Plugpack or battery vs microphone problem

Discussion in 'The Projects Forum' started by bluewave, Feb 4, 2012.

  1. bluewave

    Thread Starter New Member

    Feb 4, 2012
    8
    0
    I would be very grateful for any suggestions on this strange problem (maybe not strange to someone who is not a newbie!)

    I am building a sound-level measuring stage that feeds the ADC of a PIC.

    The attached circuit works fine when run from a 12V battery. But if I use a 12V plugpack (I've tried a number of them), the opamp output often starts to drift high - approaching maximum sometimes. If I remove the microphone (electret) from the circuit, the problem disappears (so doesn't seem to be power supply noise simply punching right through to the opamp). Not shown - the opamp power supply is also bypassed by 0.1uF.

    Very weirdly - if I touch a single point of the circuit with my finger - problem also disappears. Lots of different points work - 0v, opamp output... you name it, if I touch it, problem fixed!

    The 12v and 5v power rails look very clean, although my scope is low bandwidth so I wouldn't see RF stuff. Even attaching my scope seems to often fix the problem.

    The choice of opamp I've used etc has a lot to do with what I had lying about.:) The PIC seems perfectly happy with either plugpack or battery supply.

    I have tried ferrite beads, more bypass caps, etc, no luck. Any ideas would be fantastic!

    :confused:
     
  2. bluewave

    Thread Starter New Member

    Feb 4, 2012
    8
    0
    ...Doing some reading, I believe my problem has something to do with leakage current through my plugpacks - I am bleeding the leakage to earth when I touch the circuit. The plugpacks are 2-pin type, with no earth, like most are.

    By floating my scope, I have been able to measure the 12v supply when the circuit is earthed and when it is not - there is no particular difference in noise.

    Can anyone say how the microphone 'knows' if it is referenced to earth?


    :confused:
     
  3. PaulEE

    Member

    Dec 23, 2011
    423
    32
    I read this post about four hours ago and scratched my head. What the heck is a plug pack? Is that just a DC wall wart sort of thing?
     
  4. bluewave

    Thread Starter New Member

    Feb 4, 2012
    8
    0
    Yes, the ubiquitous Wall Wart.
     
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