big ear, fm transmitter.

[audioguru2]

Hi Steven,
It's good that you got your LM386 amp to work. The only thing I recommend for headphones is to add a 1K resistor across the output (where the speaker is connected now) to charge its output cap so if you ever plug the headphones in with the power already turned on, you don't hear a big bang.

If you don't mind listening to the hiss from the LM386 with the 10uF cap added to boost its gain, then you don't need a good mic preamp.

 

[steven2]

thanks audio guru now i just need a headphone amplifier to amplify the output sound a good battery operated one

 

[audioguru2]

But the LM386 IC will be a fairly good battery-powered headphones amp, especially if you use a quiet preamp with it if you don't like its hiss.

There are some headphones amp projects on the web called Headbanger and Super-Headbanger. They are battery-powered and use LM386 ICs. They are rated pretty good. They use negative-feedback around the output cap to extend its bass response and reduce its bass distortion. You don't need the extra feedback for your parabolic mic, unless you want to clearly hear the guy 3 streets away play his drums. ;D

 

[steven2]

audio guru ive seen that head banger site but when i look at circuits the first thing that gets me is some of the parts are hard to get and ive never herd of some

 

[ante1]

Hi Guys,

How come this thread shows up twice the width of my screen??

 

[audioguru2]

Your screen shrinks from waiting so long for this site to load. ;D
After I hit "reply", I ate lunch, watched some TV shows and even built half a project. I had a look and it still didn't finish loading. So I washed my car in the freezing cold outside. With my glasses all fogged-up, I typed this. Now I gotta wait again for it to post. Oh, man!

Maybe the site is overloaded from Steven posting his "big ear" in so many topics! Maybe John's cardboard "bull-horn" overloaded it!
[move] ;D ;D ;D ;D ;D ;D ;D ;D ;D ;D ;D ;D ;D ;D ;D ;D ;D ;D ;D ;D ;D ;D ;D ;D [/move]

 

[Dazza]

Hi

It's just downright annoying, the pages were loading that fast yesterday, I nearly fell off my chair in disbelief .

Well it was good while it lasted :.

That's my whinge for today, I feel much better now ;D ;D ;D.

 

[steven2]

audio guru and dazza ive had the same problems with downloading other topics to like mine that take time to dwonload at first i thought it was the speed of my cpu at processing the task of opening or downloading that was part of the problem, dazza your cool dude, good information takes time to download even in picture form , but sometimes its a niusence but at leat you dont get the page cannot be found exscuse, when you wait to see my postings there is something odd going on before i got here it was takeing to long for the page to open or download and on my 3rd try it did it faster , so what was it , im baffeled ,audio guru can i increase sensitivity of the electret mic more than that with the 10k resister you recamended what value resister can i use to do it

 

[ante1]

Hi Guys,

I had a few beers (between each page) while waiting for it to load, but after 5 or 6 pages (can

 

[steven2]

ante was that a few beers or few cartons of beer , i took it into concideration, that some of the bug transmitters use a 47k load resister for the electret mic so i tried it and got more sensitivity even though i want to get as much as i can so much that i could hear a flea circus on a passing dog. anyhow the potentiometer i used for the actual electret mic preamplifier is strange as when i turn it fully to get the highest sound and when i press on it the sensitivity increases, even though im useing a small 250,000 candle power spotlight dish its better to use, a speaker horn cover as the sound gathering with it was good and its pritty good to

 

[Dazza]

No more whingeing from me :-\,

after my last post I was unable to get any further than the homepage, all I got was (page not found).

I hope mixos fixes the problems soon, before ante becomes an alcoholic .


[move] ;D ;DAnother page too load, and another beer to drink ;D ;D[/move]


Is This A Bad Thing 8)

 

[ante1]

Steven,

I have changed the routine now, I use Pepsi instead! How big is that spotlight dish? Have you got a picture of it?

 

[audioguru2]

Hi Steven,
An electret mic draws a fairly constant current through the resistor that feeds it power. Different part numbers of mics draw different amounts of current but most are 0.5mA. An electret mic needs at least 2V across it to work properly. You could measure the voltage of yours.

Ohm's Law dictates the maximum value of the resistor that feeds power to an electret mic. You start with your 9V battery, that is actually only 6V at the end of its life. From the 6V you subtract the desired minimum mic voltage of 2V. Then Ohm's Law says the resistor is the voltage difference of 4V divided by the current and the answer is 8K. If you replace the battery when its voltage drops to 7V, the resistor can be 10K. If your mic draws less current then the resistor can be higher for more sensitivity, but causes other problems:
1) Frequently mics are used with fairly long cables. Cables have capacitance. The RC network of the mic's resistor and the cable's capacitance rolls-off high audio frequencies. The lower the value of the resistor, the less effect cable capacitance will have at high audio frequencies. But yours is probably OK with your short cable.
2) Amplifiers such as the LM386 have input noise current. The noise is higher when the source impedance (your volume control) is a high value. But the volume control loads-down the output of the electret mic and its value must at least be equal to the mic's resistor.
3) Resistors have thermal noise. The higher the value, the higher the noise.

So you must decide between having a very sensitive mic with a lot of noise because its resistor and volume control are too high, or lower sensitivity with less noise. I think that 10K for the resistor and 10K to 25K for the volume control are a good compromise.

 

[steven2]

thankyou ante and audio guru at the moment i have a 47k load resister and 10k pot still in it and if i press down on the pot the sensitivity jumps a little so i want to increase it more let me geuss if i change the pot to 25k and decrease the resister to 10k will that do it, the dish is only small

 

[steven2]

View attachment 36356

 

[Dazza]

Steven, have you considered The positioning of the mic in the dish, it can make an enormous difference 8).

To find the focal point, position the dish towards the sun and use a thermometer to find the hottest spot, which of course is the focal point, be careful because you can easily destroy your thermometer , if you don't have a thermometer just use your finger, when you say ouch you found it ;D. Than place your mic just above this point facing in towards the dish, this you probably already know .

 

[Dazza]

I nearly forgot to mention, you can increase the sensitivity by placing a small dish maybe from a torch, or maybe a (flashlight) hard to come by in Australia though : ;D, and place it just above the mic facing inwards.

You should view the construction of your dish, the same as if you were constructing a high-performance antenna. After all this is basically what it is 8).

 

[steven2]

gazza thats a good piont there, i never thought of that useing the same sunlight phocal piont to get great sound phocus into the mic but then i would have to find a good way to suspend the mic in that pposition without useing anything that could get in the way of incomeing sound waves, or block it to much,

 

[Dazza]

steven or is it gteven ;D ;D,

Just use light gauge wire, a coat hanger . Bend one end to an L-shape then secure it to the dish wall with sticky tape, then bend it across to the other side and do the same, I would say you would need three legs. I'm sure you get the idea. Once you're happy with the position of the mic, you can remove the tape and permanently glue it.

 

[audioguru2]

Hi Steven,
I've given up with booze and beer. It was fun, but now I like to always see and think clearly.

The "toy" big ears use a dish that is 60cm in diameter. Stadium horn tweeters use a horn (dish) even bigger. Maybe your dish is too small to properly reflect the long wavelengths of sound.

In a government scientific demo place they have a "whispering galley". It has a huge 4 or 5m dish at one end and another that is hundreds of metres away. Whispering into one dish can be heard clearly at the other dish, like majic! They use a video link to show each end that are so far apart you can't see.

It's freezing up here now (winter in Canada). Don't sweat too much down under!