Fun with Preamp Installation, or What to do at 4am when it's still not working.

Discussion in 'Pickups & Electronics [BG]' started by Justyn, May 20, 2004.

  1. Justyn

    Justyn Supporting Member

    Jun 24, 2002
    Richmond, VA
    So I picked up this older model OBP-1 preamp thinking I had three different basses for which I was considering a preamp upgrade and surely I could fit it into one of them. I decided on dropping it into my Fernandes APB-100 fretless five, which is about the best bass that no one's ever heard of. Sounds great, but I'd been thinking I'd like a little more oomph from the low end boost.

    Enter Aguilar.

    So...copious notes were written before removal of the existing preamp since I've learned from experience that the phrase 'Oh, I'll remember where that goes' is probably the most dangerous phrase in the decidedly brief history of preamp wiring. Took out the existing pre and started putting the Aggie into place. Got it all wired up and...nothing...touching the volume knob resulted in a lovely buzz, and I could actually boost the bass and treble on that same buzz (which at least let me know I'd followed the diagram right as far as wiring the preamp), but it was still just a buzz...not particularly useful.

    Back down to the basement and the soldering pencil.

    Hours later I've resoldered wires to the jack in about ten different combinations and removed the blend pot from the circuit, wiring one of the two pickups directly to the volume pot and leaving the other one unwired completely just to simplify the circuit and make sure the jack is properly wired. Eventually I I get things to wear I can hear a very faint tone out of my bass...great...the jack is wired...time to put the blend back in and add the second pickup.

    Only when I wire it all together, I get nothing but more noise. So I unwire the blend pot and the pickup again and go back to the single pickup/volume pot combo and it works again....very faintly, but it works.

    And then it hits me. Just to check my theory I swap out the stock pickup for a seymour duncan I have lying around and out comes clear, full tone. A quick look at my original preamp notes confirms my theory:

    The pickups are active. The battery powers both the preamp and the pickups, so when I yanked the stock pre and wired in the aggie, power was cut to the pickups.

    With this stunning realization, I went to bed. It was about 4:30am and the birds were starting to chirp and that sound always reminds me of all-nighters in college were if you were still up when the birds were out it meant you a) only had a few more hours to finish that paper and b) were going to be one tired hombre for the rest of the day.

    So...I realized today that I might be able to wire an additional battery to power the pickups. I used to have a set of active SDs that I never installed because I was unfamiliar with how to properly wire the active pickups into my existing preamp circuit, but when I consulted the diagram on the Duncan site, I got a good idea of how I can make all this work.

    All this because I just *had* to put that Aggie preamp in something...

    So yeah, I'm tired, but, as Dexter says, it was a good day for science.

    Stay tuned for chapter two once I put operation add-a-battery into practice.

  2. Edwcdc

    Edwcdc I call shotgun!

    Jul 21, 2003
    Columbia MD USA
    Good luck!
    It's amazing how much time passes when you work on projects like this.
  3. Justyn

    Justyn Supporting Member

    Jun 24, 2002
    Richmond, VA
    And how adept my soldering has become:)

  4. I LOVE working through the night on anything hands on or technical. It's what i love and NOTHING can pull me away from doing something with my hands...i even love the trial and error part. I just love those sort of situations. The greatest feeling in the world for me is standing back at the end of a days work and reflecting on what i built/fixed/improved....Its just an awesome feeling, like nothing else i have ever felt.
  5. Benjamin Strange

    Benjamin Strange Commercial User

    Dec 25, 2002
    New Orleans, LA
    Owner / Tech: Strange Guitarworks
  6. Justyn

    Justyn Supporting Member

    Jun 24, 2002
    Richmond, VA
    I think I also dropped five pounds in the process as my work table is, of course, in the basement, and my amp is up on the second floor in my little studio, and of course, rather than bring my amp down to the basement for this particular evening of soldering ('wrenching' for you bike mechanics out there), I think I traversed those two flights of stairs up and down about two dozen times. My little dog couldn't wait for me to go to work this morning so she could finally get some sleep.

    If nothing else, I got to listen to the new Tortoise album a bunch of times as well. Picked that up along with the new Scofield Trio disc. Good stuff.

  7. John East

    John East Commercial User

    Jan 10, 2002
    Oxford UK
    Owner of E-Pro & East UK
    > So...I realized today that I might be able to wire an additional battery to power the pickups.

    Well you shouldn't need an additional battery, just splice the pickup +VE power in wires into the +VE from the battery which goes to the pre power in.

  8. Justyn

    Justyn Supporting Member

    Jun 24, 2002
    Richmond, VA
    Indeed. Initially I ran a separate battery for the pickups (the original battery, for which there's a seperate box) and then two more for the pre just to keep things isolated so if one part worked and another didn't I could troubleshoot a little easier. I sat down last night to work on it again and though I couldn't get the blend pot to work (I could get the neck pickup or the bridge pickup, but at the center detent I got no sound at all) I was able to simply substitute individual volume controls for each pickup, and while I was doing that I rewired the battery arrangement so I've only got two in there now all hooked together so they power both the preamp and the pickups per your advice above.

    Sounds fantastic.

    Thanks for the help,