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  • Schematic question?

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    My question is the following, if the bright capacitor C76 of the attached schematic will allow high frequencies to bypass the potenciometer VR3 , will the capacitor C75 cancel this effect?

    If i remove both C75 and C76 from the schematic it will be the same of having both C75 and C76 in circuit?

  • #2
    Originally posted by Rod View Post
    My question is the following, if the bright capacitor C76 of the attached schematic will allow high frequencies to bypass the potenciometer VR3 , will the capacitor C75 cancel this effect?

    If i remove both C75 and C76 from the schematic it will be the same of having both C75 and C76 in circuit?
    No, it won't be the same with the capacitors removed. The effect will only cancel when the caps are bridging equal resistances i.e when the pot is set to 500k 500k. When the gain pot is set low there will be treble boost and when the gain pot is set high there will be treble cut.

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    • #3
      Ians rave,
      The "standard" (C76) bright cap boosts the highs at low gain settings and has little effect at high gain settings - because of the corner frequency (a pole in eng. speek) formed by the top portion of the pot (wiper to top) resistance and the capacitor value.

      The added C75 rolls off the highs at high gain settings and has little effect at low gain settings.

      So the answer to Rods's question is NO - well almost NO, at least the answer is NOT significantly.
      C76 is "active" at low gain settings (when resistance from wiper to top is maximum) and C75 is active at high gain settings (when resistance from wiper to bottom is maximum).

      The higher the gain the more abrupt the various clipping,clamping and saturating of the following overdriven stages. This means higher order harmonic distortion products (with correspondingly higher frequencies). Even harmonics are always good as they are octaves of the fundamental, of the ODDS - 3rd harmonic is good for a bit of "edge", 5th is OK but above that you run into some ODD harmonics (from the faulty memory it is 7th 9th 11th) which are musically "quint". Figures as little as 0.001% of these 7th, 9th etc. harmonics sound absolutely awful

      ASIDE: which is why some Solid State Hi Fi Amps with 0.001% distortion figures can sound horrible while a tube amp with 0.5% distortion can sound glorious).

      You need to get rid of these high order harmonic products, and limit the following stages production of them, by knocking off some high frequencies at the gain control. Which is just what C75 is doing.

      In any moderate to high gain channel, if you are considering the bright cap (C76) then match it with a C75 like the example above. It is good design. Less important and probably not needed for a Low Medium gain Channel with a bright cap.

      Cheers,
      Ian
      Last edited by Gingertube; 02-12-2017, 07:26 AM.

      Comment


      • #4
        This particular schematic is from the Marshall DSL15C. (also used in the DSL40C)

        Thanks for the explanation of the role that the two caps play.

        (This as a favorite 'take them out' of "modders".Then they wonder why the get all sorts of high freq junk coming through at high gain settings)

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