Need help on op amp guitar distortion effect

Discussion in 'The Projects Forum' started by EarlAnderson, Nov 13, 2011.

  1. EarlAnderson

    Thread Starter Member

    Nov 13, 2011
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    i am building a guitar distortion effect with a single LM308 op amp, and i for the life of me can not figure out how to get enough distortion from it. i set the gain to about 200 and i still get very little distortion, and it sounds like crap. does anyone have any suggestions on how i can get more, better sounding distortion.:confused:
     
  2. MrChips

    Moderator

    Oct 2, 2009
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    The simple way to create distortion is to clip the signal with two back-to-back diodes and then amplify.
     
  3. Audioguru

    New Member

    Dec 20, 2007
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    The LM308 is so old that it is not made anymore.
    There are thousands of "guitar fuzz" distortion circuits on the internet and most use diodes to clip the signal instead of amplifying too much and letting the output of an opamp clip the signal.

    The LM308 drops frequencies higher than 2kHz when its gain is as high as 200 so it can't produce the high frequency harmonics of distortion.

    If you use a better and newer opamp that has good high frequency response then it can produce lots of distortion.
     
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  4. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    I remember building a "fuzz box" for the guitars in my band back around 1969. It had two controls: one was "volume" and the other was "attack" which boosted the amount of distortion. It was a two transistor circuit no op amps at all.. I think it ran off a 9V battery.


    I'll bet there are schematics on the net.

    I might even have a copy somewhere of mine.
     
  5. EarlAnderson

    Thread Starter Member

    Nov 13, 2011
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    yeah at first i thought it had something to do with the power supply (i was using a single rail power supply), but your point does make sense.
     
  6. nomurphy

    AAC Fanatic!

    Aug 8, 2005
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    Roger Mayer fuzz.
     
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  7. EarlAnderson

    Thread Starter Member

    Nov 13, 2011
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    Alright i tried everything and for some reason, still no distortion. I have attached a schematic of my design. if anyone sees any problems or flaws with my design, please tell me so i can fix them.


    P.S. sorry about the image quality, i was gonna scan it, but my scanner isn't compatible with a mac, so i had to take a picture of it instead;)[​IMG]
     
    Last edited: Nov 15, 2011
  8. Audioguru

    New Member

    Dec 20, 2007
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    With your 100k gain control at max (100k) then the gain is 101 times the input level.
    The diodes will clip the output when it tries to exceed about 0.65V peak.
    The tone control will do nothing until its resistance is almost zero (the very low output impedance of the opamp) when its muffled sound will become a more distorted and a lower level sound.
     
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  9. EarlAnderson

    Thread Starter Member

    Nov 13, 2011
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    okay so how could i raise the output impedance, or what could i do with the tone control to make it work better
     
  10. rogs

    Active Member

    Aug 28, 2009
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    Your circuit looks very similar to the famous MXR distortion + circuit of 30 years ago. That worked really well.
    See that schematic here: http://www.freeinfosociety.com/electronics/schemview.php?id=562

    Two suggestions on your circuit:

    - Forget the tone control. Let the amplifier provide that.

    - Try using germanium diodes. That's what the original used. Means the unit will 'clip' more easily, and will probably sound less harsh.
    Not essential, but worth experimenting with.
     
  11. EarlAnderson

    Thread Starter Member

    Nov 13, 2011
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    yeah my circuit is pretty similar to that isn't it. both designs are very simple. but for some reason on my design, im not getting any clipping at all, yet rather a more harsh buzzy sound. i think it's coming from the 1M resistor connecting the power supply to the noninverting input on the op amp
     
  12. EarlAnderson

    Thread Starter Member

    Nov 13, 2011
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    okay i can see what you mean there, but for some reason, i don't get any smooth clipping at all, but rather a harsh "buzzy" sound that just doesn't sound anything like a fuzz or distortion. i think it's coming from the 1M resistor connecting the power supply to the noninverting input on the op amp, because when i take the 1M resistor out, the buzzing stops, but i still don't get any clipping. i might try replacing that 1M resistor with a 500k and add a preamp stage before the main op amp so maybe ill get a higher voltage on the output, and therefore more clipping. if you suggest anything different please tell me.
     
  13. EarlAnderson

    Thread Starter Member

    Nov 13, 2011
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    the attached is a schematic of my new design. it is the same as my old one, except i have added a preamp before the main circuit.i did this because i found out that if a guitar pickup has, lets say, 100mV output p/p, than with my original design with a gain of 100, i only got a 1V output, which is a little low for driving silicon diodes. i added the preamp with a gain of 3 to turn the 100mV into 300mV so i get an output of about 3V p/p. i haven't tried building this yet, but hopefully it will bring some results (good ones)
     
  14. Audioguru

    New Member

    Dec 20, 2007
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    The original circuit produced a "buzz" probably because its unshielded wiring picked up mains hum. Its input impedance was high at 500k ohms to avoid loading down the pickup's level and keeping its high frequencies.

    Your new preamp has a very low input impedance of only 5k ohms which shorts and reduces the level from a high impedance guitar pickup a lot and it also cuts the shrill high frequencies that an electric guitar needs. Its input is properly biased at 4.5V but it has a DC gain of 3 so its output will be saturated as high as it can go and it will not work.
     
  15. Audioguru

    New Member

    Dec 20, 2007
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    Your math is wrong. 0.1V x 100= 10V, not 1V. Maybe your gain-setting resistor values were wrong or the tone control was shorting its output.
     
  16. EarlAnderson

    Thread Starter Member

    Nov 13, 2011
    166
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    But this just doesn't make sense. If I'm getting an output voltage of more than a volt, than I just don't know why I'm not getting any diode clipping on the output. I have double checked, even triple checked my original schematic after I made all the corrections, and I just don't know what the **** is wrong. If I'm getting a gain of 100, than I shOuld be getting, more than enough to clip the signal. This is just proving to be a huge ******* pain in the ass so far
     
  17. Audioguru

    New Member

    Dec 20, 2007
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    The circuit is not designed properly. It cuts low frequencies:
     
  18. EarlAnderson

    Thread Starter Member

    Nov 13, 2011
    166
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    oh okay i get what your sayin. with the 47nf capacitor and the 1k resistor, that gives me a rolloff frequency of about 3.3 kHz, so as the frequency drops, so does the gain. so if i replace that 47nf capacitor with a 4.7uf capacitor, it gives me a rolloff frequency of about 40Hz, so the gain should stay the same until it goes below 40Hz, so that should filter out a lot of the "noise" from the feedback loop, which is just what i'm looking for:D
     
  19. Audioguru

    New Member

    Dec 20, 2007
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    The capacitor in the feedback loop does not "filter noise", instead it cuts the gain at frequencies below its calculated value.
     
  20. EarlAnderson

    Thread Starter Member

    Nov 13, 2011
    166
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    Right. well i built this circuit with all of the modifications done to it, and FINALLY i get some distortion from it, although not much. i think the diodes aren't really "responding" that well to the low output impedance, however im not quite sure.
     
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