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Different Pot values between preamps?

Discussion in 'Pickups & Electronics [BG]' started by berklee46, Jun 20, 2004.


  1. berklee46

    berklee46 Supporting Member

    Dec 19, 2000
    MA
    I just got a Bartlolini NTMB preamp for one of my Status basses to try out option of having a variable midrange control. I see that the pot values for the Bart are different from the stock Status preamp. Is this something I should be concerned about?

    Here are the values...

    Status stock preamp:
    Volume: 10K Log
    Blend: 200K Log
    Treble: 10K Lin
    Bass: 10K Lin

    The Bartolini:
    Volume: 25K ohm
    Blend: Dual 250K
    Treble: 50K ohm Linear
    Bass: 50K ohm Linear
    Mid: 50k ohm Linear

    It's already installed and seems to work, but now I'm wondering if it's not performing up to it's full potential because of the differences?
    Thanks for any info.
    Scott
     
  2. David Wilson

    David Wilson Administrator Staff Member Administrator Supporting Member

    Oct 14, 2002
    Lower Westchester, NY
    Are your status pickups active? IIRC, a lot of them are. If so, you might want to get a 25K or 100K blend pot in there instead of 250K.
     
  3. berklee46

    berklee46 Supporting Member

    Dec 19, 2000
    MA
    These pickups are passive as far as I know. I had the original single-coil ones replaced with Rob's new 2003 humbuckers:
    From the Status site:
    What change would I notice with a 25K or 100K blend pot as opposed to the 250K that's on the Bart preamp now? Or does it not need to be changed if the pickups are passive?

    Also, the differences in the volume/treble/bass pots don't really matter much?

    Thanks again for the help.
     
  4. David Wilson

    David Wilson Administrator Staff Member Administrator Supporting Member

    Oct 14, 2002
    Lower Westchester, NY
    duh, I just noticed in the first post that it said 200K blend (that's unusual, I read it as 20K). That means it was meant for passive pickups.

    The other pot values (i.e. bass/treb/mid) are determined by the preamp itself so I wouldn't worry about it.

    The vol pot in preamps depends. You have preamps like the OBP-3, where they give wiring diagrams with master vol before the preamp. In this case, the pot values DO matter and should be adjusted for passive / active impedance (250/500 vs 25/100) and adjusted to taste (250k/500k) for single coil / humbucker.

    In preamps such as the J/U-Retro and the Demeter, there is an active buffered blend going on. In this case, the impedance of the incoming signal doesn't matter so they can be used with any pickups without change. With non-buffered blend preamps, the K value of the blend pot does matter and should be adjusted as above. So, why doesn't everyone use buffered blends then? Because they require power to operate, so you can't do a true active/passive bypass and if the battery fails you're up SC.

    The bartolini's put the master volume AFTER the preamp output. At this point, the signal from your pickups has been adjusted for whatever eq and so the signal requires a 25K vol pot.

    As to the whole master volume before/after the preamp, I guess the idea is to maximise the input gain to the preamp and adjust post-processing as opposed to applying eq to a lower volume signal when vol adjust is done pre-preamp.

    That's just my non-EE take on it, anyone else is welcome to chime in and correct/add.
     
  5. berklee46

    berklee46 Supporting Member

    Dec 19, 2000
    MA
    Thanks for the help David, I appreciate it.
    It still hurts my head, but I do have a slightly better grasp of the concepts. :)

    Scott
     
  6. A9X

    A9X

    Dec 27, 2003
    Sinny, Oztraya
    David is pretty well on the money with what he said, but I'd like to clarify some points;

    Ideally, it's best to have the impedance or the following device be several times larger than the one preceeding it. For example, a 10k pickup usually works into a >100k (often 500k/1M) pot in a passive setup, so as not to load down the coil and get the maximum output. In an active setup, there may be a buffer/amplifier immediatly after the pup, which will have an output impedance of likely a few hundred ohms max. Take the output of this device and then feed it into a pot, and the pot only needs to be, say 10k. There are engineering benefits to doing things this way, that I'll add later if you're interested.

    With the tone control pots, these values in an active pre will be determined based upon the capacitor(s) that they work with, to determine the frequency at which the controls actually work. In your example, moving from 10k to 50k will move the eq points an octave or more (sorry not to be specific, but the equations are not coming to my sleep fogged brain) away from where the designer intended. They may be useful in this new setup, but they might not too.

    David said earlier
    <i>As to the whole master volume before/after the preamp, I guess the idea is to maximise the input gain to the preamp and adjust post-processing as opposed to applying eq to a lower volume signal when vol adjust is done pre-preamp.</i>.

    Not quite right. Having the volume control after the active stages allows for a better signal to noise ratio. All electronics have an inherent amount of noise, which for arguments sake is constant. So you want the max signal into the preamp (until it overloads at least) to get the maximum signal to noise ratio. With a pot before the preamp, at any setting except full on, there is less than the maximum possible signal getting into the preamp, so the SNR is lower.
    As I mentioned in the first paragraph, an amplifier will have a lower output impedance than a passive pup and pots etc. A lower output impedance at the bass guitar jack, has benefits in further increasing SNR, as well as hum pickup etc, especially in environments like a stage where there are lighting dimmers and other devices which can throw out a lot of electrical/RF (radio frequency) rubbish which is easily picked up.

    I hope that's at least sort of clear.
     
  7. David Wilson

    David Wilson Administrator Staff Member Administrator Supporting Member

    Oct 14, 2002
    Lower Westchester, NY
    Thanks, that's what I was trying to say and you said it much more eloquently :)