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Mic_amp_mod_needed_if posible

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zolz

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Hello am new here and am also new to electronics.

I wanted to create a sort of spy mic but a don't have the knowledge or many components to keep up with the diagrams out there, I came across this from my learning lab and maybe it is possible to change a component to be suitable for normal nature like sounds. Currently I can only listen to high pitch sounds lol from the angry fire ant. I would like to use a headphone instead of the speaker and a real electret mic not a earphone.

Here is the circuit diagram: (Electronics Learning Lab (User's Guide - Workbook 1) p.74
**broken link removed**
Mirror1:
**broken link removed**

Image hosted at: https://imageshack.us/
 

It is against the law and it is immoral to spy.
The circuit you found is horrible and is very old. Where would you find a ceramic microphone today???.

Use an electret microphone and a preamp for it then the preamp feeds an earphones amp.
 

Hi ...
Why complicated simple staff...
Try make this:
 

    zolz

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Thanks for that diagram I'll see if I can completed. It is more for me to learn.

I just want to spy mother nature.

_____________________________________________________________________
Orders that kill 1000s are transfer over the telephone wire do you blame Alexander Graham Bell?
 

I guess, no one can explain to me, what's the 47 ohm/10uF good for? (In the circuit as shown, not in a possible different one).
 

The 47 ohm resistor is an series with the emitter of the transistor to provide DC negative feedback so that the large range of current gain and large range of base-emitter voltage of each transistor have a reduced effect on the operating point.

The 10uF capacitor bypasses the emitter resistor so that the AC gain is as high as without the emitter resistor.

You don't need the resistor and capacitor if you could use a transistor with a narrow range of current gain and base-emitter voltage but they are not available unless you test and sort thousands of transistors yourself.
 

    zolz

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The said negative feedback would be in effect only with a base voltage divider, but not in the present circuit. That's why I asked regarding the circuit as shown. The circuit author apparently already heard about bias point stabilisation by DC feedback, but didn't understand it completely.
 

FvM said:
The said negative feedback would be in effect only with a base voltage divider, but not in the present circuit. That's why I asked regarding the circuit as shown. The circuit author apparently already heard about bias point stabilisation by DC feedback, but didn't understand it completely.
You are correct. Increasing the emitter's DC voltage with the 47 ohm resistor has a very small effect on the DC operating point because the base voltage is not fixed with a voltage divider but is made by negative feedback and adjusts itself.
 

**broken link removed**

Look in the Audio section of Fred's pages.
<als>
 

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