Get Rid Of Unwanted Frequency from Audio InPut

Thread Starter

Adfly

Joined Dec 31, 2019
27
I want to know that can I get rid of all the unwanted frequencies like humm buzz 50hz ground loop rfi etc using a balanced differential audio pre amplifier
i am getting very high unwanted frequencies into my electret microphone even after it is connected with a shielded wire.
 

AnalogKid

Joined Aug 1, 2013
8,251
Please give us some information.

Microphone make/part number
mic amp circuit schematic
photo of the circuit board in its enclosure
anything.

ak
 

MrChips

Joined Oct 2, 2009
19,915
First question is: What is your power source?

Use a 9V battery and test for hum.

Edit: OK, I see that there is no hum when laptop is running off battery? So hum is coming for AC mains pickup.
 

Thread Starter

Adfly

Joined Dec 31, 2019
27
First question is: What is your power source?

Use a 9V battery and test for hum.

Edit: OK, I see that there is no hum when laptop is running off battery? So hum is coming for AC mains pickup.
my source is 9v battery Sir
yes sir you got it no rfi or nothing when laptop is not connected to the charger as soon as I connect the charger to the laptop it starts picking up the rfi
it is my requestto you kindly give me the solution for this because its been 6-7 seven month i am not beign able to get rid off this

Solution: Don't use the charger while recording.
i Cannot do this sir but my laptop battery does not give good backup of 3 to 4 hr only it gives 1hr
even if i follow your solution then what if i want to use it in desktop computer there i cannot remove the power supply
 

Thread Starter

Adfly

Joined Dec 31, 2019
27
Both inputs of the opamp are receiving unfiltered noise from the power supply.
Then the opamp amplifies the noise.
thank you very much sir can you pls give me the modified schematic where the corrections required so that I can rebuild it
i mean to so what to do with the areas where you have indicated with red inks

Where are you getting 5V supply to power the mic and preamp?
Use an RC low pass filter on the 5V supply line.
thank you very much for your solution but i am not good with these filters so can you add the filter to my schematic
 
Since you used a 9V battery that has a 22uF capacitor in parallel then the power supply missing filtering does not cause the noise.
The capacitor must be at the circuit, not at the battery.

Is the preamp built on a Mickey Mouse solderless breadboard where all the messy wires are antennas that pickup interference?
Are the shielded audio cables the extremely cheap Chinese ones from The Dollar Store or the same from Radio Shack?
 
The opamp will not work if you connect the +2.5V bias voltage at its pin 3 to the +5V positive supply.
Or the electret mic might not work if you connect its 1k biasing resistor to the +2.5V used to bias the opamp input.
 

Thread Starter

Adfly

Joined Dec 31, 2019
27
The opamp will not work if you connect the +2.5V bias voltage at its pin 3 to the +5V positive supply.
Or the electret mic might not work if you connect its 1k biasing resistor to the +2.5V used to bias the opamp input.
what should i do there sir can give me the correct schematic
i removed the 0.94uf but no improvement
 
If you solder 1 to 2 then the opamp tries to amplify 22 times the +5V or +2.5V feeding R2 which causes it to stop amplifying.
The circuit is wrong anyway. The low value for R1 kills the output level from the mic and the opamp gain is too low for a mic.

Here is how I amplify an electret mic:
 

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Thread Starter

Adfly

Joined Dec 31, 2019
27
Since you used a 9V battery that has a 22uF capacitor in parallel then the power supply missing filtering does not cause the noise.
The capacitor must be at the circuit, not at the battery.

Is the preamp built on a Mickey Mouse solderless breadboard where all the messy wires are antennas that pickup interference?
Are the shielded audio cables the extremely cheap Chinese ones from The Dollar Store or the same from Radio Shack?
22uf capacitor is on the circuit sir
I hv built the circuit on a pcb sir
 

Thread Starter

Adfly

Joined Dec 31, 2019
27
If you solder 1 to 2 then the opamp tries to amplify 22 times the +5V or +2.5V feeding R2 which causes it to stop amplifying.
The circuit is wrong anyway. The low value for R1 kills the output level from the mic and the opamp gain is too low for a mic.

Here is how I amplify an electret mic:
I just want to confirm sir should i build this circuit tomorrow will pickup any interference even after using shielded wires as i explained my problem
I think with this circuit i can get rid off the rfi? Shield wires are not cheap sir it is only a meter length with good quality
 
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