Active bass -- EQ knob clipping internal preamp?

Discussion in 'Pickups & Electronics [BG]' started by momsmomsmomsmom, Feb 18, 2021.

  1. momsmomsmomsmom


    Jul 24, 2008
    Hi all,

    I have a custom-built active bass with 2 pickups (Bartolini Music Man and a piezo) and volume, blend, treble, and bass knobs.

    When I turn the bass knob anywhere near maximum, I get pretty severe distortion on the low strings.

    I have a fresh battery in there, I'm running the bass direct in to my audio interface with plenty of headroom. So it seems like the internal preamp must be clipping.

    My dad built the bass almost 20 years ago and doesn't remember the electronics used (the preamp is unlabeled). He was a classical guitar maker at the time, and this was his first time making an electric instrument. I don't remember if the distortion has always been there.

    I see from this thread that cheaper preamps have low headroom. Is this a matter of upgrading the preamp? Any recommendations or specs to look for if so?

    Worst case I can just keep the bass knob turned down, but I do like how it sounds cranked when it's not clipping.

  2. dwizum


    Dec 21, 2018
    I wouldn't imply that cheaper means less headroom. Often the same chips are used in cheap and expensive brands of preamp.

    That said, changing preamps may solve your problem. If you are dead set on needing a gigantic signal, look for an 18v preamp, ideally one with a chip that can operate rail to rail.

    You may want to do some basic setup checks on the instrument. What's the pickup height like for the magnetic pickup? Try lowering it on the bass side of the instrument. If it's too close to the strings, it'll be sending a bigger-than-needed signal to the preamp.

    Of course, the other way to look at this is that you have a solution already: turn the controls on the bass down. If you need more bass in your sound, turn the bass knob on the amp or (external) preamp up instead. I know it probably sounds attractive (along the "bigger is better" line of thought) to add MORE headroom so you can have MORE signal, but there comes a point at which you may be running the risk of missing the point of the overall system design of an electric instrument and amp. The circuit in the bass really has no other intended purpose beyond getting the signal strong enough to make it to your amp, and giving you some degree of flexibility with eq controls. You don't need 18v of headroom on a bass. No one does. It's arguable that 9v is probably already too much. You've got an amp (or whatever you're plugging in to) that's designed to amplify the signal in order to drive a speaker or other interface. It's borderline silly to try to pack a ton of power into the instrument itself considering that.

    Also, it's really cool that you're playing something your dad built two decades ago! I hope there comes a point at which my teenage kids are playing my instruments when they're in their 30s or 40s.
  3. momsmomsmomsmom


    Jul 24, 2008
    Thanks a lot for the input and perspective. I agree with your philosophy and am definitely not looking for a hotter signal for its own sake. (Turning the instrument volume knob down doesn't seem to reduce the distortion, otherwise I'd happily do that.) I just really like the tonal balance of the direct sound with the bass EQ knob cranked; even just a quarter turn lower really thins it out and takes super aggressive EQ (like, 30dB boosts) to compensate in the DAW. In my songwriting/production workflow I'd love to have a 90%-there bass sound with no knob tweaking in the box. But yeah, just keeping the knob lower is still probably the simplest (and certainly the least invasive) approach.

    Good thought about pickup height; I just tried dropping it substantially but it didn't help much. The distortion also happens with full piezo for what it's worth.
    dwizum likes this.
  4. dwizum


    Dec 21, 2018
    Can you show us a photo of the preamp? Someone may be able to ID it out of curiosity.

    I'm curious about the overall layout as well. Most times, when mixing a piezo and mag pickup(s), you need a buffer on the piezo prior to the preamp.
    Clark Dark likes this.
  5. momsmomsmomsmom


    Jul 24, 2008
    Here are a few. It's a bit of a rat's nest in there, so let me know if any other angles would be helpful. Thanks again!


    Attached Files:

  6. Piezos will generated considerable signal voltage, and if the preamp has any amount of gain, especially in the EQ stages, it's not unreasonable to expect some clipping. Is this a commercially bought preamp, or something home built? Perhaps the gain structure of the preamp needs to be reduced to match the pickups output levels.
  7. momsmomsmomsmom


    Jul 24, 2008
    It's commercially bought but I don't know what kind, just a black box with no markings. Here's a photo with the case off, on the off chance it's recognizable.

  8. The TL074 will allow about a 3.5 to 4 volt peak signal powered by a 9 volt battery, so it wouldn't take much gain to clip the output.
  9. BaileyMan

    BaileyMan Supporting Member

    Jan 8, 2012
    San Francisco
    Would that imply that swapping out the chip might help the OP's issue?
    byacey likes this.
  10. Nope; either the feedback resistors need to be reduced to lower closed loop gain, and/or raise the power supply operating voltage. Raising the power supply voltage increases the headroom before clipping, but clipping the amp input stage is a concern too, so it's better just to reduce the gain to more realistic operating levels.
    momsmomsmomsmom and dwizum like this.
  11. dwizum


    Dec 21, 2018
    That looks pretty home made to me, but I guess you never know. If you were to show us a few good closeup photos of both sides of the board, we might be able to follow the traces and tell you which resistors to swap for what alternate value(s).
  12. momsmomsmomsmom


    Jul 24, 2008
    Yeah, I assumed he bought it but I guess it does look pretty DIY. Anyway, here are a few closeups. Let me know if you need any other angles. Thanks so much again for your help! 4F2DECA9-A970-4DAA-B13E-6C2053CF274C_1_102_o.jpeg E74F7968-B374-41E3-9E52-582EDCDD69CB_1_102_o.jpeg AC20AADC-25F0-484F-839F-B748D40E4BBC_1_102_o.jpeg 352E8B6A-FFDB-4002-BFFD-BD088F23DA9F_1_102_o.jpeg