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A/E Bass - Remove Preamp, Make it Passive?

Discussion in 'Luthier's Corner' started by SamHD, Nov 22, 2004.


  1. SamHD

    SamHD

    Nov 22, 2004
    Hello, this is my first post here. Looks like a great place, and I plan on spending several company hours here :D

    My question is:

    I purchased a Carlo Robelli A/E Bass from Sam Ash a while ago for a very good price. The reason for the price break was because the preamp was not working. Since I just wanted the Acoustic part of it, I bought it.

    Now, I want to fix it up so I can plug-it into an amp. To my realization, preamps cost more that what my bass is worth.

    I spoke with a local shop, and they told me that I could possibly just remove the Preamp, and play with just the pickup, and he called it making it Passive. He did mention that it might not sound very good, but I want to try it anyway.

    I want to do this myself without taking it in. Does anyone here know how I'm supposed to wire the pickup to make it passive?

    I have no idea. Without the preamp, I'd be left with the under-bridge pickup, and the jack. Should something (something electronic) go in between the two?

    Of course, a link to a diagram would be highly appreciated! :)

    Thanks,
    Sam
     
  2. FBB Custom

    FBB Custom TalkBass Pro Commercial User

    Jan 26, 2002
    Maryland
    Owner: FBB Bass Works
    I have no idea what the specs of the preamp and transducer are, but the undersaddle pickups are generally very high impedance. This means that if you wire it directly to your jack, it will sound wimpy and trebly, probably.

    Piezo pickups are usually buffered (brought to a lower effective impedance) inside a preamp unit so that they can drive an amp with the correct response. There are commercially available buffers out there, but as you have probably found, they run $50+
     
  3. SamHD

    SamHD

    Nov 22, 2004
    Thanks for the reply FBB. I can live with a fix around $50... it's the $200 preamps that I don't think is worth spending on my particular bass.

    I'm new at this, but from what I understand, you think that this could possibly work if I had a buffer between the pickup and the jack, right?

    What is a buffer exactly? Are they rated a certain way (like resistors, etc), and how can I gauge which type I need.

    Sorry for the dumb questions, but I really don't know much about some components.

    Thanks again for the reply.

    Sam
     
  4. FBB Custom

    FBB Custom TalkBass Pro Commercial User

    Jan 26, 2002
    Maryland
    Owner: FBB Bass Works
    Buffers are like preamps without the EQ. Bartolini makes one called the MPB-1. I think it runs in the $60-80 range. Most manufacturers of piezo type pickups sell a buffer/preamp solution.

    They are rated by their input and output impedances. The MPB-1 takes a 3Mohm input and buffers it down to 3Kohm. The major difference in piezo preamps is their input impedance since they are generally geared to match the piezo they are sold with. You'll have to figure out what your own pickup's output impedance and hunt down a buffer/pre that comes close.

    Will Carlo not sell you a replacement?
     
  5. SamHD

    SamHD

    Nov 22, 2004
    Thanks for clarifying, FBB. I think I'm starting to understand this now.

    I tried to reach Carlo Robelli directly, but nowhere could I find a direct number. After searching the web for a while, I figured out the Sam Ash was the primary contact for them. I called their 800-tech line, and spoke with a tech there. His advice was to return the bass to the store for a replacement, but then I told him that I purchased it "as-is", so then he recommended that I'd get a Fishman or similar product for it. Apparently, you can't buy replacement parts for Carlo Robelli's... Seems odd, but that's OK, as I didn't pay much for the thing anyway.

    In regards to the pickup's impedance.... I'm assuming that it is measured with a standard Ohmmeter, correct??

    I'm going to try that this evening and see what kind of numbers I get from it, so I can narrow-down the buffer I need.

    Thanks again for your help.
     
  6. luknfur

    luknfur

    Jan 14, 2004
    DIXIE
    FWIW:

    don't know squat about undersaddle pickups or what the variations on a preamp used with them may be but you can pickup up a used Bart NTBT for about $50 with a trim pot that you can control gain with and maybe adjust it for what you need.

    Otherwise, I've got a Bart DAT3.1 (or 1.3) buffer laying around here somewhere that I have no use for and would let go cheap in you're in the US and would have any use for it.