Hello everyone,
I'm developing a device that requires two speakers and a microphone wired to a TRRS audio cable. The speakers are directly soldered to the cable and work just fine, however, I am having trouble getting the microphone to work without noise. Below is the circuit diagram for the microphone, it is a Knowles FG-23652-D65. Here is the datasheet. Due to the lack of documentation for recommended circuits and such I was unable to figure out what is wrong and how to fix it. Any help is appreciated!

Some background information:
The TRRS audio jack is connected to a Samsung usb-c to 3.5mm audio adapter. A DC bias voltage for the microphone is provide through the TRRS mic port, which is the same port for the microphone output signal. R1 and C3 make a low pass filter to power the microphone while C2 is a DC block for the audio signal from the microphone output.

 

The mic is very expensive. The mic datasheet says it should have 1.3VDC across it. Change the value of R1 until it gets 1.3V.
R1 and C3 are not a lowpass filter, they cut low frequency interference.

You said "noise" but did not describe it. Is it low frequency hum, high frequency hiss or squealing?

 

Hello, thanks for the reply Here is the FFT of the "noise", it is obtained when recording silence.
For what I am doing I need the mic signal to be very clean, you can observe these periodic peaks in the signal across the frequency spectrum.

And yes, I've made sure that the voltage across it is correct, the USB-C adapter actually adjusts itself by detecting the impedance of the mic.

Note: I have not calibrated the mic so ignore the fact that the noise is below 0

 

Are you sure the mic is the source of the noise?

 

That is interference, not random noise. A mic dont doo dat.

 

Hmmm...could it have anything to do with the electrolytic capacitor (it's about 1/2" diameter) or mechanical vibration (the speakers are right next to the microphone). Essentially, all of these components are housed together in a compact device. Would this potentially be the issue?

Edit: it probably wouldn't be mechanical vibration because the noise is still there when the speakers aren't playing

 

For the capacitor diameter I meant 0.5 cm not inch!

 

Is the noise present when the mic is not connected?

 

That's it Yaakov! I don't have an oscilloscope but it is in fact due to electrical noise in my phone. I noticed that if I turn my screen off during recording it goes away! Strangely it also seems to go away when I touch the ground wire with my fingers.

 

It looked like digital noise. I suspected it was from the phone.

Glad you tracked it down but now you have a different problem!

 

Yes I do! Would changing the circuit design solve the issue? My circuit knowledge is very limited, but I'm thinking of using an op-amp to boost the signal from the microphone output.

 

What are you doing with the microphone signal?

 

40 to 50 years ago transistors were used as a mic preamp. Since then a low noise audio opamp was used instead.

 

The reason I need very low noise is because I'm trying to measure a specific frequency that is around the level of the current noise floor when you take an FFT. I want to make the noise floor as low as possible which is why I am thinking about amplifying the microphone output signal.

 

The reason I need very low noise is because I'm trying to measure a specific frequency that is around the level of the current noise floor when you take an FFT. I want to make the noise floor as low as possible which is why I am thinking about amplifying the microphone output signal.

You may need to filter the input since you have a good chance of overloading the phone's input if you amplify the microphone's output. A bandpass filter would allow you to avoid that.

 

I will try that, thank you for your help!!!