MICROPHONE PREAMP HELP

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 for the instrumentation amplifier is given
below:

http://www.allaboutcircuits.com/vol_3/chpt_8/10.html

 

[martin griffith]

On Tue, 15 Feb 2005 17:39:58 +0100, in sci.electronics.design martin
griffith <martingriffithX@Xyahoo.co.uk> wrote:

http://www.allaboutcircuits.com/vol_3/chpt_8/10.html
1) what is the DC on the amp output?
2) is it AC coupled, input and output?
3) do you have resistors to ground on the inputs, say 10K
4) do you have a scope?
5)what are the voltage rails
6) why do "they" still make the 741?
7)is the mic balanced out?
8) how long is the mic cable?

have a look at TI's INA217, or use NE5532/4's


martin


"Facts are stupid things.." -- Reagan, '88
Oh I forgot, what value of R have you selected?

martin


"Facts are stupid things.." -- Reagan, '88

 

[martin griffith]

On Tue, 15 Feb 2005 17:42:45 +0100, in sci.electronics.design martin
griffith <martingriffithX@Xyahoo.co.uk> wrote:

On Tue, 15 Feb 2005 17:39:58 +0100, in sci.electronics.design martin
griffith <martingriffithX@Xyahoo.co.uk> wrote:

I've put a little cct on how it should be wired , ie all the bits they
left out of the original article here
http://uk.geocities.com/martingriffith/audioDiffAmp.zip

martin


"Facts are stupid things.." -- Reagan, '88

 

[P L U M !]

Thanx all for replyin....I was too stupid and dint put a capacitor in
the input and also dint usse a couplin capacitor....thanx to
grifftith....thank u all thank u