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Requesting Thoughts On This Small Guitar Amp Circuit

Discussion in 'Electronic Basics' started by phaeton, Apr 13, 2005.

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  1. phaeton

    phaeton Guest

    Hello again,

    I've recently built a small guitar practice amp circuit found here:

    I've seen mentioned in ths ng before, and
    there are a few players here, i see. I'm betting that someone has come
    across and possibly built this circuit themselves, or that it's simple
    enough (and probably generic enough) that most of you can look at it
    and understand it :) (i'm still working on understanding *how* things
    work as a whole, myself).

    Anyways, it soldered it up and it works. I've put it together exactly
    how it is in the diagram, except that for the output i've got it
    driving a 40W 6"x9" 4-ohm speaker instead of headphones. It's
    surprisingly loud for being 1/2W- not earth shaking, but you have to
    raise your voice to talk over it. Not expecting stellar sound quality,
    yet the distorted guitar tones are pretty good for what it is- probably
    better than most really small, solid state practice amps.

    However, speaking of harmonic distortion, there is a LOT of it. It's
    great for rocking out (Toni Iommi In A Box!) but i can't clean it up at
    all. There is no volume or gain control for the circuit, so it relies
    on the guitar's volume knob for net output. Turning down from the
    guitar will reduce the distortion just a little bit, but it's still
    very present all the way down to "0". There is a noticeable change in
    overall volume by toggling the switch (between pins 8 and 1) but the
    relative level of distortion doesn't change. The guitar i'm using is a
    stratocaster with stock, passive singlecoils, and i'm not using any
    sort of effect pedal or preamp with it. Just guitar-->cord-->amp.

    Distortion is great, and desireable for some guitar applications, but
    seeing how this OpAmp was possibly created with Walkmen, Discmen and
    other small audio circuits (Cheap T.V.s?) in mind (or not), is this
    level of distortion sound excessive? Or does it sound as if i may
    have overheated something whilst soldering? (I'm getting better, but
    i'm not perfect yet). Or have i overlooked something re: speaker type
    and compatibility? (I won't lie, it was less than $10 and it's targeted
    at the piss-poor car audio market).

    Any thoughts or direction are appreciated. I apologize that this is so
    wordy and long-winded. I pared it down as much as i could, but
    articulation is not one of my strengths either :-/

  2. Lord Garth

    Lord Garth Guest

    Okay, take a look at the datasheet:

    The part is not a high performer but the -4 version can output 1 watt of
    although that was into a 32 ohm load with a 16 volt supply. Using a lower
    impedance speaker isn't going to help.

    Looking further, they show a .2% THD at a paltry 1/8 watt output.

    Take note that power amps usually have a voltage gain of less than 1, they
    designed to deliver large currents to the speaker for power. There are
    designs that might serve you better though most of what I find is toob

    The Total Harmonic Distortion is 10% ... quite poor.
  3. What kind of distortion are you hearing, describe the sound please.
    That input circuit may not be the best for guitar.
    You need to give the guitar a load which it is suited for.

    Put a 500k - 1M resistor from the guitar output to ground. Before C4.

    You may need a preamp stage before this power amp to get better sound.
    Look up buffer stages, booster circuits, input stages for guitar
    effects boxes.

    To get a soft limiting stage before this power amp, consider a diode
    limiting stage, or a cmos unbuffered logic gate, a jfet stage, etc..
  4. dB

    dB Guest

    The TBA820M, also in an 8 pin dil package, will develop a watt in 8 ohms.

    There's nothing wrong with your articulation.
  5. phaeton

    phaeton Guest

    Lord Garth sez:
    Well, there's a novel idea! ;-) These are actually the -3's, now that
    i pull them out of the tray and look at them. I had read in another
    writeup (not this pdf, but another document on National's website) and
    saw the "0.2%THD" bit, but i suppose i didn't see the "at 1/8W" part,
    nor the "10% THD" part, or the "32-ohm" load part. Silly me...

    At what point does THD become noticeable?
    I've got some toob amp schematics too, but i'll mess around with the
    solid state stuff for awhile first ;)

    Roger Johansson says:
    Soundclips are worth 1000 words: (707KB)

    The amp up all the way, the guitar's volume up all the way. Apologies
    up all the way to the Yardbirds. The switch between pins 1 and 8 is
    closed (gain == 200). A distorted sound here is expected, no biggie... (557KB)
    Same as above, but with the guitar volume turned down to 30%.
    Interestingly it didn't sound as distorted when i played it back, but
    the distortion is still there. Apologies turned down to 30% as well. (670KB)
    The switch between 1 and 8 opened (gain == 20) and the guitar volume up
    all the way. You would expect distortion here, and note that there's a
    little less (for obvious reasons). (637KB)
    Same as above, but with the guitar volume turned down to 65%. As you
    can hear, the resonance of the guitar itself (it's a solidbody guitar)
    is about as loud as what's coming out of the amp. Still some
    distortion there. If i turn it down any lower it seems to fall under
    some threshold and nothing comes out at all (which is ok, it's a
    simple, cheap toy ;))

    Then i tried it using a stompbox (Ibanez Tubescreamer) that wasn't
    switched on. The TS does not have true bypass switching, so AFAIK in
    "bypass" mode it still acts like a small preamp: (972KB)

    There is no distortion coming from the pedal, it's all amp. This is
    with gain at 200, volume all the way up, etc. I would expect
    distortion here too, and this is fine, but it remains mostly the same
    as i turn the guitar down.

    I'll try the resistor, like you suggested.

    dB says:

    I was looking at an 8W TDA2002v circuit before this one, but decided to
    go with the smaller of the two ;)
    You are so kind!

    In fact, all of you are so kind.... it's starting to look like i've
    asked some really naive questions, esp regarding the docs.... oops....

  6. Lord Garth

    Lord Garth Guest


    I'm sure the perceptible THD level is different from person to person
    with most easily able to hear 3%.

    Notice that the gain switch is placing a cap across the internal 1.35k
    resistor in the input circuitry. Recall that power amps are have a gain
    that is typically less than one...your guitar is simply over driving the
    amps input. The datasheet shows that you can add a variable gain
    as opposed to two levels, that should help you control the distortion.
    I forgot whether you were coupling the guitar to the amp through a
    cap or directly. You should be using a small cap as that would block
    any DC offset.

    To make the LM386 a more versatile amplifier, two pins (1

    and 8) are provided for gain control. With pins 1 and 8 open

    the 1.35 kW resistor sets the gain at 20 (26 dB). If a capacitor

    is put from pin 1 to 8, bypassing the 1.35 kW resistor, the

    gain will go up to 200 (46 dB). If a resistor is placed in series

    with the capacitor, the gain can be set to any value from 20

    to 200. Gain control can also be done by capacitively coupling

    a resistor (or FET) from pin 1 to ground.
  7. The sound you get in those samples have 95-99% distorsion, but
    they sound good, for being such a simple amplifier.
    I don't think it is possible to hear the resonance of the body through
    all that distorsion, but I can be wrong.
    Strange actually. Maybe there is some non-intended noise gate function
    in the circuit.
    There are many versions of the TS, but you probably have one with the
    fet switching, so you have both input and output buffer active when it
    is in bypass mode:

    Look at these web pages.

    I think the sound is very different when the TS is connected, so there
    is distorsion from that too, but a good kind of distorsion I would say.

    The amplifier is running far too high in volume if you want clean sound.
    There can be some other source of distortion too.


    Don't use the 200 times amplification, that is far too much, unless you
    want a really distorted sound.

    Put a volume control at the input of the power amp, so you can control
    the volume. A 500k pot would be a suitable load for the guitar.

    Try replacing the loudspeaker with earphones to see if the sound is
    better that way.
    That will allow you to run the amp at much lower level and you get
    a cleaner sound.

    If you do not want to use the TS all the time you should build an input
    stage for the power amp, suitable for guitar. Copy the first transistor
    stage in the TS schematic. This could be the solution to get much
    cleaner sound, because I suspect your amp input is not a good input
    circuit for a guitar.

    See that 510k resistor from the base of that transistor to +4.5V?
    That is the input impedance for the transistor stage, and it is a
    good load for the guitar.
  8. When you strum hard on the guitar the output voltage from the guitar is
    a few Volt, lets say 2Volt max.

    To get maximum output from your loudspeaker you need a few Volt, maybe
    5 or so.

    So you need only a voltage amplification of 2-3, not 200.

    If you need 200 there is some mismatch in the input circuit which is
    wasting most of the signal from the guitar.

    The amp only has to amplify the current, not the voltage very much.

    Set the amplification to 10-20, and use a volume pot to set the input
    Use a 500k pot so you can connect the guitar directly to the
    amp if you like.

    To test the amplifier without the guitar you could connect a small
    radio, mp3 player or whatever to the input of the amp and see what
    happens when you give it a clean input signal.
  9. phaeton

    phaeton Guest

    I don't think it is possible to hear the resonance of the body
    Actually I was referring to the acoustic sound of the guitar being
    picked up by the mic directly. While recording, the amp was at my
    feet and i was sitting down, and if you listen closely you can hear the
    pick and strings snapping.
    yeah, that's exactly what it acts like- an overzealous noise-gate with
    a high threshold. I would have just said "noise gate" but i didn't
    know if you were a player and/or would know what i meant ;)
    In this case it is a TS-15....a mid-90's era "Soundtank"- style. Not
    the best example of a TS, i know. I've never looked at the schem for
    this one, but i'll assume you are right- it's got a "bounceless" type
    of switch, which could very well be a momentary switch and some FET,
    and is definately not a true bypass type.

    (I've also got a TS-10 with the classic "Jack separating from the PCB"
    syndrome, and i intend on cloning a TS-9 and/or TS-808 later this
    summer. I promise not to bug you guys about those ;))

    I like the distorted tones overall. I still have yet to look at your
    links, and try (all) your suggestions, but I will (long story short,
    i have limited time at home to actually play with things)

    Perhaps i should have done this first anyways- built one up (even if a
    duplicate) on breadboard and just experimented on it. I'd like to do
    this anyways.

    Thanks to y'all for all the suggestions and tips!

  10. Lord Garth

    Lord Garth Guest

    Have you tried the variable gain pot rather than the switch?
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