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Spector TonePump - Vol / Blend Wiring

Discussion in 'Pickups & Electronics [BG]' started by PaulMacCnj, May 19, 2011.


  1. I have a Spector ReBop4 with the TonePump and EMG HZ pups, and want to convert the Vol / Vol controls to Vol / Blend. There are a few diagrams on the Internet (including Spector's website). There are a few things that I'd like some comments about.

    1) The blend controls are stacked audio-taper potentiometers. Is there a reason why this can't be replaced with a single linear-taper potentiometer? The pups would be wired to the ends and the wiper terminal would be the output to the other circuitry. Has this been tried?

    2) Using a vol - vol control, when both are set to the max setting (0 resistance seen by the pups), the pups are in direct parallel connection with each other. This isn't a problem because they have enough internal resistance?

    3) Stacked blend control wiring typically has a connection to ground. Like with vol - vol, this results in shorting the pup outputs to ground for no output. Like #2, this isn't a problem because they have enough internal resistance?

    4) Vol - vol controls are wired on the input side of pre-amps (i.e. TonePump). Vol - blend controls are wired with the blend on the input and the vol on the output. Is there a reason why the signal chain isn't blend --> vol --> pre-amp input?
     
  2. SGD Lutherie

    SGD Lutherie Banned Commercial User

    Aug 21, 2008
    Bloomfield, NJ
    Owner, SGD Music Products
    Blend controls are not stacked audio taper pots. They are stacked M/N taper pots, or A/C taper. A/C taper is audio, reverse audio. But the best taper is M for one section and N for the other.

    What this taper does is end at the center detent, so for a 500k blend, you will get 500k from the wiper (center lug) to one outside lug, and close to zero Ohms at the other. Each stack is in reverse.

    So you wire it up so the side that reads zero is the signal, and the high resistance side goes to ground. One trick to get a smoother blend is to not connect the grounds.

    You cant use a single linear pot.
     
  3. I did some more analysis on the single linear pot blend and figured out why it would not work. At any point when the voltages are the same from the pups and the linear pot is at 50%, the cancellation would result in no signal. There may be other reasons, but there's a big one.

    SGD Lutherie: Thanks for the taper clarification! Ungrounded would be simply not connecting it to the stacked pot at all, but leaving the other connections as is, correct?

    Here's a link to Spector's vol - blend wiring diagram:
    http://www.spectorbass.com/downloads/TonePump_2PUP-BAL.pdf
     
  4. As David pointed out, blend pots are not A/A taper.

    Trying to wire a pickup to each side of one pot with the wiper as the output is a TERRIBLE idea. As you adjust the pot toward the center, you will be placing a series resistance between the pickups and the output, effectively attenuating them.

    Insertion loss is a problem with both the volume-volume setup and the volume-blend setup. When the pickups are in parallel, the output impedance drops, giving you a reduction in volume.
    The impedance and resistance of the pickups is utterly irrelevant, but the actual impedance drop can be defined as 1/([1/Z[SUB]1[/SUB]]+[1/Z[SUB]2[/SUB]])

    I have no idea what you are talking about.
    The pickups get shorted to ground, and a resistance equal to the rating of the pot, plus or minus variations in tolerance, is applied from the output to ground.

    It can be preferable to have the volume pot working over a low impedance signal, rather than a high impedance signal.
     
  5. SGD Lutherie

    SGD Lutherie Banned Commercial User

    Aug 21, 2008
    Bloomfield, NJ
    Owner, SGD Music Products
    Ungrounded you would have series resistance between the pickup and hot. So at the center both pickups wold be on, but when you solo one, you are putting the resistance of the pot in line with the signal, thus attenuating it. But you aren't shorting it to ground.

    You can use any blend pot schematic, but you have to check to see which side goes to the hot with a meter.

    STEWMAC.COM : Instructions for Blend Pots

    It doesn't matter that you have a preamp. Just replace the volumes with the blend.
     
  6. Thanks for the info. I'll be getting to this mod soon. I jammed with the bass last night and vol-vol just isn't for me.

    Another question, instead of vol-blend how about a vol-vol with a master volume? I've seen posts in other threads about how vol-blend results in 50% signal from each pup with balanced blend instead of 100% each with vol-vol on max. It's said that this affects the tone. yes? no?
     

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