I have one of those average cheapo computer microphones that connects into the sound card via pink 3.5mm jack.

I wanted to find out how much voltage can I expect from it, to get a ballpark value I could use for my opamp noise detection circuit.

The microphone works fine when connected to the PC.

And so I hooked the microphone to a scope, expecting to see my voice visualized on the screen when I talk, but all I am getting is a flat line.

I looked at how other people do it, and they merely just hooked the scope directly to the jack of the microphone, just like me, and it is working just fine for them.enter image description here

So, what am I doing wrong here?

Thank you


An electret microphone requires a DC bias current sourced from typically 5 volts through a 4k7 resistor to the microphone. This is a normal feature within sound cards for the microphone input and you may not be aware of that: -

enter image description here

If your microphone jack plug has three connections on it one may incorporate an internal resistor already.

  • \$\begingroup\$ I was indeed not aware of that. I know that microphones which need external power exist, but I thought that the sound card has no power on the microphone slot, so the microphone must be dynamic. I will try to add a 5V battery in series with the resistor to the mic and try the scope again. \$\endgroup\$ – Askerman Aug 19 '18 at 17:03
  • \$\begingroup\$ Why don't you make a break-out cable so you can connect your scope to the microphone whilst it is plugged into the PC. In that way you can measure the DC current and open circuit DC voltage from the sound card too. \$\endgroup\$ – Andy aka Aug 19 '18 at 17:05
  • \$\begingroup\$ Hooked up 5V through a 5K resistor and now it works like a charm. Thank you. I assume i should add the cap there as well to remove the 5V DC from the output? \$\endgroup\$ – Askerman Aug 19 '18 at 17:11
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    \$\begingroup\$ If all your are doing is feeding a scope then just AC couple the scope by pressing the appropriate button on the scope. It does that same as putting an external capacitor in line. \$\endgroup\$ – Andy aka Aug 19 '18 at 17:15

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