Mic amplifier noise

Question

I build a mic amplifier for a standard hands free (Apple 4-pole jack). However, even without anything plugged into the jack there's audible noise at the output (MIC_IN in diagram). It sounds like white noise to my ears.
Increasing C13 to 100 uF almost completely eliminates the noise. However, to my surprise, removing C7 also does that. Please help me understand the forces at play here and what a proper fix for the noise is.
The VCC3 supply has lots of digital noise in it. The circuit is using 0805 / 0603 resistors and capacitors.

roc1 · Answer

I figured out the issue. The circuit amplifies the power supply noise (in Vcc) by R13 / R9 times. Therefore, it is advantageous to have to have good power supply filtering and/or higher mic supply voltage.
@Andy aka's to add more filtering in the supply will help. An input resistor is not required. More details here: https://www.ti.com/lit/ug/tidu765/tidu765.pdf

Andy aka · Answer

You might want to consider these things: -

Without the input resistor the 220 nF has an impedance of 723 ohms and this means that your mid band gain (1 kHz) is 150,000/723 = 207. At 10 kHz the gain will be 2070 and you will hear a lot of noise. Try a 10 kohm resistor in series.

Marko Buršič · Answer

Don't supply the OPAMP and its input circuitry from voltage reference, latter has to have a high feedback gain to remain low drift, so it picks up noise if you extend a long wire from it. Use AVCC instead.

Answered by Marko Buršič on January 2, 2022

Mic amplifier noise

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