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Building an ULTRA sensitive PC MIC

Discussion in 'Electronic Projects Design/Ideas/Reviews' started by Peter_wadley, Apr 10, 2007.

  1. Peter_wadley

    Peter_wadley New Member

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    By the way,

    The manager thought the circuit was great :)

    it was also my lucky day...

    It turns out the store was getting rid of a large portion of their electronic components..

    Long story short: I struck an under the table deal with the manager and paid $20 for over $200 worth of parts..

    -100's of capacitors
    - chokes, coils, and inductors
    - 20 buzzers and chimes
    - LEDs
    - switches
    - lamps
    - bayonet holders
    - fans
    - stepper motors

    It was comparable to christmas.. sorting through a huge box of brand new parts... picking whatever I felt like...

    I have a question about 2 lights I grabbed..

    They are XEXON STROBE bulbs and have 3 leads.. I have no clue how to hook them up!

    Later
    Peter Wadley
     
  2. Peter_wadley

    Peter_wadley New Member

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    Oh yeah forgot to mention...

    He gave me a 12v 200ma wall wart and said he didnt want to be switching 9v batteries...

    So I knew right away I would need to deal with the buzzing that comes with these pieces of S***

    When I first plugged it in, the circuit was unusable because of the noise.. so I ran it through a 7809ct 9v regulator.. this cleaned it up quite a bit but it still was humming..

    I then added a .15uf bypass capacitor and it now sounds completely clean.

    Is this the proper method of dealing with the hum?

    Did I choose the proper size capacitor.. is there a formula (seems like there is formula for everthing!)..

    Thanks again
     
  3. audioguru

    audioguru Well-Known Member Most Helpful Member

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    Look at Xenon Flash Circuit in Google. There are many links to circuits and articles.
     
  4. dave

    Dave New Member

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  5. audioguru

    audioguru Well-Known Member Most Helpful Member

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    The hum from the cheap power supply was connected directly to the input of the opamp then it was amplified 101 times.
    Reduce the hum with a resistor-capacitor filter before it reaches the opamp's input like this:
     

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  6. Peter_wadley

    Peter_wadley New Member

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    Is it the standard practice to have a small cap (.1uf) and a large cap (100uf) for bypass filtering loud supplies?

    I seem to always see .1uf and 100uf..

    Does the .1uf rectify small changes in voltage whereas the 100uf rectify the larger change/waves of voltage change?

    hope my that makes sense :|

    Edit: Does the .1uf cap that is on the opamp go to ground on the other side, thanks!
     
  7. audioguru

    audioguru Well-Known Member Most Helpful Member

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    100uF is a good filter for low frequencies. It is electrolytic and is made by winding an insulated foil around and around that adds inductance which makes it a poor filter of high frequencies.
    A 0.1uF ceramic disc capacitor has low inductance so it is a good filter for high frequencies. Many voltage regulator ICs have a 0.22uF ceramic disc capacitor in parallel with a big electrolytic capacitor at their input.
    A 1000pF ceramic disc capacitor is a good filter for very high radio frequencies. Many radio circuits have three capacitors in parallel across their suppliy with the three different values.
     
  8. Peter_wadley

    Peter_wadley New Member

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  9. TheVictim

    TheVictim Member

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    One of the smaller modules I have for my prototyping system consists of a 100uF, .1uF, and .001uF cap in parallel mounted in a paint pot. I use it whenever I'm testing any kind of audio amplifying circuit.
     
  10. Peter_wadley

    Peter_wadley New Member

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  11. ozarkshermit

    ozarkshermit New Member

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    Hi all:

    I'm responding to this rather old thread, since I need a good electret AGC preamp. After much checking, I think I'll go with the circuit developed by Audioguru in post # 15.

    I have developed a small circuit board (with the modifications shown), using the TL071. I'm ready to etch the board, but don't want to make something no better than what I presently have. I am interfacing the mic to a speech recognition chip, (HM2007), and have been using a small pre amp I made using a LM358, which works OK, but does not have AGC. I'm not looking for "super sensitivity", just something that will allow me to speak with slightly different voice levels. As I said, the circuit I am using works OK, but it's hard to speak in a consistent volume, which can sometimes result in recognition errors. I plan on using a POT for R3 (100K fixed resistor in the circuit as it is shown), to adjust and "tune" the output to the HM2007. The Mic was also picking up a lot of electrical interference before I added a shielded cable, and that seemed to cure most of those problems. I have to add a 9 volt regulator to the circuit, since I do not have that voltage available. I will heavily filter and de-couple the regulator part, so hopefully that will not present problems.

    Just looking for any other advice from the experts. Digital I can handle, but I'm not the best at analog.

    Ken
     
  12. audioguru

    audioguru Well-Known Member Most Helpful Member

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    I selected a TL071 opamp because it is low noise, has very low distortion and has a bandwidth that is high enough for hi-fi audio.
    An LM358 is designed for low power so it has a lot of noise (hisssss), a lot of crossover distortion and a bandwidth narrow like a telephone.
    An opamp does not need a regulated power supply. Filter its reference voltage instead with a resistor and capacitor.
     
  13. Torben

    Torben Well-Known Member

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    Hi Ken,

    I thought you needed a preamp with AGC? This circuit has no AGC--it's just an electret preamp. It is a nice sounding and very sensitive little unit but yeah--no AGC at all. The circuit you're talking about was my first try at an electret preamp which I posted a few years back and Audioguru corrected (see Peter's link to the original thread). I just never got around to posting the corrected circuit after Audioguru told me which bits I'd stuffed up. :)


    Regards,

    Torben
     
    Last edited: Feb 8, 2010
  14. ozarkshermit

    ozarkshermit New Member

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    Torben:

    Thanks - - - after I looked more closely at the circuit I realized it was alost identical to the one I used with the LM358 and yes, I definitely want something with AGC.

    I've been surfing till my eyes age glazing, and have found a few circuits with AGC, (not sure how good they are) but now am considering a MAXIM 9814 - it looks like it would work. Only problem is the surface mount package, but then it's about time I started trying my hand on making PC boards for them.
    After I digest some of this for a while I'll decide what to do hopefully.

    Ken
     
  15. ozarkshermit

    ozarkshermit New Member

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    On second thought, after looking more closely at the dimensions of the MAX chip, there is no way I could use it - I don't have a microscope !

    Ken
     
  16. audioguru

    audioguru Well-Known Member Most Helpful Member

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    Look in Google for Audio Compressor Circuit. There used to be NE570, NE571 and NE572 ICs that worked well but they are obsolete. Most discrete circuits use a Jfet to attenuate signals that are too loud.
     
  17. ozarkshermit

    ozarkshermit New Member

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    Audioguru:

    I managed to locate some NE571's on eBay, might order some - but am leaning more to using the circuit using a TL072 with the FET and associated components. The only problem (for me) would be the split 15 V supply. Since this is being added to an existing circuit, I do not have the liberty of split supplies. Is it possible to use that design with a single supply?

    Ken
     
  18. Torben

    Torben Well-Known Member

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    Hi Ken,

    When you say ". . .the circuit using a TL072 with the FET and associated components", what circuit do you mean?

    You can indeed use a TL072 on a single supply--that's what the circuit from post #15 does, for instance (well, with a TL071 which I had in there but but that's just half a TL072*). In fact there's a sticky on this very topic which includes Audioguru's famous diagram explaining how it's done: http://www.electro-tech-online.com/threads/basic-opamp-circuits.35258/

    Do you already have a schematic for the AGC portion?

    * -- Truth be told I wound up using a 5532 which is why there was a compensation cap on the op-amp--Audioguru crossed that cap off the diagram though since a TL071/81 doesn't need it.


    Cheers,

    Torben
     
  19. audioguru

    audioguru Well-Known Member Most Helpful Member

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    Here is a hard limiter circuit that uses a single supply voltage. It might not work if the Jfet is at its spec limits. I would use a Jfet with tighter limits like a 2N5485. This circuit is not optimised for low distortion.
     

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    Last edited: Feb 9, 2010
  20. ozarkshermit

    ozarkshermit New Member

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    Torben:

    Here is the circuit I was referring to

    Fast Audio Peak Limiter

    It was highly recommended by Audioguru

    Ken
     
  21. anjana.larka

    anjana.larka New Member

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    My circuit is not working : (

    dear audio guru,

    i have made this circuit as per schematics with TL071 and ur modifications, completed last night. Circuit seems working ok ass pre amp but it is not sensitive as i have to eat mic to get sound from my amplifier.

    Circuit images are attached here with if you plz shed some light on it...

    Basically i want to use it with my computer so that it can pic my voice commands from some distance but it is not working in that way : (

    can i use simple a cell phone mic or a hand free mic in palce of this whole circuit.. i am thinking in this way ... but this circuit sould work what you say :)

    thanks in advnace...
     

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