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did something wrong with opamps?

Discussion in 'Electronic Design' started by [email protected], Mar 3, 2005.

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  1. Guest


    I'm trying to set up an OPA1632 differential op amp. I've got an
    unbalanced dynamic microphone hooked up to the inputs, however when I
    power the chip by a 9v battery (9v+ terminal to V, ground terminal to
    -V), apparently noise is getting sent into the microphone, as I can
    hear static comming from the element and it'll pop when I plug the mic
    into the jack.

    Is there something I'm obviously doing wrong? or would you need more
    information to figgure out why this is producing such odd results...

  2. Tim Wescott

    Tim Wescott Guest

    What are you doing with the OCM input to the amp? What are you using
    for input and feedback resistors? Do you have feedback resistors at all?

    So I guess the answer is yes, we need more information. Answering the
    questions above will be a help, posting a schematic of what you're
    doing, either here in ascii art, on your website with a link, or on
    alt.binaries.schematics.electronic will be a big help.
  3. Guest

    Here's a schematic I drew up in paint. I've got nothing on the OCM
    input, though I've patched it to the Vcom pin on my analog to digital
    converter (PCM1804) and had the rest all hooked up, and the same thing
  4. Active8

    Active8 Guest

    What are you using it for that you need a diff output/input? I'd
    ground/common one mic input, so there goes the single batt. Right
    now, your mic/line is probably a big antenna picking up everything
    you see when you touch your scope probe, if you have a scope. Go
    beyond the first page of the spec sheet and note the filter caps on
    the supply and the output.

    That "pop" is usually caused by not having a ground path for the
    inputs. The input caps which you don't show, but those feedback caps
    are probably the culprit, should have a path to ground/common on the
    mic side.
  5. On 2 Mar 2005 20:34:18 -0800, in
    Why are using it as a mic amp?

    wrong choice AFAICS, they only spec it up a gain of 10 or so, whats
    the o/p level of mic, maybe 5mVpeak

    a dynamic mic has a low source impedance, so you are really degrading
    the signal/noise by having those1Kohms in series.

    Go for the SSM 2017 architecture, works fine, second sourced by TI and
    AD and Thatcorp( i furget the part nums)


    "An eye for an eye makes the whole world blind"
  6. Guest

  7. James Meyer

    James Meyer Guest

    It may be perfectly normal. A dynamic mike can be a very sensitive

    Try a couple of tests and report back.

    Test 1, With the mike disconnected, use an ordinary multimeter to
    measure the voltage across the amplifier's input connections.

    Test 2, Use a single 1.5 volt flashlight battery in series with a
    resistor to test the mike. Start with 10K or so for the resistor and keep
    increasing its value until the mike makes the same volume of "static" noise when
    you connect it to the battery that it does when it's connected to the amp.

    Test 3, If you have access to an oscilloscope, measure various points on
    the amplifier to see if it is oscillating at some frequency much higher than
    ordinary audio.

  8. On 3 Mar 2005 19:32:14 -0800, in
    Why did you choose the this opamp?


    "An eye for an eye makes the whole world blind"
  9. Guest

    Because it was suggested by Texas Instruments for use with my
    particular ADC (PCM1804). I've put in a samples order for the parts
    you've suggested however.

    @ James Meyer: I'll work on those tests once I get home and report back
  10. On 4 Mar 2005 06:29:31 -0800, in
    I'd start off with the circuit shown in fig 44 of the 1804 data sheet,
    its pretty standard . ( see the AKM and Cirrus datasheets)
    You could put lower THD opamp in , say AD797, but I doubt if you would
    hear any difference.
    You would still need a mic preamp to get the sensistivty ie 30 to 60dB
    of gain.


    "An eye for an eye makes the whole world blind"
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