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How to make my dynamic mic pre-amplifier circuit work?

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Hello everyone, recently I'm making a sound box that have a sub-woofer sound system that works pretty fine, but for my usage I need to add a dynamic mic input which needs a pre-amp. before sending the mic signal to the sub. aux.

I found this circuit and tried to apply it, I didn't get anything at all, but the surprising thing that when I connected an electret mic, it worked like a charm !

So, what is the problem I'm missing !

if anyone have any Ideas or better circuits for my purpose please feel free to give any ideas, thanks :)

Note : even the electret mic didn't work before I canceled the capacitor after the mic input, I'm using uA-741NC @ 12V.

10 Replies

iceng (author)2016-06-07

Obviously your dynamic-mic is not putting out enough signal level compared to the



What op-amp are you using ? What voltage ?

steveastrouk (author)iceng2016-06-08

Gain of 100 with a single ended 741 is pushing it rather.

iceng (author)steveastrouk2016-06-08

I agree a 100 gain with an old style op-amp.

iceng (author)2016-06-27

Hawk-Eagle contacted me by PM which cannot do images, He said

"now I want to connect it to the sound system transformer so one power
source is needed, the transformer has 3 wires out, 2 of them 12V which
exactly what I need, and the middle is the center (GND), as the
transformer has an AC output I added 4 IN4007 to the pre-amp. to solve
the problem and every thing just workes till I connected it with the
system on one transformer, the system made a noise like buzzing or
machingun sound XD
even If I connected the +eve wire only not the GND ...

So what is the problem, can you please help me solve that problem ?"

What I understand is you added a rectified ac power supply.

1a] Is this a separate isolation mains to 6-0-6 vac output ?

1b] Or is it the commercial sound system power transformer ?

2] Also how much filtering capacitance are you using to smooth the AC ripple ?

Hawk-Eagle (author)iceng2016-06-29

it's a 12-0-12 power supply, and I discovered later that the sound system uses -ev voltage so any connection needs special steps which I don't know, so I shortcuted the way and used another 12V adapter and just connected the grounds

Hawk-Eagle (author)2016-06-08

I'm using uA 741 @ voltage = 12V

Hawk-Eagle (author)iceng2016-06-09

Thank you, really thank you, It worked finally, the voice is too low but it worked, I'll change some values to increase the gain and then I'll some filters.

You're a hero man ;)

iceng (author)Hawk-Eagle2016-06-09

Glad for you, hi-gain single op-amp can oscillate

(consider adding a second gain stage only 10and filter in the feedback )..

And thanks for the BA...

Hawk-Eagle (author)iceng2016-06-09

Kay, I'll put this in mind ;)
and you're welcome :D