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

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.

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iceng10 months ago

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



What op-amp are you using ? What voltage ?

steveastrouk iceng10 months ago

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

iceng steveastrouk10 months ago

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

iceng10 months ago

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)  iceng10 months ago

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) 10 months ago

I'm using uA 741 @ voltage = 12V

iceng Hawk-Eagle10 months ago
Hawk-Eagle (author)  iceng10 months ago

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 Hawk-Eagle10 months ago

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

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

And thanks for the BA...

Hawk-Eagle (author)  iceng10 months ago

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