low noise op amp's

Discussion in 'General Electronics Chat' started by gusmas, Sep 13, 2009.

  1. gusmas

    Thread Starter Active Member

    Sep 27, 2008
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    orite guys , any suggestions for a low noise op amp, going to use it to amplify vocal signals. and what gain should i use for the opamp, the op amp wil be configure as none inverting amp, the mic will be a minium of 50cm from the person and a max of 1.5meters.
     
  2. Audioguru

    New Member

    Dec 20, 2007
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    The new LM4562 has the lowest noise spec that I have ever seen.
    The amount of gain required depends on the sensitivity of the mic and whether the person is whispering or is screaming.
     
  3. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    The old Signetics 5532 and 5534 were always the industry standard for low noise audio, still pretty good.
     
  4. ELECTRONERD

    Senior Member

    May 26, 2009
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    I would try a low noise in-amp.
     
  5. Audioguru

    New Member

    Dec 20, 2007
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    The old NE5532 and NE5534 opamps are pretty good low noise audio opamps but the new LM4562 has almost half their noise.
     
  6. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    I'll have to check those out.... if only the part number didn't start with "LM".

    I should have gone over to the amplifier guys and begged for some samples while I still worked there.
     
  7. t06afre

    AAC Fanatic!

    May 11, 2009
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    I would say, unless you have high end microphones, and cabling. Most OPamps with a decent bandwidth will do the job.
     
  8. gusmas

    Thread Starter Active Member

    Sep 27, 2008
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    Is there a way of determening the output voltage of a mic without connecting it to oscilloscope or do i need to build
    A amp and just work it out from there
     
  9. Audioguru

    New Member

    Dec 20, 2007
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    Mics all have almost the same sensitivity. It depends more if the person is whispering or is screaming and also their distance to the mic. A mic preamp must have a level control.
     
  10. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
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    The Burr-Brown OP37 is great. Used for lots of instrumentation applications. A bit pricey, but worth it.


    Eric
     
  11. gusmas

    Thread Starter Active Member

    Sep 27, 2008
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    ok just out of curiousity i designed a amplifier.
    LM324N
    None Inverting Amp.

    Assume:
    Av = 1000
    Rf = 10kOhm

    Calculating Ri:
    Av = 1+(Rf\Ri)
    Ri = 10 Ohm

    below is pictures of my simulated results. Now my question is what determines the freq response of my circuit or is it just the IC itself.
    and is my design method correct to design a Non inverting amp with a gain of 1000.

    the mic i am using is a AMQ60G Electret Condenser Mic Insert.

    The last pic with block diagrams is just what i am planning to do with the signal i wont go in details why i want to do that i just need to know if it is possible.
     
    Last edited: Sep 15, 2009
  12. Audioguru

    New Member

    Dec 20, 2007
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    The input pin 3 of the opamp is floating so it will float high and the output voltage will always be high.

    The electret microphone is not powered so will not work.

    The datasheet for the lousy old LM324 opamp shows that its response drops above only 400Hz when its gain is 1000 and its supply is 10V.

    It will just amplify its own noise anyway.
     
  13. ELECTRONERD

    Senior Member

    May 26, 2009
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    Right, I would get a instrumentation amplifier. They have higher CMR than an op-amp which is what you need concerning noise. They are a little more expensive but it's worth it in the long run.
     
  14. rjenkins

    AAC Fanatic!

    Nov 6, 2005
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    The LM324 is not a good amp for low-noise audio.

    Have a look at the INA126, I've used that in the past as a low noise mike preamp, followed by an NE5534 for extra gain before digitisation.

    I've found a list which includes a few low noise amps here:
    http://electronics-diy.com/downloads_pdf.php?s=operational_amplifiers

    The OPA134 / 2134 / 4134 look interesting?

    ps. Don't try to get too much gain in a single stage, the frequency response drops as the gain is increased.
     
  15. Audioguru

    New Member

    Dec 20, 2007
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    No.
    CMR is rejection of common-mode hum picked up through a power supply or from an unshielded input cable.
    CMR has nothing to do with the internal noise of an amplifier circuit.
     
  16. ELECTRONERD

    Senior Member

    May 26, 2009
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    Guess what? If you have 1dB of noise on both inputs, the in-amp will reject that noise. I never said anything about "internal noise." I am saying that in-amps have a higher noise rejection than op-amps.
     
  17. Audioguru

    New Member

    Dec 20, 2007
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    The problem with noise is the internal noise of the amplifier circuit.
    The lousy old LM324 quad opamp is almost the noisiest opamp ever made. Rumble and hisssss!
     
  18. gusmas

    Thread Starter Active Member

    Sep 27, 2008
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    Ok ill read a bit about those op amps, i am very much intrested o n how u digitised the signal, jst simple adc , i am guesing nt
     
  19. gusmas

    Thread Starter Active Member

    Sep 27, 2008
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    what the prices on INA126 and NE5534 , im from south africa so i probaly need to order them.
     
  20. t06afre

    AAC Fanatic!

    May 11, 2009
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    Hi Gusmas
    Google the term "gain bandwidth product" and you will find out why you have such a poor bandwidth. A solution will be to use several gain stages (as mentioned by rjenkins). If you cascade 3 stages with gain 10, you will get a total gain of 10*10*10.
    Ps could not find your microphone on google, is the number correct?
     
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