Bright cap across volume vs bright cap across mixing resistor

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Johndandry, Jan 1, 2018.


  1. Johndandry

    Johndandry Supporting Member

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    I have an amp with a pair of 270k channel mixing resistors. I wanna add a touch of brightness..what difference would it be between adding a bright cap on the volume control vs a cap across the channels mixing resistor?
    Thanks
     
  2. pdf64

    pdf64 Member

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    The latter (ie cap across a mixer resistor) may be referred to as a peaking cap.
    A peaking cap will tend to make one channel brighter at the expense of making the other channel duller, all else being equal. It will tend to reduce stability.
    Whereas a bright cap (assuming no peaking cap) will have minimal affect on the other channel and won’t affect stability.
     
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  3. xtian

    xtian Member

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    Also, and please check me on this: a bright cap on volume pot only affects the sound when the volume pot is set low, and the higher the volume pot is turned, the less the bright cap affects the tone. The treble peaker affects the tone, no matter the setting of the volume pot.
     
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  4. Muttlyboy

    Muttlyboy Member

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    I didn't know that there would be an instability issue with a peaking cap.

    Would a smaller resistor in series with the peaking cap help with the instability?

    (For instance, a 56k resistor in series with a 500pf cap, and that assembly in parallel with one of the 270k channel mix resistors)

    I also didn't know that putting a cap across one channel resistor would effect the tone of the other channel...why would that be?
     
  5. Badside

    Badside Member

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    Such a cap increases treble frequencies and as such may get you over that threshold where oscillation now occurs. That said I have built many Marshall type amps with treble caps in all sorts of place and it never caused issues except when I was using a PCB not designed for the amount of gain I was trying to get.

    I have the treble cap on the mix resistor on a switch on one of my amp, and there is a definite treble cut on the other channel when engaged. That is because the “other” mix resistors allows a path to ground via the other channel’s volume control. So with a cap across it, your high frequencies have a direct path to ground via the non-bright channel volume control unless that volume is on full. Basically, it’s a useful mod when combining channels like we do on Plexis, but it makes individual channels too different and too interactive imho.

    What kind of circuit is in this amp? And are you trying to make the whole thing brighter or just one channel?
     
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  6. Badside

    Badside Member

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    It affects the tone at all but the two extreme settings. Thats one of the reasons it’s hard to get a clean tone with a Marshall that has a 500-1000pF bleed cap, but it’s also how they get that really crunchy tone at mid gain. I tend to go with a smaller cap, 120-240pF. No cap at all can sound dull due to Miller effect, but can help take down bright guitars.

    Although the mix resistor cap is not as volume dependant, it’s effect will be affected by the way other channel volume as per my post above.
     
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