Guide to making this amplifier?

Discussion in 'General Electronics Chat' started by Alan410, Apr 16, 2014.

  1. Alan410

    Thread Starter New Member

    Apr 16, 2014
    4
    0
    Hi all, first post.

    So I am trying to recreate this amp that is here:

    http://runoffgroove.com/littlegem.html

    But the problem is that it looks complicated and I don't know what each of the components are (Apart from the LM386 chip)

    Also, can this create distortion using pin 1 and 8 of the 386, and if so, can I have a potentiometer and a switch to turn the distortion up (and off).

    I plan to turn this into a portable guitar amp.

    Thanks in advance.

    P.S Mods, if I put this in the wrong place or made other mistakes please help me correct them, I am new :p
     
  2. ericgibbs

    Senior Member

    Jan 29, 2010
    2,499
    380
    hi Alan,
    I would say 0.5Watt for a guitar amp will leave you disappointed with the volume.

    Pins 1 and 8 set the gain of the LM386 amp.

    E
     
  3. Alan410

    Thread Starter New Member

    Apr 16, 2014
    4
    0
    So, Do I just get a direct connection from pin 1 to 8 with a resistor? So confused.
     
  4. ericgibbs

    Senior Member

    Jan 29, 2010
    2,499
    380
    hi,
    If pins 1 and 8 are not connected, the LM386 has a Gain of 20 times.

    If a 10uF capacitor is connected from pin 1 to 8 , the Gain will go upto 200.

    The usual way is to have a variable resistor, say 10K and a 10uF capacitor in series between pin 1 and 8. The variable resistor [ volume control] would vary the Gain from ~ 20 to 200 as required.

    If you link pins 1 and 8 you will get a distorted sound output...

    E
     
  5. Alan410

    Thread Starter New Member

    Apr 16, 2014
    4
    0
    http://runoffgroove.com/lgmk2-pcb.pdf

    I found this schematic that bridges 2 LM386 chips.

    I can't make custom boards, so could you make me a schematic with just a protoboard?

    Another question,

    Those capacitors, are the polarised, ceramic, etc etc. I don't really understand the difference between them.

    And for the JFet, can I just use any JFet or that one specifically.

    Thanks for helping me.
     
  6. tracecom

    AAC Fanatic!

    Apr 16, 2010
    3,869
    1,393
    The JFET is a preamp that boosts the output from the guitar to a level that is adequate for the LM386. Without the JFET, you are having to increase the gain on the LM386 too much, and getting distortion. Before you give up on this amp, you should build it with the JFET.
     
  7. #12

    Expert

    Nov 30, 2010
    16,247
    6,744
    The MPF102 is rated for 2ma to 20 ma. That's a wide range!
    You can use a lot of different jfets.
    The goal is to get a middle level voltage on the source by adjusting the resistor.
    I'd try for 3 volts. That allows for 1 volt low on the power supply, 2 volts for the transistor to use, and 6 volts peak to peak for the signal. That's enough unless you have really hot pickups.

    When you have the bias right, use the .047 capacitor to a 10K audio taper volume pot circuit. That will give you a range from 33 Hz (lower than a bass guitar) up.

    ps, this jfet does not do gain. It relieves the guitar of the load required to drive the LM386 chip and keeps the high frequencies from loading down.
     
    tracecom likes this.
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