I'm trying to use the factory bluetooth microphone of my truck, problem is that when I use the factory microphone with my aftermarket deck the level is low and nobody car hear what I'm saying.

I've decide to build a mic preamp to boost the signal. I've use this https://www.sparkfun.com/products/retired/8872 , I've tested with my laptop soundcard and it's working fine, but as soon I plug it on the mic input of my headunit it get all distorted.

The mic input of the deck send a constant 3.3v, this is probably the cause of my problem, but have no idea how to deal with the issue.

Can this preamp be modified to deal with the voltage at the input or do you have another idea to increase the signal from the factory mic.

thanks

 

Do you think an isolation transformer could do the trick?

 

A capacitor will block the dc and pass the signal. Choosing a value would be the next step.
If yo can provide specific info on what you are doing others can help you better.
Feeding the output of a mic preamp into the input of a mic preamp is likely to result in a distorted signal. You would be over driving the second preamp. At the least you need a gain control on the output of the first preamp.

 

3.3V at the mic input of the deck is there to power an electret microphone element. Your preamp has an adequate output coupling capacitor with the correct polarity to block this.

The problem is that your preamp has 35 dB of gain where you need only about 6 dB. On the preamp, change R4 (220K) to 10K and see how that works out.

ak

 

Perfect ill give that a try and report back

All I'm trying to do is to use the factory mic in the truck to play nice with my aftermarket radio. Why not simply use the deck mic? I do sound/installation competition and using the factory mic will give me more points and will not look out of place.

Right now I'm powering the circuit with a 9v batt, but will be using a 7809 if everything work as I want.