LM380 oscillates on positive

Discussion in 'General Electronics Chat' started by jmoffat, Nov 29, 2015.

  1. jmoffat

    Thread Starter Member

    Jul 18, 2012
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    I breadboarded the audio amplifier in the circuit below. My O'scope indicates that it is oscillating on the positive half of the sine wave. Any ideas? I used the inductor symbol to represent the speaker. If anyone knows where I can find a LTspice model for a LM380 please share. Also what would be a reasonable model for a 2" speaker?
     
  2. dl324

    Distinguished Member

    Mar 30, 2015
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    Do you have the power supply decoupled? Is the oscillation frequency low enough to be audible?

    Post a schematic for those of us who might be using a device that doesn't have LTspice installed.
     
  3. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    Can you post a readable schematic?

    Do you have the resistor/capacitor combination on the output of the LM380?

    LM380_output.png

    Bertus
     
  4. jmoffat

    Thread Starter Member

    Jul 18, 2012
    42
    1
    Yes the power supply has a 220 uf cap which I moved to be physically closer to the opamp.

    What is a good way to generate a jpeg of my schematic?
     
  5. dl324

    Distinguished Member

    Mar 30, 2015
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    You need a ceramic capacitor for good high frequency decoupling.
    Depends. Are you using a schematic editor? Or drawing on paper?
     
  6. jmoffat

    Thread Starter Member

    Jul 18, 2012
    42
    1
    I will have to find a way to generate a jpeg or something that can be displayed easily.
    and yes I do have the resistor / capacitor in place. Ironically the oscillation diminished
    substantially when I disconnected it. This led to me dress the leads differently. Problem solved!
    I never thought lead dress would be critical at 1KHz!
     
  7. dl324

    Distinguished Member

    Mar 30, 2015
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    Breadboarding can introduce all sorts of coupling and parasitics.
     
  8. RichardO

    Well-Known Member

    May 4, 2013
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    Your signal may be at 1 KHz but the amplifier has gain at hundreds of kHz or more. This gain at high frequencies with some feedback can cause oscillations from the parasitics dl24 mentioned.
     
  9. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    Make a screenshot of the simulator you use and post it here.

    Bertus
     
  10. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    @jmoffat , Is it the LTSpice sim that is oscillating, or is it the real circuit?

    Can you post the spice model you are using?
     
  11. Bordodynov

    Active Member

    May 20, 2015
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    There is no symbol, but a symbol of the resistor can be used.
     
  12. Roderick Young

    Member

    Feb 22, 2015
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    In post #3 by Bertus, there is a 2.7 ohm plus 0.1 uF snubber on the output. Do you have that installed? In 1979, I worked on a commercial product where the original engineer did not put in the snubber, and there were severe problems. The LM380 worked as an amplifier, but was very soft. My theory was it was putting out most of its power at the high frequencies of the ringing (oscillation). Once the 0.1 uF cap was installed, the output became normal.
     
  13. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    As a rule of thumb, audio power amp chips, old and new, are notorious for breaking into oscillation. They are very sensitive to ground impedance (long wires/thin traces), decoupling, etc. Almost all have reference designs in the datasheet or app notes, and that board layout is not just recommended. Often it is the only thing that will keep the device under control.

    ak
     
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  14. eetech00

    Active Member

    Jun 8, 2013
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    I made a symbol and created a spice model.

    LM380.png
     
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  15. Bordodynov

    Active Member

    May 20, 2015
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    eetech, join please symbol element and test scheme.
     
  16. eetech00

    Active Member

    Jun 8, 2013
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    Hi Bordodynov

    I'm not sure what you mean....please clarify.
     
  17. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    Attach the files.

    ak
     
  18. eetech00

    Active Member

    Jun 8, 2013
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    Hi

    LTspice files attached.
     
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