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Copying Op Amps to Make Mic Amps

Discussion in 'Electronic Design' started by D from BC, Mar 17, 2007.

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  1. cledus

    cledus Guest

    There are graphs for distortion on Page 9. I overlooked it on the first
    pass. My bad.
  2. John  Larkin

    John Larkin Guest

    With a typical voltage gain of 30 million and a typical gbw of 75 MHz,
    the LT1028 should be pretty good. The datasheet does have thd curves,
    hitting numbers like 1 ppm at 1 khz, g=-20, 20 v p-p out. Not bad!

  3. John  Larkin

    John Larkin Guest

    Sure. 75 MHz / 1000 is only 75 KHz of available gbw. Two stages of
    gain, 32x each, and the distortion would be back in the single-digits
    of ppm.

    I use these opamps in nmr gradient amps, to amplify the current shunt
    signals. To reduce thermal effects in the shunts, it's best to keep
    their resistance and hence voltage drop as low as possible, and we
    need ppm wideband noise, so a good opamp really pays off. My first
    stage out of the shunt is an LT1028 running gains like 15. The
    feedback resistor values are so low (to keep the Johnson noise down)
    that the 1028 would have to source a lot of current into them and
    transiently self-heat, so I put another, cheaper follower opamp inside
    that loop, so it's the one that gets hot. At that point, one
    appreciates the fact that the 1028 has a compensation adjust pin. Nice

    Maxim made a MAX1028 for a while, but LTC made them drop it, somehow.
    Analog Devices now has a similar part.

  4. Eeyore

    Eeyore Guest

    Which is where using op-amps exclusively falls down. I can't think of many
    situations where I use them for > 30dB gain. Rarely above 20dB in fact.

  5. Fred Bartoli

    Fred Bartoli Guest

    Eeyore a écrit :
    And what's wrong with compound?

    I have a fA input bias amplifier with a 100 gain that have 100MHz GBW
    Pretty easy in fact.
  6. Eeyore

    Eeyore Guest

    What's wrong with discretes ? I design with discretes quite a lot in fact. Another
    of my specialities is high power audio amplification. ICs can't do that either.

    Irrelevant to audio. I'm sure it's fine for what you need it for though.

  7. John  Larkin

    John Larkin Guest

    You're not going to get a clean gain of 1000 from a single discrete
    stage, either.

  8. Eeyore

    Eeyore Guest

    I'm not trying to am I ? The 'worst' example I posted was ~ 40dB from a single stage
    (the remainder came from the op-amp) using the compound pnp-npn arrangement.

  9. Andy Peters

    Andy Peters Guest

    I think that the mic preamp has reached its state of the art, and
    further improvements are probably found in reducing the cost while
    keeping the performance. It's not all that difficult to build a mic
    preamp that measures flat from single-digit hertz to 200 kHz over a
    reasonable gain range.

    Anyways, I think the most obvious reason for differences in the sound
    of mic preamps is simply the input loading. Change the input
    impedance and how it reacts with a dynamic mic changes.

  10. Eeyore

    Eeyore Guest

    That's one of the ways. Any 'boutique' mic pre I may ever make will have selectable input Z.

    The other main things influencing their sound is transformer colouration and (for old designs) the small but
    measurable amount of THD from the Class A stages.

  11. John Larkin

    John Larkin Guest

    Four transistors is not a single stage!


  12. Eeyore

    Eeyore Guest

    It is in my book. If they'd been ordinary darlingtons would you still have quibbled ?

    Ppffftttt !!!

  13. Phil Allison

    Phil Allison Guest


    ** The * PROBLEM * is that source impedance alters the HF response of the

    A given design may be "flat as a tack" with a 600 ohms mic as the source
    but peak by 6 dB at some high audio frequency when changed to 200 ohms.

    Then, if a modern condenser mic with say 20 ohms impedance is used, the HF
    peak goes right off the wall.

    Getting rid of that damn tranny was the best thing to ever happen to mic

    ........ Phil
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