Connect with us


Discussion in 'Electronic Design' started by [email protected], Feb 15, 2005.

Scroll to continue with content
  1. Guest

    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
  2. Ruediger

    Ruediger Guest

    Hello Karthik,

    we use some sennheiser mic's with .. .humm.. I think condenser-capsules with
    an integrated FET... aside of the phantom-power we plugged them directly
    into the PC and use the 20db Boost option nearly all AC97 (or better )
    compliant soundcards offer.
    I have not much experience with electret-capsules, but AFAIK their output
    after the phantom-power is more or les similar to the one of
    condenser-capsules. Try plugging it directly into the PeeCee and swip on
    all boost-options in the computer.

    Just my 2 Euro-Cent of thoughts
  3. On 15 Feb 2005 04:16:30 -0800, in
    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


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


    "Facts are stupid things.." -- Reagan, '88
  5. Bob Eldred

    Bob Eldred Guest

    Your description makes very little sense to me. How does the gain drop when
    you move the mic closer or further from a speaker? That is unclear gibberish
    that makes no sense. Reading between the lines and looking at the schematic
    in the url, I'm guessing that your problem has to do with DC offsets in the
    system. That is, DC from the mic or from you circuit is driving the amps
    into or near saturation under certain conditions. When this happens, there
    can be no signal output. Furthermore it will probably go through a region of
    severe clipping and high distortion.

    For openers, the pre-amp must be AC coupled with capacitors blocking
    possible DC voltages that can exist. There is no sound signal in the DC, and
    therefore, no point in amplifying it. Secondly, get rid of the 741's, they
    are totally inappropriate amplifiers for audio purposes. Use a 5532 or
    something similar for low level audio. Thirdly, a differential
    instrumentation connected amplifier can work but is probably unnecessary. A
    simple, single ended gain of 20 to gain of 40 stage should suffice.
  6. Try these guys

    BTW my mic preamp for my AKG condensers is set to 46 dB gain which is
    200. PC sound cards have very little headroom so external level control
    and metering would be needed for setting the actual gain in your

    741 'opamps' are OK for somethings. Audio isn't one of them. I use them
    as multi legged thumbtacks.
  7. P L U M !

    P L U M ! Guest

    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
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day