LM386N Audio Amplifier picking up local FM station

Discussion in 'The Projects Forum' started by Mitch conrad, Apr 10, 2013.

  1. Mitch conrad

    Thread Starter New Member

    Apr 10, 2013
    7
    2
    Hi I'm pretty new to electronics, still reading through the volumes of info on this site.

    I decided to take a break from reading and start building some simple circuits.

    So I made a basic audio amplifier with the LM386N using a schematic on the datasheet. I plugged it into my computer and strangely enough im picking up signals (92.7 MHz) from a local radio station that's being amplified. You can also hear it faintly if its not plugged into a audio source.

    [​IMG]

    I used different values for the capacitors and I didn't include a 10 ohm resistor after the .01uf cap cause I don't have one.

    Another thing is I used speaker wire and a old headphone jack to make my audio input, I think that's is where I'm getting the leak-in. Could a low-pass filter fix this?
     
  2. #12

    Expert

    Nov 30, 2010
    16,257
    6,757
    Yes, the leak-in is at the input. Keep your wires neat and ground the bottom end of pot 1.

    Just a ragged estimate, Put 75 ohms after the wiper of the pot, in series with the input signal, then a 1nf ceramic to ground, then the signal goes into the amplifier (to cut the high frequencies). The distance from 20Khz to 97 MHz is so wide that a guestimate should work.

    You really must put a few ohms before the .01 cap on the output. It's a frequency roll-off to match the speaker, so 16 ohms will do. Anything from 8 to 16 will be kinda OK. And learn this: Many amplifier chips hate a capacitive load. You have done a no-no.
     
    Last edited: Apr 10, 2013
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  3. Mitch conrad

    Thread Starter New Member

    Apr 10, 2013
    7
    2
    Thanks #12, you confirmed my suspicions. I didn't know why a 10 ohm resistor was needed ill put a small value there in its place.
     
  4. #12

    Expert

    Nov 30, 2010
    16,257
    6,757
    ps, it's called a Zobel circuit...the impedance of the speaker and a capacitor to drain the very high frequencies rejected by the inductance of the speaker coil.
     
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