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Total beginner Audio Amplifier (LM386)

Discussion in 'Electronic Design' started by Rene, Apr 17, 2004.

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  1. Rene

    Rene Guest

    I am trying to build an audio amplifier using the LM386, since I have no
    idea what am I doing, I wired the chip as good as I could and it worked but
    its very noise. I am certain that I am doing something wrong, especially
    because I have no idea where all the grounds in the diagram should go.

    Could you please look at the picture on the link below and tell me were I am
    screwing up? Please look at the battery and speaker jack connections.

    Thank you.
  2. Joerg

    Joerg Guest

    Hello Rene,

    Some grounds appear to be going only to the input jack, others to the battery.
    Connect all together at shortest possible path and see what happens. Best would
    be to have a ground plane on your experimental board.


  3. Tim Wescott

    Tim Wescott Guest

    Yes, generally _all_ the ground symbols go to the same place, unless
    they're different shaped or otherwise annotated (and sometimes they go
    to the same place anyway).
  4. You absolutely need a big electrolitic cap, in aprallel with say a 100 nF
    poly of ceramic on the supply line.
    1000uF would be nice...
    Also watch you input lines, things may be oscillating.
    I have seen the input pot used in a different way, look up application notes.
    Oh, I have this old National Audio/Radio handbook, nice book!
    Here they have the LM 386, and that input you use has a low pass with 1 k
    and 2200 pF! before the external lead and pot.
    Also they use a small ferrite bead with 2 turns in parallel with a 47 Ohm
    resistor in series with the pen 5, and then 5nF to ground...
    So oscillation ! Do you have a sope?
  5. Rene

    Rene Guest

    But if all the grounds go to the same place (say the battery ground) then I
    would only be able to connect one of the wires coming out of my speaker jack
    (The one that gois in the 10k pot), where would I connect the other one??
    Please check out the new link:
  6. Tim Wescott

    Tim Wescott Guest

    The wire with the '?' goes to the negative terminal of the battery, too.
    When you do this your circuit is referenced to the computer ground.

    BTW: you want to use the ground lead of the computer speaker jack. This
    is the long part (not the tip) if it's a 1/8 inch audio jack, or the
    outer part if it's an RCA-type coaxial.
  7. John Jardine

    John Jardine Guest

    You need a hulking great electrolytic capacitor across the battery
    terminals. Something like 470uF would be OK.
  8. Seth Koster

    Seth Koster Guest

    The lm386 has, I believe, a 10%THD (total harmonic distortion),
    this means it will be noisy no matter what, but the most obvious
    problem to me is that the grounds are hooked up wrong. Take the
    negative lead from your input jack (negative from the computer cord)
    and hook that up to ALL grounds, then hook up ALL grounds to the
    negative lead from your battery.
  9. The ground side of the computer audio jack and
    the - (inverting) input of the LM386 should be
    connected to the same ground as the rest of the

    I would also suggest that you put a 1 microF
    capacitor in series with the input (from PC jack
    to 10K pot), a 10microF capacitor from pin 7 to
    ground and a power supply de-coupling capacitor
    of several hundred microF from pin 6 to ground.

    The LM386 data sheet is at:

    I have made a few changes to your schematic.
    Send an e-mail to nospam AT
    if you want me to send you a copy.
    Replace " AT " with "@"


  10. N. Thornton

    N. Thornton Guest

    Theres 2 things missing there. Your ground connections are connected
    in 2 sets, with no connection between them - they all need connecting
    together. And I'd put a 1000uF capacitor across the 2 power lines.

    Regards, NT
  11. In addition to (or reinforcing) other replies:

    1. I had great difficulty eliminating noise in a rather similar
    circuit using the LM380. Major contributor was indeed the
    configuration of ground wiring. But sorting that was more trial/error
    than following any fixed rule.

    2. One complication you may have was that input and output sockets
    were earthed via chassis. And in my case I had two *sections* of
    chassis which were electrically connected only when screwed together,
    unless I added a connection wire.

    3. Large electrolytic across supply, and 100n across pins 2 to 6.

    4. Extra ripple suppression with 10uF from pin 7 to ground.

    5. Screened cable if input socket more than couple of inches from pot,
    and from pot middle to 386. (And then experiment to find optimum: both
    or one end of screen grounded? To where?)
  12. Since discussion is going all about LM386, it's a nice place for me to

    Is Slew Rate of an Op-Amp a good parameter, to be taken into
    consideration while designing an audio amplifier.

    If yes, why don't the datasheet of LM386 carry its Slew rate.

    Or maybe LM386 is solely designed for audio power amplification, so
    designers don't have to worry lot about its SR.

    Thanks a lot

    Best regards,
    Animesh Maurya
  13. Tom Bruhns

    Tom Bruhns Guest

    Oh, my. Well, ALL the grounds must be connected together, and your
    diagram shows some connected to the 'puter and some to the battery,
    etc. Best, lacking any other info, is to connect them all in a "star"
    configuration: pick one point that you call ground, and run each
    connection through as short a wire as is reasonable to that point.

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