Help with settings - Page 3 - Home Theater Forum and Systems - HomeTheaterShack.com

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post #21 of 23 Old 03-16-13, 01:16 PM
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Location: Hillsboro, Oregon
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Re: Help with settings

I had two problems, one is the Mic I am using only goes to 50 Hz and the second were some switches on my mixer that cut 16 db per octave below 75Hz. I changed the switch settings, and am building an electret mic that will go from 10Hz tp 20k+ Hz. The element is only $1.00, from Digikey, and two resistors, one cap, and a 9 V battery held in place by some Velcro. I used a small $2.00 box from Radio Shack, then cut one end from a shielded cable. By my choice of resistors and cap I can change the sensitivity and output impedance. I made sure that everything was shielded. Everything for $5 to $10.00
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post #22 of 23 Old 03-17-13, 06:29 AM
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Re: Help with settings

Interesting idea. I was looking at the idea awhile back, stopped when it came to the thought of heating up a soldering iron - I am getting lazier by the day.

A minor detail - for real accuracy, you will have to calibrate your homemade mic against a known reference (known all the way down to whatever lowest freq you want to be able to measure). The difference between "actual" and "typical" on those mic capsules can be quite a bit at the frequency extremes. Also, the values of series capacitors will limit that low frequency response.
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post #23 of 23 Old 03-17-13, 01:52 PM
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Re: Help with settings

Very true. In my case I do not care if it is 83 db or 73 db. I am just looking for it to be even across the spectrum. The Element is across both the load resistor in series with The battery. 9 volt is better than 1.5 volt. Between the load resistor and the element is the output cap to the cable. the other side of the cable is attached to the element and the other side of the battery.
The output impedance is set by the load resistor and should be the same as the input impedance of what you are going to plug it into.
The low cut-of frequency cutoff Fc is determined by the value of the cap using C =1/{(2) *(pi)*( Z)*(Fc)}, where Z is once again the input impedance. Shield everything on a two wire plus shield cable. and use a TRS (Tip, Ring, Sleeve) plug (aka stereo plug), tip to output cap, ring to battery element connection, and sleeve to shield in cable and shield in housing. A good shielding can be made by using aluminum foil. strip an inch or two of a section of stranded wire. tape it to the foil with shipping tape cover all the foil with tape. attach the other end of the stranded wire to the cable shield, and line the inside of the parts container on all sides leaving 1/8-1/16" around openings for transducers switches etc.
I added a switch, LED and LED biasing resistor to turn off the battery and the LED reminds me that it is on.
I used an Fc of 10Hz and the results were great
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