SIM800L GSM module humming noise

[mohamedesalem]

Hello everyone,

I am trying to play audio over the call by using sim800l

I am using this "DFPlayer mp3 module" to play audio and connected its AUDIO OUT to the MIC IN of "SIM800L GSM module" and i hear a humming noise.

I tried to put those capacitors as in this schamatic

as recommended in the datasheet to amplify the audio and it reduced the noise a little bit but it still there.

After a lot of searching a found that this noise may be because of a ground loop, but i don't have the reasons of the ground loop i use the same power source and i grounded the two modules with the same ground.

I also tried to use shielded and insulated cables between the two modules but also nothing happened, and the noise remains

I tried to make a low pass filter using resistors and capacitors like this schamatic , But this makes the sound distorted.

I also tried to decrease the sound level from the DFPlayer, and the sound actually decreases, but the noise remains.

I also tried to change the gain from the SIM800L's AT+ commands but nothing happends

After another search i found that maybe the audio from the DFPlayer is very high level and the SIM800L needs the signal from an electret microphone. Can that be the problem? and what can be the solution?

NOTE: The sound from the Dfplayer module is very clear when i connect it to a speaker AND also the sound is very clear on the other call when i connect the SIM800L with an electret microphone.

THANKS SO MUCH FOR YOUR TIME I REALLY APPRECIATE IT

[xfpd]

Electric or audio feedback (unwanted amplification) through the microphone?

[mohamedesalem]

kindly can you explain more what do you mean?

[xfpd]

You have a microphone which might cause an audio loop called feedback.

Audio feedback

Try isolating your microphone by surrounding it with cloth.

[mohamedesalem]

sorry, i think you miss understand my question.
i just was testing with the microphone and it was a clear audio by the way, but the main issue was the noise between the "DFPlayer mini" and sim800.

not the microphone

[xfpd]

Shorten the jumper wires.

Have only one ground potential. Ground loop occurs if two devices are grounded together, but both devices have another ground path (through a metal housing, grounded cable, et c.). Referring to your picture, try disconnecting a ground jumper wire from the device connected to the USB, and make the ground wires of the other device (or devices) connect to the USB ground. The goal is a single ground bus for all devices or a star-bus, but not a cascading bus.

[mohamedesalem]

thank you for your reply.

i tried to do as you mentioned and connect the gnd and vcc of the two modules in the same point on the out of the power source, and unfortunately nothing happend.

but there is a weird thing happend i connected two potentiometers "one on the right wire and one on the left wire " of the audio out as in this photo

and i endedd with two values for them "the audio is somwhat clear in this point" '73.7k ohm' and '21.5 k ohm'

when i tried to replace them with a fixed resistors with the same values and they didn't work.

i really don't know what is the problem

[Tordens]

1. The microphone voltage is much smaller than the module output voltage. You need to modify the hardware. You are saturating the microphone input.

2. Also, check the psu. Use a battery, not psu from mains and see if it any difference.

[mohamedesalem]

For future people, The problem was because the DFPlayer audio is very high and made for speakers, and the sim800l needs only an audio from an electret microphone so i put two potentiometers between the two lines of audio between the dfplayer and sim800l till the noise DISSAPEARED

Good luck