Home Forums GuitarPCB Build Support MKC – low gain

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  • #4892
    Anonymous
    Inactive

    Hello everybody!

    I recently finished an MKC pedal. While it sounds fine basically, I noticed that it doesn’t have a lot of gain, especially when comparing it to another Klon-style pedal I borrowed from a friend.

    So I hooked up my oscilloscope and found out that the gain stage IC1B only has a maximum gain of 3! According to this site it should have a gain of 100 at 1kHz:

    https://www.electrosmash.com/klon-centaur-analysis

    I already checked all resistor and capacitor values, changed the opamp, checked for continuity and resoldered some solder points. I also tried lifting R13 and R7, but still the same gain.

    Here are the voltages at IC1:

    1: 4,66V

    2: 4,66V

    3: 3,66V

    4: 0V

    5: 4,65V

    6: 4,66V

    7: 4,69V

    8: 9,29V

    Any idea what might be the problem?

    Thanks!

    #4894
    Billy
    Moderator

    It should be a low to mid gain pedal R15 is the resistor to change you can do the ultra drive mod you’d wire in a dpdt switch with the stock 47K on one side and a 4K7 on the other which gives you masses of gain

     

    So check R15 for cold joints etc initially

    #4895
    Anonymous
    Inactive

    Hey Billy, thanks for your reply!

    I am really talking just about the second Opamp (IC1B) here. With a gain of 3 it hardly pushes the Ge-diodes into saturation. R15 is just a summing resistor which has nothing to do with the gain of this stage, so changing it would just make the preceding signal louder and push the second opamp into saturation (which is not what we want, bearing in mind that the original had an even higher headroom in this stage!).

    I know it’s not supposed to be a high gain distortion pedal, but between 3 and 100 there’s quite some difference! The circuit around IC1B uses basically the same values as the original, so it should at least be in the ballpark….

    #4896
    wilkie1
    Moderator

    The circuits are not the same.  The GPCB MKC adds a 100K resistor at R26 which changes the max gain of that stage to about 5.  (1+ 470K ((R10))/ 100K ((R26))+15K((R9))=5.1.

    Obviously, if you wish to increase the gain of that stage, you may reduce or jumper R26.

    The circuit should still exceed unity output level with the stated values.  If it does not, then there be a different issue.

    #4905
    Anonymous
    Inactive

    Thank you very much! I overlooked that R26 affects the gain of this stage, now it all makes sense…

    I jumpered R26 and now it sounds like a Klon to me! 😉

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