Made an electret mic preamp circuit, but how to eliminate mains hum?

The circuit's purpose is to be able to listen to heart sounds and output it in a PC and the heart sound frequencies usually fall under 20 to 100hz. The problem is when i plug it in my laptop, i start to hear the hums. But when my laptop is just powered by the battery it completely removes it. Is there another way to eliminate the hum without having to run my laptop on my battery? 

-max-3 years ago

Mains hum is always a pain in the butt to remove, so try this so we know it is not the power supply causing it: take just the ground lead of the power supply for the laptop, and connect into the mains ground in the house, then plug something into the mic slot and record like you said you were tring to do. Does the ground connection cause the hum? Then it may be that your heart monitor is not properly grounded and the input to the mic is actually electrically connected to your body or something. You means you body is acting like a large antenna just picking up all sorts of junk from your surroundings, especially the mains power surrounding you!

I have actually measured myself on my oscilloscope, and I see a dirty 60Hz waveform peaking around ~10 volts! A piece of sheet metal (electrically floating) with some electronic stuff sitting on it is able to generate ~20 volts peak to peak!!!

If the 60Hz noise only occurs when the laptop is actually charging, then it is the power supply and you will need to add more suppression in-line. 60Hz suppression is not easy to fix but I do not think that is the problem anyway since it is rare that SMPS's have 60Hz ripple issues. (that usually only arises in heavily loaded linear power supplies.)

Show your circuit: One thing it probably needs is some decent suppression components.

iceng3 years ago

First shield your mike and its cord (( signal and supply ))

Then twist the wires to avoid magnetic induction.

You may need to add a notch-filter for 60 Hz to halt the mains and

a band pass ie a band lock out for 100 Hz and up.

Or you can build a really knockout decoupling rock steady power supply for your preamp.

Your body is an antenna for all the AC wiring in your home and when you bring an unshielded mike near you it picks up your body induced AC.

When on battery and not grounded anywhere, the laptop and body both are at the same potential and there is no differential signal to amplify.

Most HiFi amplifiers have enclosure shielding with extra DC filtering for the preamp inputs section.