LM386 guitar amp = radio?

Discussion in 'General Electronics Chat' started by daroc26, Feb 13, 2010.

  1. daroc26

    Thread Starter Active Member

    Apr 4, 2009
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    Hi All,

    I built the Little Gem Amp on a breadboard and it plays somewhat as expected. However, now it is picking up a local radio station.

    Could anyone explain this?

    Thank you![​IMG]
     
  2. hgmjr

    Moderator

    Jan 28, 2005
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    You say now it picks up the station. Was there a time that it did not pick up the radio station?

    hgmjr
     
  3. daroc26

    Thread Starter Active Member

    Apr 4, 2009
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    Yes.

    Today, I added a SPST On-Off Switch, a 1k resistor and a green LED.

    I'm not sure how this would make it pick up a radio signal.
     
  4. hgmjr

    Moderator

    Jan 28, 2005
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    Where did you place the added components? The presence of the LED could be providing the rectification that when combined with the high gain of the amplifier pulls the radiostation audio out of the noise.

    hgmjr
     
  5. daroc26

    Thread Starter Active Member

    Apr 4, 2009
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    I have a long breadboard. On the left hand side is the lm386 circuit and components and on the far right I put the resisted and LED and power.

    Crummy webcam pic after the break...
     
  6. flat5

    Active Member

    Nov 13, 2008
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    Did you also add a guitar chord and/or guitar?
     
  7. hgmjr

    Moderator

    Jan 28, 2005
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    I believe what is needed is bypass capacitors on the power supply. If you have a large electrolytic capacitor and a 0.1 ufd or 0.01 ufd capacitor in your parts stash then put both of them across the power supply to filter it.

    hgmjr
     
  8. daroc26

    Thread Starter Active Member

    Apr 4, 2009
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    Yes, I should've mentioned...the guitar is connected when the radio signal comes through.
     
  9. daroc26

    Thread Starter Active Member

    Apr 4, 2009
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    Okay, that would make sense! I'll give that a try!

    Thank you for your help!
     
  10. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    If the power supply pin bypasses dont do anything, and I predict that they won't, I may have a fix. Building the circuit into a metal box will go a long way in fixing it. Putting an RF bead on the input wire, and possibly shunting the input pin on the PA with a 100pF or 470pF may help, too.

    Question: How close to you is the nearest MW AM broadcast transmitter? You are using a shielded lead between the guitar pick up and the amp, right? Shield is connected to Pin 4, right?
     
  11. daroc26

    Thread Starter Active Member

    Apr 4, 2009
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    I'm not sure about the nearing AM transmitter...Radio is still coming across, after putting a 350uf cap across the power supply. When I'm ready to build I'll use a metal box for sure.

    Thanks for the suggestions!
     
  12. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    How about the shielded wire question?
     
  13. hgmjr

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    Jan 28, 2005
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    In addition to the 350ufd you will need to put a 0.1 ufd or a 0.01 ufd bypass capacitor across the power supply very close to the power pins on the LM386.

    hgmjr
     
  14. MikeML

    AAC Fanatic!

    Oct 2, 2009
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  15. hgmjr

    Moderator

    Jan 28, 2005
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    You are probably right but it is a cheap experiment nonetheless. These bypass capacitors are a good thing to provide as a general rule.

    hgmjr
     
  16. flat5

    Active Member

    Nov 13, 2008
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    I'd start by putting a 0.001uf cap from the input (pin2) to ground. If not successful, add the rf bead at the input socket. A resistor to ground at the input may be a good idea to protect the IC.
    Maybe a 1 megohm or less.

    Short leads and a metal box will certainly help,
    but you need some input conditioning.
    Use a good guitar cord with clean plugs.
     
  17. retched

    AAC Fanatic!

    Dec 5, 2009
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    Could you opto-isolate before preamping? It would seem if you isolated the guitar cord from the rest of the circuit, you would reduce possible antenna.
     
  18. flat5

    Active Member

    Nov 13, 2008
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    Hi retched. Then you would need a preamp to drive the opto. The radio could sneak in from that preamp. Sneaky RF :)
    You would just be adding complexity and distortion. I think.
     
  19. nomurphy

    AAC Fanatic!

    Aug 8, 2005
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    Please see the attached PDF, all the below should help with AM noise to one extent or another.

    The power supply inductor value is not critical, but try to find one around 10 uH that has 1/2 ohm to 1 ohm DCR.

    The LM386 has a 50K input resistor, the added resistor and cap on the input help match guitar cable capacitance (among other things).

    Since you're using this to drive speakers, adding the inductor at the output Zobel network may help (see note in schematic).

    I did not show the two pots or pins 1 & 8, but when using pins 1 & 8 then pin-7 should be bypassed as shown.

    Your layout and grounding is also critical, but that is another topic entirely.
     
  20. flat5

    Active Member

    Nov 13, 2008
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    Good work but if you want to go to that extent then include an RF bead (or two) on the input.

    Edit: Thinking for a minute I realize a musician may play anywhere - including next to an antenna farm or a trucker's party with lots of CB radios.
     
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