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A/P switch question

Discussion in 'Pickups & Electronics [BG]' started by thiessen3.14, Aug 24, 2012.


  1. thiessen3.14

    thiessen3.14 Supporting Member

    Nov 22, 2002
    wichita, ks
    When using a Bart NTMB, a 25k vol. pot is called for. When installing an active/passive push/pull volume pot, a 250k is used. I understand why, in passive mode, a 250k is necessary. But, how does this affect the active sound? It seems odd that a 250k pot could be substituted for a 25k pot. Is there enough adjustment on the trim control on the preamp to account for this? And, one more question: does the passive/active switch cause a "pop" when switching between modes?

    Thanks in advance.

    Matt.
     
  2. ctmullins

    ctmullins fueled by beer and coconut Gold Supporting Member

    Apr 18, 2008
    MS Gulf Coast
    I'm highly opinionated and extremely self-assured
    You can do it two ways - you can use the 25k volume pot between the preamp and the output jack, or you can use the 250k volume pot between the pickups and the preamp. The latter allows for a preamp bypass for passive mode.
     
  3. It doesn't. The pot runs before the preamp, so that the signal impedance that it controls remains the same whether the preamp is in the circuit or not. The addition of a pot parallel to the preamp input does drop the input impedance a bit, however, which will darker the sound a bit, as the resonant peak shifts downward.

    You're moving the position of the pot in the circuit. A 25k volume only works in the circuit because the output impedance is significantly lower at the preamp's output. Try to run the passive signal through that load, and it will basically just short out. The 250k pot sees the same input impedance at all times.

    Sometimes they do. You can reduce switch pop with fixed resistors.
     
  4. thiessen3.14

    thiessen3.14 Supporting Member

    Nov 22, 2002
    wichita, ks
    as i look at these diagrams:

    [​IMG]

    [​IMG]

    ...it seems to me that in active mode, the volume is in the same position that it is in the "switchless" or active only position, i.e. after the preamp. am i missing something?
     
  5. That diagram is incorrect. The output of an active/passive switch can only feed a volume pot if the pickups are active, so that the output impedance is similar in both modes. This is not the case, however, as a 250k pot is specified. If you try to feed a preamp through a 250k pot, it will function like an on/off switch with very little usable range. Conversely, as, I mentioned earlier, if you feed a high impedance signal through a 25k pot, it's going to get loaded down.
     
  6. thiessen3.14

    thiessen3.14 Supporting Member

    Nov 22, 2002
    wichita, ks
    so, since bartolini has it wrong, how should it be? another thought: is it possible that the 5k gain trimmer has enough range to make the preamp work with a 250k vol?
     
  7. The volume pot goes before the active/passive switch. This is quite simple; don't overthink it.

    It doesn't matter what the gain pot is set to, the output impedance will remain the same.
     
  8. ctmullins

    ctmullins fueled by beer and coconut Gold Supporting Member

    Apr 18, 2008
    MS Gulf Coast
    I'm highly opinionated and extremely self-assured
    L6M won't steer you wrong; that's for sure. But sometimes he uses big words that make my brain fuzzy. So see if this helps:

    Ignore the volume pot on Bartolini's diagram. Instead, route the yellow wire to the push/pull switch as indicated on the second graphic that you posted. The signal from your blend control (or pickup selector switch, or vol/vol, or single pickup) comes into the middle lug on the right; the ......


    Oh, wait.

    The second graphic looks okay to me when the switch is in passive mode, but shows a 250k pot coming after the preamp in active mode. Or am I reading it wrong somehow?

    Anyway, L6M won't steer you wrong....
     

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