# LM358 Mic Amp Component Selection

Discussion in 'General Electronics Chat' started by PointyUK, Jun 1, 2014.

1. ### PointyUK Thread Starter New Member

Sep 22, 2013
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I have returned to an old project and just need a bit of advice about component selection. This was the original thread but I have done away with the 555 and am feeding the LM567 ouput to an ATTiny. I have the code written and working great. I have the circuits designed and built for my device and it works as expected, so now I am just trying to understand what each part does, and how to make it as good as possible.

I am much better at programming and digital electronics than the analog side, so any help is greatly appreciated.

Here's the LM358 part of the circuit...

I found this guide here, which helped explain it a little.

So first question is about the mic. I am using this one, which says that RL should be 300R to 5K. In my circuit I have been using 10K and it still works, why?

Next up is the caps, that guide says their values are calculated based on the frequency range of input signals. How do I calculate them? I am wanting to listen for tones between 1Khz and 3Khz.(You can't see my C2 in the picture but I am using a 10UF)

Lastly his circuit doesn't have a resistor between C1 and the negative input of the LM358, while most of the other circuits I found do. What does it do and does it's changing its value do to the circuit?

Thanks for taking the time to read, and any help is much appreciated.

Regards,

Les

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2. ### THE_RB AAC Fanatic!

Feb 11, 2008
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Because it's not critical. Most of those electret mics work fine with 10k. If it makes you feel better, swap the 10k resistor for a 4k7 (4.7k) one.

I'm assuming C2 is the output pass capacitor, like in the earlier thread here;

The value of pass capacitors is not that critical for audio frequencies. To put it in simple terms; if the cap is too small, then lower (bass) frequencies will have trouble getting through the cap. But your choice of 10uF is fine for audio frequencies and will work great for your range of 1kHz to 3kHz.

That resistor is part of the LM358 circuit that controls the gain (the amount of voltage amplification of the signal).

I googled for a simple example diagram to show you, and found this one;

The gain is a function of R2/R1, so if R1 is missing (ie; R1 = zero ohms) the gain is very high.

it is only a few paragraphs and explains it quite well.

3. ### PointyUK Thread Starter New Member

Sep 22, 2013
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Thanks for the info and link. I am trying to get as much gain as possible so the tones are picked up at a reasonable distance. That's why I changed the resistor values from the original circuit, although I didn't realize that R1 was part of the gain, I thought it was all done by the VR (R2 in your pic)

Regards,

Les

4. ### THE_RB AAC Fanatic!

Feb 11, 2008
5,435
1,305
THe LM358 you are already using has two amps inside. You can chain the second (currently unused) amp as another "gain stage" to give more gain.

Just be aware that for very small sound levels the electret mic will probably output more noise than the actual tone signal.

There will be a limit to how small the tone sound can be, and increasing the gain may not benefit you that much.