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LM386 (audio amp) oscillates

Discussion in 'Electronic Basics' started by Bill Bowden, Apr 26, 2007.

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  1. Bill Bowden

    Bill Bowden Guest

    I have a LM386 that oscillates when the volume is turned up too far.
    It's operating with the default gain of 20 and there is a RC on the
    output as recommended in the app notes (10 ohms in series with .05uF).
    The oscillations will stop if I add a .05 cap across the input pot,
    but that reduces the signal and rolls off the high end. There is a
    1000uF cap across the power terminals

    Seems to be set up right, but makes too much noise when the volume is
    cranked up.

    Any ideas?

    -Bill
     
  2. Have you got a capacitor between ground and the bypass pin 7?

    Have you got a bypass capacitor between ground and the
    supply, pins 4 to 6?

    Does the speaker ground end tie back to the ground pin, or
    to somewhere else on a ground bus?

    You might try replacing the .05 uF output filter capacitor
    with a pair of .022 uF or .027 uF or .o33 uF caps in series
    across the supply pins of the chip, with the 10 ohm resistor
    connected to their common node.
     
  3. Eeyore

    Eeyore Guest

    This seems to be the most common complaint on the sci.electronics groups.

    Why are you using that POS ? It was dreadful when it was introduced.

    Graham
     
  4. I have a cheapy radio shack amplifed speaker that uses one. It has a huge
    amount of gain and never motor-boats or oscillates. It has enough gain that
    you can attach it directly to the output of a NE602 mixer and hear audio.
    That's allot of gain. :)
     
  5. John Fields

    John Fields Guest

     
  6. Eeyore

    Eeyore Guest

    I've never needed to nor do I wish to. It's rubbish and I have no use for it. My
    preference going back to days of old was TI's SN76023. You could make one of
    those perform surprisingly well !

    I'm sure it would present no problems to an expert in audio amplifiers like
    myself though. After all, my 2kW amplifiers work beautifully and have
    class-leading performance figures and need I add, totally unconditional
    stability.

    Graham
     
  7. Bill Bowden

    Bill Bowden Guest

    No. The app notes show pin 7 open in the 26dB setup.But it looks like
    a good idea to use a 10uF- 50uF anyway. I will add a cap and report
    results.
    Yes, 1000uF
    It's close to the chip, and one side of the board is all ground plane,
    so it's pretty much zero ohms wherever you go.
    -Bill
     
  8. Jamie

    Jamie Guest

    try putting a non inductive cap across the rails as close to the
    leads as possible., you can still keep the 1000 there.

    .1 uf ceramic might do it.
     
  9. I really like the Panasonic V series stacked mylar film
    capacitors for low ESR hefty bypass use. The 1 uF at 50
    volts is about a quarter inch cubed. But if space is very
    limited, a 10 uF 25 volt 1206 X5R SMT capacitor will do a
    nice job with something like a milliohm of ESR.
     
  10. Jamie

    Jamie Guest

    I have a few mica smt caps that work very nicely, they are expensive
    how ever because they are hard to make..
    When I was working at Semco as an consultant, I was part of a project
    where we automated a robotics arm that had visual to the computer.

    The arm used a mini vacuums finger to pick up the mica sheets to be
    stacked.
    This process would pick a cut piece from a pile, visually inspect it
    for defeats first, put it under a pressure plate where High voltage was
    applied for leakage test. If all of that passed, it was then pick up
    and place in a contact frame where the cap was being assembled.
    Very thin silver foil was laid between for the plate's from a mini
    spool entering from the side's of the frame..

    When the stack was complete, a push finger comes down along with a
    button lift finger that pushes it out of the contact frame and is
    compressed between these 2 fingers as it is quickly hit with a HOT
    iron (very small) to bond the foils..
    from there another HV test is done along with capacitance test which
    will very the compression tension of the fingers to make adjustments on
    the fly.
    the next process then passes this over to two other holding fingers
    where the encapsoluation is applied and harden with UV (Very Strong) UV.
    then another test is done, then they are place in little strip frames
    for later encaps and printing ..

    All of that above it done using robotics arm with attached video camera's
    into a PC using DELPHI has the graphics and guidance for the movements.
    Things were color coded on the table to make it easy to track.
    this may sound like a lot of steps how ever, the process actually was
    very fast once you got it going. the operators only needed to dump small
    piles of pre-cut mica on a pick up area. From there, the robotics did the
    rest.
    Before that time, the Japanese was the only one's making then and
    their yield was like 5 out of 10 good, and that was done by hand as for
    as the stacking.
    We also suffered the same problem, but after the automation, it was
    like 99.99% good other than an unexpected machine failure/Jam up.
     
  11. Bill Bowden

    Bill Bowden Guest

    Well, I had high hopes for the bypass cap on pin 7, but unfortunately
    it didn't help. I tried various caps (0.4uF, 1uF,22uF) from pin 7 to
    ground with no improvement. Also tried small caps across the chip
    power pins with no change.

    -Bill
     
  12. Eeyore

    Eeyore Guest

    What dielectrics ?

    How small ?

    What frequency is your oscillation ?

    What value are all your components ? Is your circuit DS006976-3 from the
    datasheet with pins 1,7 and 8 open ? Have you tweaked any values ?

    And indeed what value is your input pot ?

    I'm suspecting you're trying to get to high an input impedance and you have some
    capacitive coupling from the output to the input.

    What's the output Z of the signal source too ?

    What's the supply voltage and load impedance ?

    Are you absolutely stuck with using this chip ?


    Graham
     
  13. christopher

    christopher Guest

    *************

    "One mans ringing may be another mans oscillation."

    I solved my own ringing problem on the output of the LM386 using a
    ferrite bead. Why do I use this chip, I can walk into Radio Shack and
    find it for quick projects.


    View this ferrite on page 6 of this PDF. Many good pointers on the
    page.

    http://downloads.solarbotics.com/PDF/LM386.pdf

    good luck,

    * * *
    Christopher

    Temecula CA.USA
    http://www.oldtemecula.com
     
  14. Bill Bowden

    Bill Bowden Guest

    There is diode/fet detector circuit that drives a 10K pot that feeds
    the 386. The problem is not really oscillations but a loud roaring
    noise that seems to be related to the diode. I'm going to add a 100K
    in series with the 2 meg and a capacitor from the junction to ground.
    That should reduce feedback to the gate of the jfet. Something like
    this without the extra restistor or cap shown


    +6
    |
    | +6
    [2 Meg] |
    | |
    | |---+
    [diode]-+--g>| jfet
    | |---+
    | |------||-----+
    [270 pF] | |
    | [R] [10k pot]----> LM386
    | | |
    | | |
    GND GND GND


    -Bill
     
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