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Customizing my own preamp. I want to go in deep...

Discussion in 'Luthier's Corner' started by Trevorus, Jan 10, 2006.


  1. Trevorus

    Trevorus

    Oct 18, 2002
    Urbana, IL
    I posted this also in pickups, but I think it might be better answered here... I hope. If the double post is a problem, I will delete one.

    I have a bass with a preamp which I would like to fiddle with. It is a large circuit type (as in not surface mount) that had easily identified components. Now, the things I would like to accomplish are changing the general tone structure of the pre, and make the bass sound more lively. I know this will be a complicated venture, but I am willing to undertake it. There's a reason I am going back to school for an EE degree. I love electronic fiddling.
     
  2. So what's the question?
     
  3. Trevorus

    Trevorus

    Oct 18, 2002
    Urbana, IL
    What kind of things can I change to give the EQ a different voicing? How can I change it to sound more lively? Which components shall I replace? I can take a pic of the unit itself, if that would help.
     
  4. Try identifying the tone section of the preamp. It'll mean you'll probly need to examine the board and translate it into a bit of a schematic. You should be able to modify the frequencies that the bass/treble(/mid) by changing the values of the capacitors.

    The other thing I would try would be a bypass of the tone stack. That should turn it into a straight gain/buffer type of deal.

    Its probly also possible to change the input and output impedances by swapping a few resistors around.
     
  5. Like davese said, you're going to have to translate it into a wiring diagram. Then you'll be able to see what you have. If you had an oscilloscope and a function generator it would be helpful as well.
     
  6. tjclem

    tjclem Supporting Member Commercial User

    Jun 6, 2004
    Central Florida
    Owner and builder Clementbass
    Another place to look for help would be the MIMF forum.t
     
  7. pilotjones

    pilotjones Supporting Member

    Nov 8, 2001
    US-NY-NYC
    As said, first you have to make a schematic representation. Then you need to decide what "more lively" means to you-- boost at 3k? 8k? Then you can get a circuit simulator and see what the current response is, and what would be the effects of changing different components. Or as said, identify the tone control sectoin, and just try alternate cap and resistor values.
     
  8. You could randomly wire in various resistors and capacitors in parallel with the ones already there. ;)
     
  9. you could leave the existing preamp alone and build a new one from scratch, specifically designed to meet what you want? That is assuming, of course, that you know what you want.
     
  10. Trevorus

    Trevorus

    Oct 18, 2002
    Urbana, IL
    I'll take a peek at the preamp and maybe take some good pics. I'll see what kind of filter networks this thing has in them. I really like the preamp in my MIA Jazz Deluxe, but I don't know if that thing would translate over. I would love for this thing to have a mid control.

    I have an EMG eq built for a 5 string which just doesn't seem to bring out the character of the bass that much. The current setup is just not boosting the sounds I want. I want the bass to control a little more toward the mids, and the treble could be shifted down as well. I can't remember which way to go with capacitance values to achieve certain frequency responses.
     
  11. Trevorus

    Trevorus

    Oct 18, 2002
    Urbana, IL
    I have noticed that my pre seems to get dull when I turn it down. Hmmm, maybe 18 volt would be better.
     
  12. Rodent

    Rodent Supporting Member Commercial User

    Dec 20, 2004
    Upper Left Corner (Seattle)
    Player-Builder-Founder: Regenerate Guitar Works
    could also be your volume pot stealing some of your highs. There are ways to keep this from happening if it's the case.

    All the best,

    R
     
  13. Trevorus

    Trevorus

    Oct 18, 2002
    Urbana, IL
    I know about the cap/resistor setup on the volume, but how would this affect an active setup. BTW, here's a pic of my preamp.

    [​IMG]
     
  14. Rodent

    Rodent Supporting Member Commercial User

    Dec 20, 2004
    Upper Left Corner (Seattle)
    Player-Builder-Founder: Regenerate Guitar Works
    you wouldn't happen to also have a shot of the other side, would you?

    All the best,

    R
     
  15. Trevorus

    Trevorus

    Oct 18, 2002
    Urbana, IL
    I can get one. Just a sec.
     
  16. Trevorus

    Trevorus

    Oct 18, 2002
    Urbana, IL
    [​IMG]
     
  17. pilotjones

    pilotjones Supporting Member

    Nov 8, 2001
    US-NY-NYC
    Likely even more so. In the active tone controls, there's an RC setup on the feedback loop of an op amp or transistor; I would expect it to be at least as sensitive to R and C values as a passive circuit is.

    If you look at the Aguilar OBP-3 diagrams, you'll see that access is provided to some of the internal circuit points, allowing you to change the midrange frequency by either shorting internal components, or by adding capacitors.
     
  18. Whats the opamp it has in it? You've got a fairly simple method to swap out and change around different opamps there. Quick and cheap.
     
  19. Trevorus

    Trevorus

    Oct 18, 2002
    Urbana, IL
    It's a TL062CP. I was thinking of a TL082 in it's place. I want something with more power, and more liveliness.
     
  20. Looks like a good choice. IMO I think you'll need one with a JFET input, which the TL082 has.