Electronics

P L U M ! · Guest

Hi,
I need to build a preamplifier for a mic to connect it to a PC
soundcard. The mic I am using is a proffesional mic which uses an
electret mic capsule inside. I dont need to provide phantom power also
coz it operates on a 1.5V battery. Now, the plan is to build a preamp
with a female jack into which I can connect the mic's output jack. I
tried an instrumentation amplifier circuit which has three 741s in
it.The circuit worked fine for a gain of 10.
Although I could see a gain of 10 only when I put the mic real close to
the speaker.As I moved it farther the mic output(amp input) and also
the gain dropped.When I increased gain further, that is to 11 it worked
fine too. Then I increased it further to 20. Now the output of the
amplifier was zero! Then I tried to give a gain of 2 thru an additional
stage(a noninverting ampliflier using 741). So that would be 2*10=20
gain...But again the output was zero. Can anybody tell me where I am
going wrong???

The mic without any amplification showed an output range of 5-20 mV.
The link to the schematic of the insturmentation amplifier circuit is
given below:
http://www.allaboutcircuits.com/vol_3/chpt_8/10.html

tempus fugit · Guest

If you are plugging that mic into a line in on your soundcard, you should be
looking at a pre with a gain of more like 1000 if you are micing quiet
sources. You don't really need an instrumentation amp for this purpose; a
single dual opamp would do the job nicely. Also, 741s suck for audio. Get a
decent opamp with wider bandwidth and a higher slew rate. Even something
like a TL072 or NE5532 (both dual opamps) will sound way better than a 741.
You could build the differential amp on the page you have linked to, or get
2 of the opamps I suggested and use them to build the instrumentation amp.
Or do a search on Google for mic preamp schematics and see what you get.

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