Lowering output gain from active bass

Discussion in 'Pickups & Electronics [BG]' started by ShowKing, Jul 7, 2013.

  1. Hey guys

    I currently own 2 active basses.
    One has a gain trim pot in the cavity so i can control the output level.
    The other bass however does not have any when i look inside the cavity.
    This bass has a very hot output and I'd like to reduce the gain from its onboard preamp to match my other bass.

    Do you guys know if there is a way to do this?

    Photo of the preamp attached. ImageUploadedByTalkBass1373173297.712546.jpg
  2. Why not adjust the trimmer pot on the bass that has one? You're trying to adjust the one that is not adjustable.

    You'll need to add a trimmer pot.
  3. I have the trim pot on max on the other bass and that's still lower output than the bass without a trim pot.

    i'll see if i can find out how to add a trim pot.

    any info on doing this would be appreciated also.

  4. Ok, that makes sense.

    Try a 100k trimmer in series with the signal path, after the preamp. Take the connection that goes to the jack, and wire it to either side of the pot, and then connect the wiper terminal (center) to the jack. Leave the third terminal of the pot open, unless you cannot attenuate enough. If that is the case, ground it.
  5. Rockin Mike

    Rockin Mike

    May 27, 2011
    Why not just turn the volume knob down?
  6. mmbongo

    mmbongo I have too many basses. Supporting Member

    I'd replace the preamp, but that's just me. Lowering the volume knob is not precise and impossible to set it exactly the same twice, plus it can get moved without you knowing it. It could also affect your tone, as would adding a trimmer pot. All of my basses now have preamps with trimpots.
  7. JustForSport


    Nov 17, 2011
    A trimmer pot may alter the tone in a good way,
    if it alters it noticeably,
    or may need a small value cap in series with it to retain tone.
  8. ex-tension


    Jun 11, 2009
    If you can trace the circuit a little, we can try to find the gain set resistor and replace it with a trimpot (and a serial resistor).

    So, first, see what kind of opamp is it (probably a dual one). Then you'll try to find out which one of the two stages is the gain stage. Acording to this info we'll try to find feedback (=gain set) resistor.
  9. Munjibunga

    Munjibunga Retired Member

    May 6, 2000
    San Diego (when not at Groom Lake)
    Independent Contractor to Bass San Diego
    It looks like the preamp is glued into the cavity pretty solidly, so doing a trace could be difficult. I'd worry a little about pulling clad off the board by trying to remove it.
  10. ex-tension


    Jun 11, 2009
    You're right. If it's glued solidly, trying to remove it would damge the PCB. It looked like a double sided tape to me.

    Anyway, ShowKing asked info on doing it. I tried to guess some info on electronics. He'll let us know about the mechanics :)
  11. Steve Dallman

    Steve Dallman Supporting Member

    I would add a 100k trimpot after the preamp, wired like a volume control. It should work well, with no tonal changes. EQ tweaks like treble bleed circuits are relevant when dealing with high impedance, but the output of the preamp is low impedance, and a simple volume control you can preset will be fine.

    Disconnect the output wire from the preamp, either at the jack or the preamp or cut the wire. From the preamp connect to the end terminal of the trimpot. The wire/connection to the jack goes to the middle terminal, and the other outside terminal goes to ground. If the trimpot works backwards, swap the outside connections, but who cares. You will be setting this and forgetting it's in there.

    A 50k trimpot would probably be OK too.

    And/or, lower the pickup(s) some on the hot bass, and raise the pickup(s) on the not-so-hot-bass.
  12. Thanks for the replies everyone.

    I've never worked with electronics before so i'm a newb.

    The preamp is indeed glued on pretty strong in there.
    Ive thought about changing preamps but because ive never done it before i'm a little scared of messing it up.

    Whats the difference between 100k and 50k trimpots?
    Would any trimpot work well or should i be looking for something in particular for bass? (Links to sites would be appreciated)
    Is there any soldering involved?

    Many thanks
  13. Zooberwerx

    Zooberwerx Gold Supporting Member

    Dec 21, 2002
    Virginia Beach, VA
    JOOC, what's the bass? What's the pre and is it stock?

  14. The difference is 50k Ohms. Pots come in hundreds of different values, but you need one with a range that is appropriate for the signal impedance. I suggest 100k, because it will allow you to attenuate more, if needed. A 50k pot will not provide the same range of attenuation, but it will spread its range of control throughout the rotation, allowing you to better fine tune the level of attenuation.

    The trimmer pot should be of linear taper. Do not use log taper or reverse log taper pots, because they will be very difficult to fine tune, since small rotations cause large jumps in value, at particular points.

    Note that I suggested leaving the ground off of the trim pot, unless you cannot attenuate enough with the value you choose. This is because is is preferable to avoid loading the signal with parallel resistance.
  15. Steve Dallman

    Steve Dallman Supporting Member

    Since we don't know the output impedance of the pickup, I suggested 100k. Wired as a volume control (voltage divider) it will go from full output down to zero. With a 50k, as long as the impedance is low enough, will act just like the 100k.

    You would have a hard time finding trimpots that are anything but linear taper.