LM386 configuration

Discussion in 'The Projects Forum' started by Brogkul, May 9, 2013.

  1. Brogkul

    Thread Starter New Member

    Mar 7, 2013
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    hi all. i want to thank in advance, all the people that made this site. I've been using it for the last couple of years and it always helped me comprehend the basic concepts in electronics.

    now to my questions.
    I'm building a small guitar amplifier for this semester, I have a design I got online

    [​IMG]

    1.- can I expect to make it work using a breadboard? (I will eventually do a PCB but I need to know if I can test it first)
    2.- most configurations I've seen on books show a feedback resistor or capacitor for the op-amps. any especial reason this design doesn't?
    3.- The signal from the JFET separates into two and goes to both op-amps, one to the + input, the other to the - input. what is the result? do they add up?
    4.- from the datasheet I know pins 1 and 8 are for controlling the gain, it says that given an RC configuration will change the gain. what is the formula?
    5.- Where would I find the formulas for Av, Zin, Zout, for this configuration?

    Thank you all.
     
  2. wayneh

    Expert

    Sep 9, 2010
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    I hope you're plan is to use a rechargeable 9V? I know it may seem obvious, but you didn't mention it. Is that charger just a wall wart, or a genuine charger? Can the charger handle the load of the amp plus the charging current?

    I'm also wondering how well the 386 can reject power supply noise when your charger is plugged in. Hopefully you'll get an answer to that here.
     
  3. Brogkul

    Thread Starter New Member

    Mar 7, 2013
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    yes, it's supposed to use a rechargeable 9V battery. i was hoping to use this circuit only with the 9V battery. I'm not planning on use it with the charger directly connected to the circuit.
    would it make a difference if I just use a normal 9V battery instead of a rechargeable one?
     
  4. wayneh

    Expert

    Sep 9, 2010
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    Just don't try to recharge a non-rechargeable battery. That was my only point.

    I think a 9V won't last too long unless that speaker is a headphone. But there are experts here that can chime in.

    Another thing requiring expert comment: I'm not sure about connecting the outputs of two 386's together. I believe that's called bridging, and the concern is that there would be a DC current from one to the other even with no input.
     
  5. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    A large cap in series with the speaker will probably not be a bad idea.
    Or maybe using some other chip that is actually meant to be bridged.
     
  6. Matter45

    New Member

    Dec 13, 2012
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    Yes I agree, two 386 CPU's should do the job!
     
  7. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    Just to be clear, the LM386 is neither an op-amp, nor a CPU. It's a small IC audio amplifier, and I see no need to "bridge" two together. If you want a small guitar amp (that works) using an MPF102 and an LM386, here it is.
     
  8. Dodgydave

    Distinguished Member

    Jun 22, 2012
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  9. Brogkul

    Thread Starter New Member

    Mar 7, 2013
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    thank you all so much for the answers.
    I'm going to use the cricket amp design posted by tracecom.
    I'll post any further problems I encounter.
     
  10. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    I made some changes to the circuit and built it on perfboard. It worked well with input from an MP3 player as well as from both an electric and an acoustic guitar. Of course, it's only 1/2 a watt or so, but works fine as a practice amp.

    As often happens, I lost interest in the project after proof of concept, but I did assemble it to a speaker. I wanted to keep it portable, but still operate on 9 V, so I added a battery pack using 6 AA alkaline cells.
     
    Last edited: May 11, 2013
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  11. tracecom

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    Apr 16, 2010
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    I found another photo that might interest you. It's my solderless breadboard of the Noisy Cricket II before I made circuit changes. Even in this messy condition, it worked.
     
  12. Brogkul

    Thread Starter New Member

    Mar 7, 2013
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    the design has a jack output on the pictures, but I want a speaker as an output. what speaker would you recommend and how would I implement it?
     
  13. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    Connect the + terminal of an 8Ω speaker to C8 and the - side to ground.
     
  14. Brogkul

    Thread Starter New Member

    Mar 7, 2013
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    Thanks.
    another question. what is the use of the second switch? (the one that isn't the on/off)
     
  15. tracecom

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    He calls it a "grit" switch; it adds a little distortion, which some people like.
     
    Brogkul likes this.
  16. Brogkul

    Thread Starter New Member

    Mar 7, 2013
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    I build it on a breadboard and tested it with a signal generator and an oscilloscope. using a 50mV (peak) - 1KHz signal as input and a 10Ω resistor as a load. worked perfectly.
    I'll build it on a perfboard next week and post some pics.
     
  17. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    Congratulations. I am happy for you. Good luck with your perfboard. Don't forget that you can use the output from an MP3 player and a speaker to better hear how it's going to sound.
     
  18. Brogkul

    Thread Starter New Member

    Mar 7, 2013
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    I tried using a mp3 player and a 4Ω speaker (blastking YD10360PF) like you suggested.
    I'm getting to much noise from the speaker when I raise the volume and gain pots.
    I don't mind the low volume but I'd like a more clearer sound.
    some possible reasons:
    I'm using linear pots for volume and gain. I couldn't find log pots.
    the speaker is 50 watt, 4 inch, 5 oz. to big maybe?
    from the pics I posted: to much spacing between the components?
     
  19. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    I think that with the old LM386 it won´t get much better than that. You might try to see what happens whan you put the circuit in a metal box or !carefully! surround it by tin foil, but probably the difference will be small.
     
  20. tubeguy

    Well-Known Member

    Nov 3, 2012
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    The speaker is fine. 50 watts is the amount of power the speaker is rated to handle. Running it with less is not a problem.

    What are you using for a speaker enclosure ?
     
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