Passive tone control question

Discussion in 'Pickups & Electronics [BG]' started by dadodetres, Mar 5, 2006.

  1. dadodetres


    Dec 19, 2004
    i was wondering, in a passive tone control (lets say a jazz bass), does the tone control que cut frequancy? so when you move the knob the cut frq moves.... or the cut is set in a frequency and you control how much cut?

    I readed an article to Justion Chancellor (of Tool) and he sayd for the intro of "the patient" he used the tone control on his WAL, and it sounds like a Wha Wha, what do the Wal`s tone control have so special?
  2. Trevorus


    Oct 18, 2002
    Urbana, IL
    The tone control on the wal is probably an active preamp with a sweepable mid. A normal passive bass has it's tone control frequency set by the size of the capacitor. It allows a certain amount of treble to be shunted to the ground, and not come out of the bass.
  3. dadodetres


    Dec 19, 2004

    is it possible to make a passive circuit in which you can change the frequency of the cut with turning the knob?
  4. Trevorus


    Oct 18, 2002
    Urbana, IL
    There are variable capacitors, but they are generally very big (for bass purposes anyways).
  5. Not quite. The passive tone control is part of an overall circuit that includes the pickup resistance, inductance and capacitance, the volume pot resistance, the cable capacitance and the amp imput impedance. The overall response is an underdamped low pass filter, but, interestingly, what determines the overall cutoff frequency is the cable capacitance.

    This is what a passive tone control does at different positions of the pot (this is for typical values of a Jazz Bass pu, 250k pots and a 0.033 uF tone cap):


    As you can see, towards the end of the tone control range a resonant peak appears again but at a much lower frequency, so with the right combination of values and eq, you could probably make it sound kind of like a wah, I guess.
  6. DavePlaysBass


    Mar 31, 2004
    In a simple J or P type passive tone control, a constant capacitor value is gradually coupled into the signal path via the tone pot. When the tone pot is wide open (no treble cut), the capacitor has essentially minimal effect or loading of the pickup.

    As the capacitor is coupled into the signal path with the tone pot, the varying resistance / constant capacitance impedance creates different roll off points for the low pass filter as shown below.


  7. sotua


    Sep 20, 2004
    US Northeast
    It's a low pass filter with a boost at the cutoff frequency.
  8. rumblinbass


    Aug 22, 2003
    Wimberley, TX
    Yes, it requires a little work though. Get a rotory switch and several caps it can switch between. That will move the cutoff frequency.
  9. Pneuma


    Feb 14, 2004
    So what is the difference between using a .05uf, .025uf, and .033uf cap? Just the frequency? What frequency is for each?

    Both the graphs above look exactly the same (after acounting for the different scales, log vs linear), yet one is for a .047 (.05uf), and the other for a .033uf.