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LM386 max input spec is 400mV?

Discussion in 'Electronic Basics' started by Bill Bowden, Mar 6, 2007.

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  1. Bill Bowden

    Bill Bowden Guest

    Data sheet for the LM386 audio amp shows a maximum input limit at +/-
    What happens if you tie the input to a pot and the pot is coupled
    through a cap to a signal sitting at 5 volts DC?

    If the pot is set maximum when power is applied, the cap may pass a
    voltage greater than 400mV as it charges.

    Is this a bad thing?

  2. If you change the pot position fast enough, or the pot
    doesn't have perfect contact the whole time, yes, it is a
    bad thing. It slaps the output of the amplifier around.

    A much better way (if one end of the pot is at ground, and
    there is DC across its resistance), if you can arrange it,
    is to put the cap at the signal input end of the pot, and
    connect the wiper directly to the LM386. That way, moving
    the wiper does not change the average voltage going in to
    the amplifier.
  3. Eeyore

    Eeyore Guest

    Why are you using such a miserable device ?

  4. Bill Bowden

    Bill Bowden Guest

    Well, because it provides less than 0.3% THD, has an idle current of
    less than 5mA, provides 5 volt p-p signal into 8 ohms using a 7.2 volt
    lithium battery, and the gain is internally set to 20 so that minimal
    extra parts are required.

    Do you have a better solution?

  5. maxfoo

    maxfoo Guest

    LM4877 More output power, better THD and PSRR.
  6. And you can get a 386 in single quantities just about anywhere. If
    you can't extract it from all kinds of consumer equipment that is
    using them already (lots of modems that nobody wants, for instance, will
    have a 386).

    If he needed a better solution, as in better specs, likely he'd be
    pursuing it. But I'd say the 386 is still valid.

  7. Bill Bowden

    Bill Bowden Guest

    One end of the pot is at ground and the wiper connects directly to the
    LM386 input. The other end of the pot connects to a capacitor which is
    tied to the signal riding on a DC offset. So there is no DC voltage
    across the pot. My concern was when power is applied and the wioer of
    the pot is set at maximum (connected to the cap), a DC voltage will be
    momentarially applied to the input as the capacitor charges during
    power-on. It would just be a short spike of a few milliseconds.

    The gain is 20, and I need 5 volts p-p out, so the normal input should
    not exceed 150mV peak.

    Do you think a Schottky diode could be used from ground to the input
    to limit the signal to around 300mV? Or is that necessary?

  8. Yes. There will be a power up transient. If you use a
    capacitor on the bypass terminal, it will also produce an
    output transient at power up. If you couple the speaker to
    the output with a capacitor, and the other end of the
    speaker connects to one side of the supply, you will get a
    power up transient. (You can eliminate most of that by
    putting two equal capacitors in series across the supply,
    and tie the speaker between the middle node and the output)
    You might get the combination of these transients to
    partially cancel if you work at it.
    Since the +-0.4 input volts is the absolute maximum, this is
    probably a good idea. I can see how the -0.4 volts might
    cause problems, but I can't understand how exceeding +0.4
    volts does anything except saturate the output.
  9. Eeyore

    Eeyore Guest

    Wow ! You knocked me sideways. Is it really truly that BAD ?

    What's the significance of lithium ?

    You mean an extra 1/4W resistor ? Do you *want* or *need* a gain of 20 ?

    Any modern power amp IC.

  10. Eeyore

    Eeyore Guest

    Because clots keep designing the useless pices of shit into gear because they're
    incapable of looking beyond the ends of their noses to the open page of a modern

    The LM386 is *notorious* for ultrasonic oscillation btw. It's noisy as hell and
    distorts like a good 'un.

  11. Bob

    Bob Guest

    You don't get 0.3% THD at 5Vp-p output,
    the THD is over 10% at that output level.
    Look at the graph in the datasheet, the THD shoots
    up above 0.2watt (about 1.2v in 8ohm).

    The LM386 is a horrid amplifier.

  12. Guest

    There is some confusion in your question. "you tie the input
    of the pot" to what? The signal cannot be 5V DC, signals are
    AC by definition. Please clarify what you mean, even post
    a circuit diagram.
  13. Bill Bowden

    Bill Bowden Guest

    You tie the input (of the 386) to a pot (the wiper).
    One side of the pot is at ground and the other side is tied to a cap,
    The other side of the cap connects to the input signal which is
    sitting at a positive voltage.

    Very standard setup. How else would you do it?

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