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Generic problem with microphones,phantom powering and op-amps?

Discussion in 'Electronic Design' started by N Cook, Sep 21, 2005.

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  1. N Cook

    N Cook Guest

    Actually a repair problem for a low noise microphone pre-amp but seems to be
    a general design flaw.
    Pre-amp uses a Burr Brown INA103 very low noise instrumentation op-amp.
    In this M-Audio Omni i/o preamp and an outline design application in the
    Burr Brown book show much the same circuitry.
    The 48Volt phantom supply to the mike is protected by 6.8K limiter
    But to block the 48V DC to the op-amp there is a 10uF/100V electrolytic in
    each line directly to the inv & non-inv i/p of the op-amp .

    If , as seems in this case, a balanced line microphone with a short to
    ground is connected to such a system
    then the +48V / 0V across the elecrolytic will instantneously go to 0V
    / -48V with -48V
    directly connected to the op-amp i/p powered from +-15V rails and according
    to the databook
    can be taken to only +-12V.

    Blown input to this op-amp due to just the owner connecting a microphone.
    Anyone familiar with this, adding limiting diode pair at each input ?
    I see no point in replacing this 15 GBP/ 25 USD IC until this design flaw
    is attended to or it will happen again should a fault to ground develop in a
    mike or lead while the 48V power is on.
  2. Yes, diodes to each +/-15 V rail.

    Part of the price you pay for not using a transformer.
  3. Phil Allison

    Phil Allison Guest

    "N Cook"

    ** The use of back to back zeners to protect sensitive input devices is
    standard practice with phantom powered mic pre-amps. See figure 23 on this
    pdf for how it is commonly done:

    Hard to believe any commercial product has no protection against the common
    phantom accident.

    .............. Phil
  4. N Cook

    N Cook Guest

    Many thanks for the link. Not only no protection in this 300 USD as new,
    year 2000
    made, bit of kit but no mention in Burr Brown data book book either.
    Previous repair job concerning problem with phantom, as it was never used ,
    except when someone accidently engaged it, I totally disabled that function
    on that large mixer/amp
  5. There are plans to produce an Audio Engineering Society standard on
    input protection, but progress is slow.
  6. also look at the other "typical " method on P12 of TI's PGA2500

  7. Pooh Bear

    Pooh Bear Guest

    This is classic. I suppose Burr Brown missed the 'usual method' of protecting
    the inputs.

    Add 4R7s in series with the INA inputs and fit clamp diodes to both supplies
    supply at the INA inputs. Use decent size parts. It'll blown an 0805 R for sure

    I'm sure one of the other TI mic amp parts hows this on the schematic ( see
    PGA2500 for example ) and I'm sure the SSM parts have it too.

  8. Pooh Bear

    Pooh Bear Guest

    Quite possibly because it requires some beefy parts to be effective.

  9. Phil Allison

    Phil Allison Guest

    "Pooh Bear"

    ** Hardly a rational explanation.

    No surprise from Stevenson the Fake.

    ............. Phil
  10. Pooh Bear

    Pooh Bear Guest

    I've seen 0805 Rs fail open in that type of protection. The component needs
    to have some ability to dissipate the energy. Check out the peak current

  11. Phil Allison

    Phil Allison Guest

    "Pooh Bear"

    ** Hardly a rational explanation.

    Only a total idiot would use that as an excuse for no fitting protection.

    Pooh just did.

    ........... Phil
  12. martin griffith wrote...
    Ahem. "It is recommended that the Schottky diode chosen for this
    application be specified for at least a 10A surge current."
    Here Burr-Brown recommends ON Semi's monster mbra120 diode,,4450,MBRA120ET3,00.html

    But sheesh, that beefy leaky high-capacitance diode is spec'd to
    handle 40A for 8ms! The surge current is much less, depending on
    the series resistor used. For example, 2x 27 ohms (0.9nV of noise)
    limits the 48V surge to 2A lasting less than 1.5ms (if Cin = 47uF).
    A nice little SD103 can handle that with oodles of room to spare.
  13. Fred Bloggs

    Fred Bloggs Guest

    THAT recommends 4.7 ohms series and 1N5819 bridge rectifier...
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