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How do I bypass an onboard preamp?

Discussion in 'Pickups & Electronics [BG]' started by Archon113, Aug 11, 2005.


  1. Archon113

    Archon113

    Oct 21, 2004
    I want to find out what my active bass sounds like as a passive, as I'm considering having an active/passive switch installed or even completely missing the preamp out of the signal chain. What I want to do is connect the pickups straight to the output jack of the bass and check out the sound of the passive pickups, but I don't know how to wire it up like this. Can anyone help me out?
     
  2. David Wilson

    David Wilson Administrator Staff Member Administrator Supporting Member

    Oct 14, 2002
    Lower Westchester, NY
    ok, you don't say anything about what bass / pickups / preamp you have so all I can do is speak in generic terms - specifics will vary according to your bass/pickups/preamp. If your bass has a PCB preamp with the pots mounted on it, putting in a bypass will require more work.
    You also don't say what if any experience you have in doing this type of work.

    Your output jack has three lugs. One of those is connected to the black wire from the battery harness. Another is connected to ground - look for black wire from bridge soldered here, and probably another wire from the preamp.
    On the last lug, there will be a wire coming directly either from the preamp or a master volume control (different preamps do it differently). This is the hot signal lug.
    From your pickups, there will be two or more leads. I don't know whether you have active or passive pickups. I'm going to assume standard passive pickups.
    You should have one wire going to ground (either back of a volume pot or maybe even directly to jack - same lug as the bridge ground), and another wire going to the lugs on either blend pot or volume pot.
    Those are the pickup hot wires. Unsolder those from their original location (taking care to remember which was soldered where previously). Unsolder the preamp/master vol output wire from the hot signal lug on the jack Then solder the pickup hot leads to the hot signal lug on the jack that you identified earlier.

    Again, I've said all this using a bunch of assumptions. Which, as you know, sometimes make an ass out of u and me ;)
    So, I'd really prefer you attached a photo of the cavity before doing anything.
     
  3. Archon113

    Archon113

    Oct 21, 2004
    After reading your post and looking at the wiring of the bass, everything you assumed seems to be correct. The first picture below is of one of the volume pots. The blue wire on the right is the hot wire from the pickup, the black wire in the middle is the ground wire and the one the right connects directly to the other volume pot (I don't know what this does.)


    [​IMG]



    The second picture shows the output jack - sorry about the horrible quality. It's probably easier if I explain what's there, so here goes. The black wire coming in from the bottom (which you can't really see) comes from the battery harness. The black one from the top is connected to the preamp, and I can't seem to find a ground wire.


    [​IMG]



    Now the thing that confuses me is this: the hot wire from the pickup to the volume pot splits into two right before it connects to the pot. A small white wire comes out of the blue one and they are soldered on in different places. If I was to unsolder these, where would I connect each one on the output jack? There is a similar situation on the jack itself. As you said there are 3 lugs, and one of them has the battery harness connected to it. The other two are occupied by the preamp wire, and another white wire that splits off from the main preamp wire.

    Should I disconnect the hot pickup wires (both parts) and connect them to lugs on the output jack, with the main pickup wires going to where the main preamp wire was and the smaller ones going to where the smaller preamp wire was?

    Another question: will I have any control over the pickup volume after this rewiring? It seems like the pickups will be wired straight to the output jack, bypassing both the volume pots and the preamp.
     
  4. David Wilson

    David Wilson Administrator Staff Member Administrator Supporting Member

    Oct 14, 2002
    Lower Westchester, NY
    Those pictures are too poor quality to make anything out at all.

    re: the pickup wiring.
    Sometimes passive pickups have three wires: hot, cold and shield. The cold and shield. Can you outline exactly where each wire is soldered to.

    Of course, that's what you asked for in your original post:
     
  5. Archon113

    Archon113

    Oct 21, 2004
    Never mind, I tried my suggestion out and it worked. Thanks very much for the help though :)
     
  6. Archon113

    Archon113

    Oct 21, 2004
    How much more work would it be to be able to use the volume pots while bypassing the preamp? Although I generally keep both pickup volumes at full anyway, it would be nice to have the option to adjust the balance.
     
  7. David Wilson

    David Wilson Administrator Staff Member Administrator Supporting Member

    Oct 14, 2002
    Lower Westchester, NY
    it depends entirely on the preamp / knob configuration you have.
    Could you take some photos of the cavity/preamp/knobs, at a higher resolution than the photos you posted earlier.