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Passive tone circuit question

Discussion in 'Pickups & Electronics [BG]' started by Vanceman, Feb 6, 2018.


  1. Vanceman

    Vanceman

    Feb 14, 2007
    So. Cal.
    I built an outboard passive tone circuit to use with my Carvin LB70. As an active bass with the ability to go passive, and without any tone controls in passive mode, I felt the need to do this. It's simply a 250k pot, .047 cap (Orange Drop, only because I already had one), an input jack, and 1/4 in cable sticking out, all encased in a clear plastic round container. You can see the Orange Drop, and it looks way sexy. Cable from bass plugs into container, and the container plugs into bass. I'm pretty proud of what I built, and if I knew how to post a picture, I'm sure all would get a good laugh.

    It's a huge improvement over no tone control at all, but this thing can go REALLY dark. Almost too dark, but it is adjustable. My question: I think the pots in the bass are 500k, and I know the external is 250k, so is this an ok match, or would using a 500k pot in the tone circuit be a better match?
     
    Last edited: Feb 6, 2018
    fermata likes this.
  2. You're sapping some brightness & output from the bass by using a 250K Ohm pot.
    If it's too dark when rolled all the way back, use a 22 nF cap instead (0.022 uF).
     
  3. Vanceman

    Vanceman

    Feb 14, 2007
    So. Cal.
    I see, the easy solution! Thanks.
     
    Killed_by_Death likes this.
  4. DavePlaysBass

    DavePlaysBass

    Mar 31, 2004
    CO
    A 500K pot would give your more high end with the tone control in the full clockwise position (no treble cut). Rotation to add treble cut would be different. Less cut at first and more in the mid to later travel.

    If you are not using an audio taper pot, you may want to. It will give you a more uniform sweep during the last 50% of travel. If using an audio taper make sure your rotation direction is the same as if it was mounted on the bass (clockwise = least treble cut).

    As mentioned changing the tone capacitor value will change the response. 0.022uF will pass more highs than a 0.047uF. Keep in mind that the response will be an interaction of the passive tone control, the pickup impedance (inductance and resistance), and the capacitance of the instrument cable (longer length = more cable capacitance).

    I would use the 500K pot if you find the bass is too dark in the full clockwise tone position. I would consider changing to a 22nF if like the full clockwise brightness but feel it is rolling off too much going from the 80% to the 50% point.

    Also, if you want to eliminate the massive roll off experienced at last 10-20% of the counter clockwise side of the pot travel (full treble cut), put a resistor in series with tone capacitor. Rough calcs in my head say try a 5K ohm resistor. That would be roughly the 10% point of a audio taper 250K pot.
     
    Last edited: Feb 6, 2018
  5. Vanceman

    Vanceman

    Feb 14, 2007
    So. Cal.
    The bass is not too dark in the full clockwise, and seems ok most of the counter clockwise turning. There is a massive roll off right at the end that I find odd and intriguing, but not sure if it's a usable tone.
     
  6. megafiddle

    megafiddle

    May 25, 2011
    Does the tone circuit work normally if you keep the passive volume in the bass at "10" ?

    If the volume is rolled back, it can place a lot of resistance between the pickup and tone circuit, forming an RC lowpass filter.

    This occurs because the tone circuit is on the "wrong" side of the volume circuit.

    -
     
  7. DavePlaysBass

    DavePlaysBass

    Mar 31, 2004
    CO
    The plot below shows an audio taper pot being rotated thru its mechanical positions in 20% mechanical steps. Note that at the 0% (full counter clockwise position), the roll off is big and out of proportion to the other curves. This is because the pot resistance is essentially gone and its just the pickup feeding straight into the capacitor. Put a 5-10K resistor in series with the cap and everything will be the same except the last 20% of travel will be more consistent with the first 80%.

    L4bUQyJ.
     
  8. Vanceman

    Vanceman

    Feb 14, 2007
    So. Cal.
    I will try this when I get home. I think there are differences between full volume and reduced settings, but I can remember if I even tried full volume.
     
  9. Vanceman

    Vanceman

    Feb 14, 2007
    So. Cal.
    So, when I dime the volume, the tone circuit works normally, and sounds correct. Thanks megafiddle. For now I will keep the volume up when playing with this. However, there are some serious dark tones lurking in there, that I wanna try on What I Got by Sublime or Smashmouth's Walkin' on the Sun.
     
  10. megafiddle

    megafiddle

    May 25, 2011
    Thanks for confirming it.

    Something you might try experimenting with, start with the volume at "10" and the tone at "0". Then use the volume to adjust the tone rolloff.

    -
     
  11. fermata

    fermata

    Nov 10, 2015
    Mountain West
    .033 uF is also a great tone cap value. And there's also the trick of putting a resistor (somewhere in the 5-10K Ohm range) between the cap and ground; it limits how dark the tone roll-off gets and decreases (or eliminates) the resonant peak when the tone control is rolled off all the way.
     

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