 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. 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. So here there are some details about the implementation and things that I've done to try reduce/identify the noise: So my pre-amp circuit looks like this, where VCC is 3V, all other components are the same:Īnd it works, I hear the sound being amplified but, it's buzzing quite a lot.

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.

Crazy buzzing linein