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Opamp preamplifier design question

Discussion in 'Electronic Basics' started by MRW, May 31, 2007.

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  1. MRW

    MRW Guest

    I'm working with these microphones because I have them already:

    Assuming that the voltage output of the mic at 40dB re 20 uPa is 5
    microvolts, would using two opamp stages for a pre-amplifier circuit
    be suitable?

    I think 5 microvolts is too low, so I'm trying to step up the voltage
    to at least 50 mV. That would mean that I would need a gain of 10,000.
    But looking thru some of the opamp datasheets, I'm noticing a gain
    bandwidth of about 7Mhz for most of the opamps that I've seen so far.
    This would mean that the usable bandwidth goes down to 700Hz.

    So, I was thinking if I have one amp with a gain of about 400 followed
    by another opamp with a gain of 25, then I should be able to step-up
    my microphone output voltage to some workable level.

    What should my concerns be with using this simple two opamp preamp?
  2. Eeyore

    Eeyore Guest

    You've made some mistake somewhere about the mic output level methinks.

  3. MRW wrote:
    If you cascade two amplifier stages made with similar
    opamps, the one with a gain of 400 will have a bandwidth of
    1/16th of the one with a gain of 25. If you make each stage
    have an equal gain (100 in this case) you will get the
    highest overall bandwidth out of the pair.
  4. MRW

    MRW Guest

    Oh yeah, you're right. Sorry, I meant to say -40dB re 20 uPa. 40dB
    would definitely be a different voltage output.

  5. MRW

    MRW Guest

    Thanks, John! Is that 1/16th of the gain-bandwidth of the opamp with a
    gain of 25? Or is that 1/16th after (gain-bandwidth)/25 ?
    How did you get 1/16th by the way?

  6. 400 is 16 times 25. lets say you have 7 MHz GBW opamps.
    the one programmed to have a gain of 400 will have a
    bandwidth of 7,000,000/400= 17,500 Hz. The one programmed
    to have a gain of 25 will have a bandwidth of
    7,000,000/25=280,000 (16 times higher than the gain 400
    unit). If each of those stages were programmed to have a
    gain of 100, each would have a bandwidth of 7,000,000/100=70

    You can always roll the gain off at 20,000 Hz or some such,
    if you want, with a feedback capacitor, but within the audio
    band, the extra gain inside the feedback loop will improve
    the accuracy of the programmed gain.
  7. phaeton

    phaeton Guest

    If you are just trying to boost the signal, just a single opamp (or
    even very small transistor gain stage) will likely be about 100 times
    more than you need, depending upon:

    1) What voltage you need or prefer to run the preamp from


    2) If you are building JUST a boost circuit, or if you are going to
    follow it with any adjustable filtering, such as a simple RC frequency
    limiter or passive tone stack (active baxendall might require the
    second opamp stage)

    Either way, I can point you to oodles of schematics, but if you prefer
    to slog it out yourself, I completely understand. :)

  8. MRW

    MRW Guest

    I'd like to know about these other schematics, too. Thanks!
  9. MRW

    MRW Guest

    This gave me an idea. I'm thinking of using a feedback resistor &
    capacitor to limit the maximum voltage amplitude out of the pre-
    amplifier. I was thinking of tapping off part of the pre-ampl output
    voltage and re-directing it to a comparator. I'm also going to add a
    SPDT analog switch in series with the feedback resistor & capacitor
    and also with just a feedback resistor.

    The basis of the circuit is that if the pre-amp output voltage
    surpasses a threshold value. Then the switch enables the resistor &
    capacitor branch. But due to the RC time constant, the effects of this
    feedback branch won't be fully realized until after the time constant
    (would this be considered the attack time?). Are my thoughts correct?

    Here is a sketch of what I was thinking:

  10. What is the point of having the capacitor in series with the
    resistor? The gain will change with just the resistor.
    Also, you can just use the contact to parallel additional
    resistance with the one that sets the highest gain. There
    is no need to disconnect one resistor and substitute
    another. 1 contact saved, and a nasty noise pulse reduced,
    during the moment when there is no resistor connected. But
    to be practical, there needs to be a lot more effort put
    into the switching decision than just comparator. There has
    to be some sort of filter that prevents switching on a
    single pop, and holds that decision for a reasonable period,
    after a loud sound, before it changes its mind during a very
    brief silence.
  11. MRW

    MRW Guest

    Got any suggestions for this?

    I was thinking the RC circuit would do this. I thought the capacitor
    would store some of the charge and discharge it a certain amount of
    time later based on the resistor & capacitor value.
  12. Are we talking about an audio signal here?

    If so, you need some sort of rectifier in the signal going
    to the comparator, and the RC filter would modify that
    signal, before the comparator makes its decision. In other
    words, you need to derive an amplitude signal from the pass
    through signal, and process that signal to react to changes
    in amplitude. And you might need to obtain that amplitude
    signal upstream of the variable gain stage, so that its
    decisions will not undo its decisions in an oscillating
  13. Eeyore

    Eeyore Guest

    You want a signal limiter or compressor.

    What you suggest above will sound horrible btw.

  14. But potentially, very educational.
  15. Don Bowey

    Don Bowey Guest

    Consider the reactance of the capacitor at frequencies across the band that
    exists at the output of the amp. The capacitor will provide more feedback
    at higher frequencies than at lower ones.

  16. phaeton

    phaeton Guest

    Well, perhaps "oodles" wasn't the correct word, but....

    First take a look at this:

    I don't know what your level of understanding is, but if you get the
    stuff on this page you can build most any rudimentary audio
    amplification circuit. Ignore the stuff about clipping and distortion
    for your purposes.


    is an old single-transistor booster design with low parts count. I've
    built a bunch of variations of this schematic and have been pretty
    happy with the results. As you can see, you can also buy a nice kit,
    and on the left under "boosters" is an MFT of more circuit examples.

    Jack Orman has some circuits there, but here's a link to some kits as

    But just the schematics are there too if you want to build it without
    the kit.

    A few simple mods to this:

    and you've got a nice op-amp boost. Omit the diodes, change R4 to a
    1M linear pot and you'll get an adjustable gain of ~100. See the
    first link on how to set the gain range of op-amps. I would recommend
    a different opamp though, such as an NE5532 or TL072 for lower noise
    than the LM741.

    And, since most of these are geared for guitar use, I would change any
    input or output cap that is 0.1uF to at least 0.47uf for more bass
    response. I highly recommend breadboarding any of these and twiddling
    with them until you get the desired result for your application. What
    are you recording, anyways?


  17. phaeton

    phaeton Guest

    Errr... make that an adjustable gain of 0 to 1000. On 9V, expect a
    little distortion near the upper range of that, but by the time you
    get it turned up past 6 or so you are probably already flogging the
    hell out of your mixing console anyways. And also, if you *leave* the
    diodes in, you'll have your 'limiter' as well. Snicker Snicker

    But seriously (and I'm going to get some stern looks here), depending
    upon your application, all the distortion that gets generated from
    that could be unimportant or even desireable. Stack more diodes in
    series for more headroom (higher amplitude before clipping) and
    stagger them (even amount of diodes going one way, odd amount of
    diodes going the other way) for a less harsh distortion.


  18. Marra

    Marra Guest

    Most mics have a much higher output voltage.

    What are you plugging it into?
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