need help on tone control

Discussion in 'The Projects Forum' started by EarlAnderson, Nov 19, 2011.

  1. EarlAnderson

    Thread Starter Member

    Nov 13, 2011
    166
    4
    I am building a guitar distortion effect, and it sounds good so far, except that there are a few bugs with the tone control. the tone control only works when i turn the potentiometer all the way up. is there anyway to resolve this problem? if so, please help me.
     
  2. jimkeith

    Active Member

    Oct 26, 2011
    539
    99
    The way it is implemented here will not work well--the capacitor will have no effect until the series resistor is close to zero, at which time the amplifier probably goes into current limit attempting to charge /discharge the capacitor this limiting the slew rate.

    What I recommend is that you study the traditional tube-type guitar amp circuit and adapt it to your op amp circuit--perhaps scale down the resistances to make it more compatible and /or feed the tone control output into another high input impedance voltage follower. Most guitar tone controls have been standardized to the following circuit--they tend to be tricky to understand and defy modification as there are so many interacting variables--been there--done that.

    Check out this web page: http://drtube.com/guitamp.htm

    This schem is typical: http://drtube.com/schematics/fender/bassman100.gif
     
  3. Adjuster

    Well-Known Member

    Dec 26, 2010
    2,147
    300
    If you simply want to get your crude treble cut working better than at present, insert a resistor of say a few kΩ to 10kΩ in series with the feed into the top of the tone pot. This will give the op-amp a more reasonable load to drive. The volume pot should remain tied to the top of the tone pot, so that only the op-amp output and the diodes see the extra resistance.

    Having said that, your present circuit output comes straight from a 100k pot, which will give a high and variable output impedance. Level and tone may be affected by external loading. I would agree with the last poster that including a buffer would give a better output, and would leave you free to have a more flexible tone control.
     
  4. EarlAnderson

    Thread Starter Member

    Nov 13, 2011
    166
    4
    Okay i put a buffer on the output of the tone control. will the attached circuit work:
     
  5. Adjuster

    Well-Known Member

    Dec 26, 2010
    2,147
    300
    No, that is not likely to work well: as has been explained to you by jimkeith, when the tone control resistance is turned to zero, the capacitor will be directly in parallel with the first operational amplifier output.

    This is not a very reasonable load for the op-amp, and the low output impedance will result in a very fierce control action, all at one end of the control, as you have seen.

    At the very least, a series resistor is required after the first amplifier.
     
  6. EarlAnderson

    Thread Starter Member

    Nov 13, 2011
    166
    4
    Okay here is what i have come up with (schematic attached below). this circuit has fixed high and low cut filters. the out put from each filter is mixed together with the 100K pot. I shouldn't have any problems here, because as i turn the pot, i'm not altering the values of any of the filters, but rather mixing the output of both filters. i might have the values of the filters a little off wack, but i should have it debugged in no time
     
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