Changing Cap Value for Frequency Change

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I am building a listening device and I cant hear normal voices so I read that if I increase the value of the c11 and c12 caps that it will reduce the frequency range, c11 and c12 are 100pf (100, 101) . Can anybody help me with finding the right change to be able to hear human voices instead of all the background noises, Thank you
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Since my crystal ball is out for repair yet again, would you be kind enough to upload / link to a schematic of what you're dealing with? I can only guess that the parts mentioned are audio filter capacitors limiting frequency range, in which case I wouldn't be expecting too much in terms of miracles - but then again, we might be talking about RF instead.
Yes I can hear a phone conversation just fine but this kit I bought has things pretty simple . I am trying fine tune the frequency's so I can hear more that just traffic and trains . Occasionally I can hear the wind blowing but I am trying get the values right on some different capacitors to achieve 50hz to 5000hz
here is the schematics and yes I can hear a landline. This kit needs some help in the frequency dept. I can hear traffic and planes once in a while a bird chirping maybe some wind but no clear voices. I can hear a very very faint voice sometimes but cant make it out. the admin at the forum for this project posted "increasing the value of c11 + c12 will reduce the frequency range" so thought I would change the caps to something different and see.
"increasing the value of c11 + c12 will reduce the frequency range"
Yes it will. The -3 dB frequency is around 30 kHz (give or take, I did it in my head) with C11-C12 @ 100 pF. Increasing it to 1000 pF will put you around 3 kHz, which is probably too much roll off. Try some values in between to see if you get your desired effect.

There are simple circuits that will limit bandwidth for telephone use. Making C11-C12 larger might do the trick, if it is high frequency noise that is obscuring your hearing.

And not all microphones are created equal, either. I assume from the circuit that you're using an electret condenser microphone. There are different pickup patterns for microphones, and as far as I know cheap electret condenser microphones are always omnidirectional. A cardioid pattern microphone would cut out half the background noise right off the bat. And there are even shotgun microphones, which allow you to hear noises far away without background noise.
Thank you everybody for the help, So Fast Eddie do you think that maybe a mic change would be the first place to start? And can you give me an idea of a good unidirectional mic that would fit this board. There are currently 2 mics on board and if it is easy to go to 1 mic by itself I could change that over, I am not by any means electronic smart. Mostly I fly by the seat of my pants and get away with a lot. I am just having fun with a little project and appreciate everything from you folks. Now electric mics do not need power"Right". And that mic I ask you about would also be the same so I wouldn't have to mess with anything else. Although the admin from the Velleman forum noted in a forum question that the mic here doesn't need power but that they put those resistors in so if you went with a different mic they are already installed. This is getting to be pretty confusing, But keep it coming I need the education.
Board mounted mics aren't going to get you any kind of practical listening device. This kit you bought is cool, but it's for experimenters and hobbyists. Velleman is entry level stuff and does provide kits that let beginners learn a few things.

You should read up on microphones yourself. Cardiod mics are the most common type of mic you'll see in the field. When you see a live music performance or a live reporter on the street, you can be sure they're using cardioid mics. It's important to understand that they are unidirectional - if you hold them backwards, they won't pick up your voice but you sure will pick up background noise.

Shotgun mics are popular with DIYers. You can hear a conversation on the other end of a noisy room with them. In other words you can spy on people with them, but they have other uses too.

You may have to adjust your gain once you've found a mic. Show me some options you might consider and we'll go from there.
Your problem is not mainly "electronic" but *microphone placement*.

What is the intended use?

Sit at a bar table and eavesdrop a conversation 2 or 3 tables away?

It can be improved, knowing your *exact* use and circumstances, and some electronics filtering can be used afterwards, to improve results, but you need to pick the correct signal first.

Microphones are brutally honest, and if they pick up trains and traffic, it's because that's what is reaching them, period.

So at your table there is 80dB of train and traffic noisebackground and 60/65 dB of interesting conversation?" ... no matter how much gain you apply, the 15/20dB difference will remain and voices will still be buried in noise, only answer is to improve pickup, such as: shotgun/parabolic/very directional microphones, setting mics closer to source, using an RF/ultrasonic/infrared/laser link, etc.

Just for fun, rent and watch this excellent old movie: electronics has improved immensely; microphones not so much (if at all)
I forgot to tell you that I never did put the mics on the board I soldered and shrink tubed them to some shielded and they sound really crisp with no snap crackle pop. Since they are side by side I was reading that somebody said to split them and run a baffle between the 2. Sounded a little different but not what I am trying to get. I have an xtra kit and this time I will change a few things around and see what I can come up with. I have some jumpers and some other resistors and caps from a few electronic devices that I salvaged and I will see what I can come with. But I will be talking to you soon . thanks
I bought a audio-technica super cardioid shotgun mic and it has a dual mono output plug. I also bought a 3.5 4 conductor to 3.5 headset and 3.5 microphone adapter think I could solder it to the amp board to except the mic 3.5 jack. This is getting complicated and I am not really that smart to figure out how to wire this crap up. I think that if I get the right mic and position it properly that this op amp has enough juice to handle it. Do you have any suggestions about some mics because this amp has 2 stereo mics and I am going to have to wire something to the board. I thought that I could just hook an adapter up to the amp and plug in the shotgujn but it is looking like a little more trouble than I was thinking. I did get this other kit in the mail today that I bought from Apogee and it has 1 mic that seems tom be pretty darn sensitive . There is so many different configurations out there on how to build one that I think that once I have the right hardware it should be ok. Problem with buying these mics online is that I personally dont know what I am looking for. I went to a bunch od sites and read up on this and that and still walk away scratching my head.. Ok man Im going to bed .
Update on project

First of all I would like to thank everybody for their help and knowledge. I took this original kit (which I have 4 of) and modified the kit as follows:
1.Replaced R7 & R8 with jumpers
2.Disconnected R1 & R2 but wouldn't play sound because of mic so I reconnected them.
3.I bought another amp kit from Apogee (ML202) and plugged the 1st kit into the 2nd kit.
4.Changed all the microphone wires to 3.5 male to female jacks so I could quick disconnect the mics and try different ones to see affect.
5.Tried about 10 different mics and what a difference each mic is. some are incredibly sensitive and some are kinda just blahh. others are louder and some are very crisp. I did have a few tiny tiny mics from other sources like phones and different cameras. One of those mics is so small and so clean it is amazing. I bought a $75.00 shotgun(battery operated) with 2 different settings and has a 3.5 male jack for quick connect and disconnect, but didn't seem to be anything but very low and dull.
6.Changed the dish to a larger and more parabolic wok and that helped a lot.
7.I changed all wires to shielded and balanced.
8.Changed battery pack from AAA to a 9 VOLT.
I have a lot of power and the sound is very very clean and crisp. I can hear so many different things its incredible. The volume control just has to be on less than a quarter of a turn to reach an incredible loudness.I can hear voices better but it is very directional and only picks up things when it is pointed at the object.I was listening to crickets and when I took off the ear phones I couldn't hear them at all.I did hear a neighbor talking in their backyard 3 house away but couldn't make out a voice from next door. I could hear them but didn't seem clear or crisp for being 10 feet away. Although I wasn't pointing it right at them. I can sit in my living room and point the dish directly at a home stereo speaker and is much louder and clear than it used to be.
I think increasing c11 & c12 will help. start at 250pF and then 500pF then 750pF and see the results.I still need to reduce c2 & c4 to 0.047 to set LF to 300 Hz.I was told that the R1 & R2 are 10k but to reduce them to 15 to 20 ohms.
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Joined 2011
I was told that the R1 & R2 are 10k but to reduce them to 15 to 20 ohms.

That's too low in resistance, and the mic could be damaged. What mic are you using now?

If you are keeping the second board hooked up after the first board, only change the capacitors
in the first board. Keep the second board with the standard values.
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Last night I changed the values on c11 and c12 to 680pF because you said that 1000pF would be too much. It cut all the background noise to a minimum and I could hear somebody talking 3 houses away. I do have a 330pF and 500pF and a 750pF to try. The second amp is completely the way the original design was . The modified amp works pretty darn good just by itself and I haven't plug it in yet to the other amp. I have 6 or 7 different mics on a 3.5 female plug so I can try different ones. I am running the 2 stereo 2 wire mics that came with the first kit.(electret) they seem to be pretty clear and very responsive. I put them on a shielded cable and put them into a flashlight cartridge like a shotgun kind off. I will hook them to the dish and see what happens I will let you know tomorrow, thanks again
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