problem with lm386

Discussion in 'General Electronics Chat' started by alarassi, Apr 20, 2010.

  1. alarassi

    Thread Starter Member

    Feb 6, 2010
    52
    0
    hello guys

    i have construced a circuit that detects and amplifies heart sounds successfully!
    i wanted to add speaker or headphones to listen to the heart beat, i made a circuit of lm386 with 20 Gain that i found on the lm386 datasheet!

    but whenever i switch the power ON, high noise sound (whistle) comes out from the speaker and when i place the stethoscope on my chest the noise partially disappears!

    anyone can help me how to remove this noise sound?

    thank you
     
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Well, the first thing you need to do is to post your schematic, exactly as you have constructed your circuit. If it is exactly the same as a schematic in the LM386 datasheet, then post that. If you have made any changes, you need to indicate what is different.

    Without that kind of information, it's going to be difficult to help you.
     
  3. alarassi

    Thread Starter Member

    Feb 6, 2010
    52
    0
    attached are the circuit i used as heart sounds preamplifier, the output is fed into PIC! but i made another connection for the lm386 amplifier,
    however, i added 330nF coupling capacitor to 100K volum control. i used the same connection of the attached for the lm386 schematic
     
  4. Mike33

    AAC Fanatic!

    Feb 4, 2005
    349
    25
    Weird things start to happen when one couples into new circuits without a lot of prior thought about it!

    My guess is there is some sort of interaction (feedback) between the preamp and 386. I've had these sort of things happen, and sometimes just changing the input from "+" to "-" on the 386 helps. Maybe lower the gain of the 386 and see if you can get it working, then try upping the gain again. Could be parasitics from nearby wire runs...
    those with more experience will be chiming in shortly, though, I bet! :eek:)
     
  5. eblc1388

    Senior Member

    Nov 28, 2008
    1,542
    102
    Acoustic feedback.

    Use a longer cable for the speaker and place it at a distance away from the stethoscope as far as possible.
     
  6. alarassi

    Thread Starter Member

    Feb 6, 2010
    52
    0
    thank you guys for your help!
    i tried all the ways to told me but i still have the problem!
    yes it was somehow better when i used longer cable and place it away from the stethescope but still it gives me very high noisy (whistle) sounds!
     
  7. eblc1388

    Senior Member

    Nov 28, 2008
    1,542
    102
    Use a cable long enough to place the speaker in another room and shut the door.

    If the "noise" is still there, then it is not "acoustic feedback" but feedback due to electrical coupling between varies amplifier stages.
     
  8. alarassi

    Thread Starter Member

    Feb 6, 2010
    52
    0

    in both cases, how to solve this problem and remove the noise!?
     
  9. t06afre

    AAC Fanatic!

    May 11, 2009
    5,939
    1,222
    Try to use some head phones;) If your noise is acoustic feedback it is not much you can do about it, other that trying to avoid it. Hence using head phones
     
  10. R!f@@

    AAC Fanatic!

    Apr 2, 2009
    8,750
    759
    HF feed back is mostly due to high gain.
    the opamps starts to oscillate.

    One way to avoid this is use a high gain differential amp at the stethoscope.
    This way common mode noise can be canceled and avoid HF oscillation too.
    The high gain and the sensitive input is causing all the trouble.
    U have poor ground or input shielding problems.
     
  11. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    If the microphone can hear the speaker or headphones then acousical feedback howling is produced.
    To eliminate the feedback then simply turn down the volume control, use a stethoscope head that seals away outside sounds, use headphones that enclose your ears with a good seal or record the sounds and play them back later.
     
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