audio amplifier using lm386 HELP!

Discussion in 'The Projects Forum' started by tron, May 9, 2009.

  1. tron

    tron Thread Starter Member

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    Hello

    I am trying to make an audio amplifier. I am using an LM386 audio amplifier. I want to pick up a sound using a small microphone (link below) and hear the sound through a set of head phones. I found the circuit below online:

    [​IMG]

    for the output, i hooked up a trs connector (for stereo output). the ring and tip leads are connected to pin three, and the sleeve is grounded.

    fo the 250uF cap, i used a 250uF ELECTROLYTIC capacitor, is this okay?

    my problem:
    i hooked up the microphone to the left side of the schematic (the ground going to the dot on the left, then i have the microphone hooked to power).

    the potentiometer is also hooked up, the middle pin is grounded, then one side goes to pin 3 and the other side to the negative side of the microphone.

    what happens is i just hear a noise. excuse the explination but it makes a 'farting' sound and when i turn the pot, the sound either gets higher or lower. i play a song on a speaker nearby and i cannot hear it. is there anything i should check first? i breadboarded everything and its still not working. heres a pic of the breadboard i made:

    [​IMG]

    the microphone i used:
    [​IMG]
    question about the mic: both cables in the black heatshrink have to be grounded right?

    thanks
  2. lofi

    lofi New Member

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    mic black to ground, red to pot maybe?
  3. lofi

    lofi New Member

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    pots have input, output and ground i think

    if so then

    left mic in
    centre pot out (pin 3 in the 386)
    right to ground
  4. lofi

    lofi New Member

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    power to pin 6 on the 386 and pin 4 to ground
  5. Audioguru

    Audioguru New Member

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    Your microphone is an electret type. It won't do anything if it is not powered and connected correctly. It looks like a 3-wires one from RadioShack but they don't know and don't say how it is supposed to be connected.

    I would guess that the black wire is ground, the red wire is from a decoupled positive voltage from 4V to 10V and the white is the output that might need a coupling capacitor.

    Next time buy a microphone from a parts source that has connections instructions.
  6. David Bridgen

    David Bridgen Senior Member

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    I shall assume that you mean pin five.
    Of course. It's the olny type of that value which you are likely to find.

    The picture of the microphone doesn't help much. A view, or sketch, of the rear would have been better.

    Neverthless, it looks as if there is a single screened cable (what you refer to as "both cables in the black heatshrink"), and one single (red) unscreened wire connected to it.

    If that is so, the red wire is to bias trhe electret capsule, and the screened cable carries the signal on its inner, white, core.

    This is how the whole thing should be connected:

    [​IMG]


    The symbols hanging below the diagram of the LM386 to which you refer are Earth symbols, i.e. they mean a connection to the Earth.
    If you run this from a battery source, then those symbols are not appropriate. Unless, of course, you really are going to connect it to Earth.
  7. eblc1388

    eblc1388 Senior Member

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    @tron,

    The resistor on the breadboard at the LM386 output after the 0.05uF capacitor looks like a 10KΩ to me.

    The correct color band for a 10Ω resistor is "Brown-Black-Black".
  8. David Bridgen

    David Bridgen Senior Member

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    Yes, it does doesn't it.

    Neverthless, 10k is not the right value to use there.

    Although 10 ohms is often used, 1 ohm is the value I have seen more often.
    Last edited: May 10, 2009
  9. Bill_Marsden

    Bill_Marsden Moderator Staff Member

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    I'll have to strongly disagree with that statement, it is a convention. I've worked for Collins Radio, Alcatel as a lead tech for 25 years, and have a AA degree, that is how I was taught and how most of the schematics I've used were drawn. It is also how I draw the many I have drawn for this site.

    Ground is a tricky subject, it can be the chassis (though not always, especially with plastic cases). It might be better to say it would be the lead you would connect to earth if you were going to, but in the world of power supplies, especially dual, it is the reference by which the other voltages are measured, nothing more.

    A note to the OP: You can also replace that electret mic with a dynamic microphone or an 8Ω speaker, which also acts like a dynamic mic.
    Last edited: May 10, 2009
  10. David Bridgen

    David Bridgen Senior Member

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    Your disagreement is noted, but it is still wrong, sorry.
    You have never said anything more true in your life Bill.

    Read this http://www.davidbridgen.com/earth.htm

    I read all the British monthly "Radio" magazines since around the mid 50s. Every now and then one or the other of them would publish an article on various conventions. These included not only circuit representatrions but abbreviations of commonly used units. The Earth symbol was always as I described, with successively short horizontal lines.
    Last edited: May 10, 2009
  11. Audioguru

    Audioguru New Member

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    I think the datasheet for the LM386 amplifier should be looked at. Without a 10uF capacitor between pin 1 and pin 8 the gain is only 20 which is far too low for a microphone input signal. The capacitor increases the gain to 200.

    The 9V battery needs to have a 100uF capacitor in parallel to stop the LM386 amplifier from oscillating or motorboating due to the high internal resistance of the battery.

    The pins on the pot are connected wrong. The center pin should connect to the pin 3 input of the LM386. The pin marked "CW" should be from the capacitor connected to the microphone.
    The wires on the breadboard are so long that the LM386 will probably oscillate.
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