0
\$\begingroup\$

I wanna make a FM microphone for my guitar but the problem is that the electret mic itself output a lot of background noise even before amplifying its signal.

So obviously I need a preamp circuit but I don't need high gain or high volume. A low noise output with 1 volt peak to peak signal is ideal. But schematics I found online are relatively complicated and I don't know what they exactly do. So I would be grateful if anyone help with low noise preamp...

For the record this is FM transmitter circuit:

enter image description here

And microphone is just those ordinary mobile mics:

enter image description here

EDIT: At the moment I am just testing the microphone via PC sound card with TRS 3.5mm jack.

enter image description here

\$\endgroup\$
8
  • 1
    \$\begingroup\$ What makes you think the microphone is too noisy? Have you looked at the microphone signal? That transmitter will be pretty noisy itself. The transmit frequency of that circuit is not really stable. It will wander all over the place. Depending on how it is built, it may also pick up interference from other equipment and broadcast it along with your microphone signal. \$\endgroup\$
    – JRE
    Oct 27, 2018 at 7:52
  • 1
    \$\begingroup\$ If the noise is coming from the microphone, then adding an amplifier would make the problem worse. \$\endgroup\$
    – JRE
    Oct 27, 2018 at 7:54
  • \$\begingroup\$ The 22k resistor powering the microphone is rather high. Try using something around 2k. Like, 2.2k or whatever common value resistor you have in that range. Too much resistance in that spot could make the microphone a (little) bit noisier. Try it. It doesn't cost anything to swap out a resistor. But, I think the real problem is the circuit. \$\endgroup\$
    – JRE
    Oct 27, 2018 at 7:57
  • \$\begingroup\$ i will try changing that resistor but at the moment i'm just testing the microphone output directly via PC soundcard. it has background noise even before FM broadcast ! \$\endgroup\$
    – Mohammad mohammadi
    Oct 27, 2018 at 8:06
  • \$\begingroup\$ i don't think it is microphone self noise. i have seen these kind of mics that output decent snr signal. could it be unstable audio jack input voltage? \$\endgroup\$
    – Mohammad mohammadi
    Oct 27, 2018 at 8:09

1 Answer 1

Reset to default
1
\$\begingroup\$

PC power rails are notoriously noisy. Here's the rest of the image that you used in your question (from here):

sound card mic input

Note that any noise on the power rail is coupled directly through the bias resistor to a high-gain audio input. I would recommend adding additional filtering:

schematic

simulate this circuit – Schematic created using CircuitLab

This creates an additional low-pass filter for the microphone bias voltage with a cutoff frequency of about 7 Hz.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Not the answer you're looking for? Browse other questions tagged or ask your own question.