INA217 circuit squealing at high gain

Discussion in 'The Projects Forum' started by MisterAntony, Nov 28, 2014.

  1. MisterAntony

    Thread Starter New Member

    Nov 28, 2014
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    I have an INA217 circuit I have built and want to use for a microphone preamp. My voltage source is +-12.8v and not the 15v circuit. I am using a battery pack. Not an AC source. I also used 47uF NP 25V caps. The tech docs suggest to use NP caps if phantom power is not used.

    circuit diagram
    technical document

    The output is loud squealing noises (not too unlike a theremin!) when the resistance between pins 1 and 8 falls below approximately 200ohm. Looking at the datasheet, I do not see why this would be a problem.

    As soon as the potentiometer is turned to this point, it immediately begins to squeal and you cannot hear my voice through the microphone. It's not a gradual effect. It's immediate. Otherwise, the circuit seems to work great although not quite as loud as I had hoped (the 15V source would make it louder, yes? I was just using what I had on hand.)

    Why is this happening? I could obviously replace the 8.2ohm resistor with a 200ohm to avoid this, but that would exclude almost half the gain (figure 1 in the tech docs has a gain/resistance table.)
     
    Last edited: Nov 28, 2014
  2. MisterAntony

    Thread Starter New Member

    Nov 28, 2014
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    0
    I am not seeing the edit option (sorry for double post!! please point it out if it's there) but this may help you guys understand what is going on:
    test audio

    A couple "testing" checks at about as high as I can go before the squeal. It sounds good to me. Then I turn it up to the squeal. Back down. Then up again.
     
  3. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    Delete button is in bottom left of each post.

    Make sure your In Amp has decoupling capacitor (0.1 uF) close the the power supply pins.
    Also, you can put a 0.1uF cap across pins 3 and 6 on the OPA 137.
     
  4. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    Also, from the datasheet - input considerations...

    Note: a 1.2 uH inductor is like 20 turns of insulated wire (22 gauge hookup wire) around a pencil.

    image.jpg
     
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