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Mic Preamp Question

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HarryL

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Hi,

I have built a mic preamp circuit that does not like the small noise cancelling electret mics that I purchased from DigiKey----they have 680 ohms impedence.
The preamp works very well with some of the larger mics that I have but has very low volume with these mics.
I know enough to build various circuits but not enough to keep me out of trouble.

Here is the circuit that I used
https://freecircuitdiagram.com/2010/11/15/op-amp-microphone-pre-amp/

I added a 10K resistor after C1 before going to the mic with no luck.

Thanks in advance for any and all advice.

Harry
 
The 10K needs to be BEFORE C1 (the LHS of that diagram).

The circuit has a 50K input impedance, so is really intended for high impedance mikes - but electret inserts aren't really low impedance anyway.
 
Yes, an electret mic needs to be powered by a 10k resistor.
A noise-cancelling mic must be touching your lips or it will cancel your voice. It must point down your throat not to one side.
The gain of the mic preamp is very low at 21 times. If the 47k resistor is replaced with 220k then the gain will be 101 and if it is replaced with 470k then the gain will be 215 times.
 
Thanks for the quick response.

A 10K resistor from vcc and it works very well.

I plan to use this preamp in conjunction with a CMOY amp for my home built audio system for my motorcycle.
The CMOY amp does a great job of amplifying my Garmin and MP3 to my helmet speakers. I now want to be able to communicate with my wife (passenger) when we take a few longer trips this year as I am retiring.

Can this preamp be used for both mics or will I need to build another circuit? Also, Is it possible to power both the preamp and CMOY amp from the bikes 12v system or will I be getting too much noise and feedback?

I know that I could have bought a bike communication system, but I love to learn new things and it is so much better when you do it yourself.

Thanks again

Harry
 
Each mic needs its own preamp that feeds the amplifier for the other person. Then each amplifier needs a mixer circuit to mix your own voice, music and intercom.

Your circuits are very simple and need power supply filters so you won't hear loud alternator whine, turn signals clicking and maybe each spark plug spark.
The 100k resistor to pin 3 of this preamp opamp and the 10k resistor that powers the mic should come through a 1k resistor from the 12V supply and then a 47uf or 100uF filter capacitor to ground.
The 12V that powers this preamp circuit should have a 0.1uF high frequency capacitor plus a 100uF low frequency capacitor to ground. The same for the CMOY amplifiers.
 
I mixed the MP3 and Garmin by adding a 2.2K resister to the output of each before going into the CMOY amp without blowing either one up and had good sound quality.
I was going to use a 2.2K resistor and then add a 10k pot to each output when I added the preamps.
I had tried to power the CMOY with the 12v bike system with noisy results. I will try your suggestions to fix that.

Thanks

Harry
 
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