Unexpected results - Passive P pups w/ buffer preamp

Discussion in 'Pickups & Electronics [BG]' started by Geri O, Jun 9, 2021.


  1. Geri O

    Geri O Endorsing Artist, Mike Lull Guitars and Basses Gold Supporting Member

    Sep 6, 2013
    Florence, MS
    I'm referring to the not-favorable results when I installed a buffer preamp into a passive Precision-style bass.

    The idea was to simply boost the output of the P-bass to match the active preamp output of my Jazz-style bass. I would like to use one transmitter with both basses. I absolutely love the tone of this bass, I'd just like more output. I've pulled the trim controls in the Jazz-style bass all the way down, but it's still considerable hotter than the P-style bass.

    So I installed a Bartolini AGB/918-2 buffer preamp into the P bass tonight. The preamp is described by Bart as an adjustable-gain buffer preamp. No tone controls. The results were not good. The tone now was very bright, very harsh. I immediately uninstalled the preamp and rewired the P bass to its stock form.

    So where did I go wrong? Do the values of the tone control and/or the capacitor on the tone control need to be considered because of the preamp? I doubt I'll try this experiment again anytime soon, but I'd like to know how to proceed should I decide otherwise.

    I looked at the John East P bass preamp which is a drop-in unit and requires an external source to charge the internal battery which is kinda odd. Besides, John East has pulled that preamp off of the market, citing a need to redesign the battery system.

    Thanx for any guidance.
     
    Killing Floor likes this.
  2. sunbeast

    sunbeast Supporting Member

    Jul 19, 2006
    Los Angeles, CA
    The Bart buffer pre has a flat response, but the passive controls in a bass do not. Standard passive 250K volume and tone controls as well as long instrument cables combine to lower the resonant frequency and treble response of the attached pickup(s), while a buffer preamp works to eliminate many of these passive losses. The “harsh” highs you hear could just be a matter of finally hearing the full frequency range of the pickup you are using. Did you put the buffer pre before or after your Tone and Volume controls? If all you are looking for is a boost in output you should be able to keep the same original controls and add the buffer pre after the original Volume control. The buffer will still eliminate the considerable high-end losses of your instrument cables though, which may or may not result in a “harsher” tone. Depending on how your Tone control is wired (whether directly to the pickup or alternatively to the output signal at the Volume control) it should still work mostly the same.
     
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  3. Geri O

    Geri O Endorsing Artist, Mike Lull Guitars and Basses Gold Supporting Member

    Sep 6, 2013
    Florence, MS
    Ahhh…if I remember correctly, I wired the preamp right after the pickup instead of after the volume control. That was suggested by the Bart’s wiring instructions.
     
  4. Crater

    Crater

    Oct 12, 2011
    Dallas, TX area
    Yes. Bartolini's wiring diagram show 25kΩ pots for volume and tone instead of the typical 250kΩ pots used in passive electronics. The tone cap value is also different, 0.47 microfarads instead the standard value of passive, 0.047 microfarads.
     
    Last edited: Jun 10, 2021 at 7:01 AM
    dkelley, BrBss and Geri O like this.
  5. Geri O

    Geri O Endorsing Artist, Mike Lull Guitars and Basses Gold Supporting Member

    Sep 6, 2013
    Florence, MS
    Man, I blew it on this attempt, then. I didn’t even observe the pot and cap values Bart listed on the diagram.

    Maybe I’ll attempt the preamp mod again later down the road. Thanx for the help, ya’ll.
     
  6. spiritbass

    spiritbass Supporting Member

    Jun 9, 2004
    Ashland, MO
    I've used the Redeemer buffer from Creation Audio Labs in a couple of basses. I didn't get the results you experienced. It's unity gain. It won't provide any boost.
     
    TrevorR, RichSnyder and Geri O like this.
  7. sunbeast

    sunbeast Supporting Member

    Jul 19, 2006
    Los Angeles, CA
    I think the assumption from Bart is that you want to take full utilization of the buffer’s ability to eliminate passive losses, so putting it right after the pickup would do this. If you want to only take advantage of a higher output and retain most of the passive tone then putting the original controls before the buffer would work better. If you do it the Bart suggested way then you change pot values because the impedance of the buffered signal means that the original pots would function more like on/off switches rather than having the gradual taper we are used to. With the controls before the buffer the preamp shouldn’t effect the way they work at all (though you will probably still notice an increase in high frequencies, just not as sharp).
     
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  8. Geri O

    Geri O Endorsing Artist, Mike Lull Guitars and Basses Gold Supporting Member

    Sep 6, 2013
    Florence, MS
    Great helpful info. This particular bass (A Mike Lull P5 with the Seymour Duncan pup designed to Mike's specs) is the brightest passive-P bass tone I've ever heard. It's very bright when practicing solo, but it sounds positively wonderful mixed in with a band or recording rhythm section with all the wonderful P bass grind I've come to know and love. I'd hate for it to be any brighter, honestly.

    The whole point of this exercise is to have the output of both basses very close. I use a Lectrosonics wireless system for my bass. This is the wireless system you see on referees at the Super Bowl and other national events. It's industrial-strength wireless (robust, quality, not necessarily any higher output), so the transmitter is VERY expensive. And it literally sounds better than a wire! As I can't afford a second pack, I plan to use the same transmitter pack for both basses. And having both basses with similar output is helpful to the soundfolks (I know the output will never be identical).

    At this point, I've decided that (A) get an external 2-channel box that I can set the gains for each channel and switch when I switch basses, or (B, and the most reasonable, logical solution...) just play one bass all night and have the other one as a spare. That's my solution for now.

    But the day ain't over.....:roflmao: :D
     
    darwin-bass likes this.
  9. Ric5

    Ric5 Supporting Member

    Jan 29, 2008
    Colorado
    I disclose nothing
    For preamps I like Audere and John East
     
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  10. clickclack

    clickclack

    Jan 20, 2018
    Hi,
    1-Install it after the passive tone pot and even the volume pot, just before the output jack. That will keep it close to the original "loaded" signal, only now with lower impedance and higher V.
    2-Or, use a booster with no buffer. Specifically(as this can be confusing even for the shops that sell this stuff), you want a booster with high impedance output.

    1. would solve your problem though.
     
    Last edited: Jun 10, 2021 at 12:02 PM
    Geri O likes this.
  11. Altitude

    Altitude An ounce of perception, a pound of obscure. Supporting Member

    Mar 9, 2005
    Denver, nee Austin
    Same. I have that buffer in two passive basses.
     
    RichSnyder likes this.
  12. darwin-bass

    darwin-bass Supporting Member

    Mar 29, 2013
    Salem OR
    Is the wireless transmitter on your belt or guitar strap? If not, perhaps a floor preamp plus A/B switch is the ticket.

    TC Spark Mini Boost?
     
  13. Geri O

    Geri O Endorsing Artist, Mike Lull Guitars and Basses Gold Supporting Member

    Sep 6, 2013
    Florence, MS
    The transmitter stays on my belt. Mute the tuner, change basses, and I’m off and running again.

    The TC Spark is looking like the ticket. Only one of the basses needs a little increase.
     
  14. Barts allow variable boost
     
  15. spiritbass

    spiritbass Supporting Member

    Jun 9, 2004
    Ashland, MO
    Got that from the original post. :cool:
     
    dkelley likes this.
  16. Killing Floor

    Killing Floor Supporting Member

    Feb 7, 2020
    Austin, TX
    I also saw the East P retro was pulled out of production. I agree, the battery situation is way different on a P because of the need to remove the pick guard. So I get why he tried a rechargeable solution. Good luck.
     
    Geri O likes this.
  17. Killing Floor

    Killing Floor Supporting Member

    Feb 7, 2020
    Austin, TX
    Another option is a volume pedal or an EQ like the Source. Maybe that complicates things. But that is actually how I use mine, a level for several instruments.
     
  18. clickclack

    clickclack

    Jan 20, 2018
    Geri, after a second glance on the subject, how about looking at it the other way? There's another, super simple solution:
    You can cut the volume on that other bass -permanently! -Such that when it's full on, the volume is very close to your P bass.

    You can do that with a single resistor. There will be very slight loss of highs(as happens with passive vol. pots) but I don't think you'll notice that or if you do, dislike the effect. Now what does that cost? A penny!:D

    R between Hot and Ground. I don't know the volume difference but start with 330K-470K maybe? The lower the value, the more signal you send to ground.

    Edit:
    LOL, yet another even simpler one: Place a marker, sticker, smtg on the Jbass Vol Pot where it matches your Pbass.
     
    Last edited: Jun 10, 2021 at 3:42 PM
  19. Geri O

    Geri O Endorsing Artist, Mike Lull Guitars and Basses Gold Supporting Member

    Sep 6, 2013
    Florence, MS
    I like the resistor idea on the hot bass. Definitely easy enough to do (for me, anyway). Thanx for the idea.
     
  20. micguy

    micguy

    May 17, 2011
    The tone of a passive bass is due to the loading of the pickup - the volume pot and tone pot (even at full) are resistive loads. The cable you use is a capacitive load. Remove that capacitive loading, and yes, it’ll be brighter.

    You can add capacitive loading by adding...a capacitor (before the buffer) Along with a fixed resistor (for some resistive loading), it’s very easy to get the same sound as without the preamp (assuming the sound you want is with the controls wide open). With the volume at less that full on a passive bass, things are different - the volume control and cable capacitance interact so your volume control also changes your tone. A buffer can fix that.
     
    Jefenator likes this.
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