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Noob trying to build a small guitar amp

Discussion in 'Electronic Basics' started by ngdbud, May 6, 2006.

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  1. ngdbud

    ngdbud Guest

    Hey guys, after searching the web for some simple schematics, I found
    this 8 watt amp that would be perfectfor me as a beginer (based mostly
    on the number of components) http://www.aaroncake.net/circuits/amp.htm
    There are just a few things that need clarifying for me.

    1) The IC doesn't have pin numbers labled and it's beyond me to figure
    them out.
    2) Only one wire is labled input. Does this mean I attach one wire to
    this input and the other to the same ground used for the negative
    power? If so, does it matter which wire I put where since sound travels
    as AC waves?
    3) This says it's an 8w amp, but would it be a good idea to plug in a
    10 watt speaker just in case? and which would give me better or louder
    sound, a 4 or 8 ohm speaker?

    I've had enough minor experience to understand the rest of this
    circuit.

    Thanks in advance!
     
  2. mc

    mc Guest

    Follow the diagram just below the schematic. It shows the IC as seen with
    the label toward you.
    Ground the cable shield rathe rthan the center conductor.
    Not sure. There should be an LM383 data sheet at
    www.nationalsemiconductor.com that will tell you more.

    Does your guitar have line-level output? I thought guitars were closer to
    microphone level, usually, and this won't be enough amplification for them
    (I don't think).
     
  3. ngdbud

    ngdbud Guest

    so is it...
    pin 5 = 12v +
    pin 4 = the wire connecting between r3 and c4
    pin 3 = 12v -
    pin 2 = connects to the positive end of c2
    pin 1 = connects to the negative end of c1

    ????

    I did my best at guessing. Did I get it right?
     
  4. Pooh Bear

    Pooh Bear Guest

    Actually they *are* there in the diagram marked LM383 Pinout. It's rather
    small though. You can find the data sheet here ( but the part's been
    discontinued btw ). http://www.national.com/pf/LM/LM383.html

    The tip of the guitar's jack plug is the input signal.
    The screen/shield goes to ground.
    Yes it matters because of how the shield works. The shield must go to
    ground.
    Good practice in case you 'overdrive' the amp for example.
    The impedance will make no difference to sound quality as such but 4 ohms
    wil be abit louder.
    Good luck !

    Graham
     
  5. Pooh Bear

    Pooh Bear Guest

    Yup.

    Graham
     
  6. Pooh Bear

    Pooh Bear Guest

    It's got a voltage gain of 100x ! That's plenty for a guitar amp, if fact
    possibly too much.

    It really needs a level control before the input.

    Graham
     
  7. ngdbud

    ngdbud Guest

  8. Pooh Bear

    Pooh Bear Guest

    That's a 'passive' tone control i.e. it doesn't use any amplifying device (
    tube, transistor or IC ). As such it's affected by the source and load
    impedances. The source impedance should be low and load impedance high for
    it to work right. You don't have that. In order for it to work
    satisfactorily it would need to be 'buffered'.

    Try googling for 'active tone control' maybe, although it has to be said
    the guitar types of circuit are interesting if only for their quirky
    'imperfections'.

    Btw - I meant to add before .... You do realise that amplifier chip needs a
    heatsink don't you ? And see the note about stability.

    Graham
     
  9. ngdbud

    ngdbud Guest

    I have an electronics book with an active tone conrtol schematic in it,
    it has treble and base adjustments but no mid range. I'll keep
    searching. And yes, I know I have to use a heatsink. The way he put it
    in caps and that this claims to be an 8 watt amplifier while the data
    sheets on the chip say seven were screaming at me that this can't be
    avoided. The note about stability... Are you talking about the one that
    says to build it on a pcb to avoid oscillation? Thats gonna be tricky.
    I don't have etching solution or clad boards or anything like that.
    after diging through my junk box late last night I found a 5 watt 250
    ohm resistor, is this god enough for R3? keeping in mind that I'm going
    to swap it with R1 like he says in the notes to make it for guitars.
     
  10. ngdbud

    ngdbud Guest

  11. ngdbud

    ngdbud Guest

    good to know, I have tons of 1/2 watt resistors, i just figured since
    it was connected directly to the ground of an 8 watt amp i might need
    a little over kill
     
  12. Pooh Bear

    Pooh Bear Guest

    I reckon after som ethought that the best answer is to use a couple of
    buffer stages and use the guitar tone controls actually.
    Hmmm... It seems he thinks then that a guitar amplifer needs no gain at
    all. I wouldn't do that. sadly this is typical of mny ppl's websites with
    hobby plans where the info is really not that great.

    Just forget that for now !

    You really don't need a 5 watt resistor either.

    How do you plan to connect it all together then ? Perfboard ?

    Graham
     
  13. Pooh Bear

    Pooh Bear Guest

    That's the kind of thing - unforunately that design has huge control
    interaction !

    Do you need this to run off a single 12V supply btw ?

    Graham
     
  14. Mike

    Mike Guest

    For stability be sure to follow his note number 7 by adding the
    ..2uf/1ohm components to the output pin of the LM383 as shown in the
    data sheet.
    Mount C3 as close to the LM383 pins as possible.

    He says you don't need a PCB to build it and he's probably right. Just
    keep all the ground connections as short and direct as possible.

    The 5watt resistor would work, but it's way bigger than you need. I'd
    go to radio shaft and see if you can find a 220ohm 1/4watt.


    I don't understand what swapping R1 and R3 is all about, but that
    doesn't sound right to me. Maybe someone who knows could clairify
    that.

    The input resistance of the LM383 is 150k, but if that is too low is
    there any reason the OP couldn't add a simple jfet source follower to
    the output of his tone control circuit to form higher Z load for the
    tone control?

    Mike
     
  15. Rich Grise

    Rich Grise Guest

    There's no need. To boost the midrange, cut the bass and treble and turn
    up the volume. ;-)

    Good Luck!
    Rich
     
  16. ngdbud

    ngdbud Guest

    A single powersupply would be nice... it's a first, I'd like to keep
    things simple.
     
  17. Lionel

    Lionel Guest

    No, guitar pickups have quite high outputs. More than enough for
    standard line levels.
     
  18. Phil Allison

    Phil Allison Guest

    "Lionel"

    ** That is just not true.

    The *average* valve or SS guitar amp has an input impedance of 500 kohms and
    a sensitivity of 10 mV at 1kHz to full power output.

    The popular Marshall " Master Model" (an all valve amp) has an input
    impedance of 1 Mohms and a sensitivity of ** 60 microvolts ** to full power
    at 6 kHz !!!!

    While some electric guitars have pickups that can deliver an "unloaded"
    output of 500 millivolts or so when crashing a full chord, the level with
    gently played single notes is only a few millivolts.

    Line level it sure aint.



    ........ Phil
     
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