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How to tame high freqs from piezo?

Discussion in 'Pickups & Electronics [BG]' started by SebbyNC, Nov 3, 2010.


  1. SebbyNC

    SebbyNC

    Mar 17, 2005
    Guadeloupe
    Hi TB'ers,

    I have a bass with Ghost piezo bridge pickups (with buffer) and magnetic pickups, they complement each other really well. However, the frequency range extends too far in the top end to my ears - I love how much the lows sound from the piezo, but the highs tend to be overemphasized.

    So, I was wondering if there's any simple way to tone down the high frequencies, maybe with a simple cap or resistor (I'm no electronician)? Adding a new tone pot is not an option, as I already have 5 pots, 2 of them being concentric. Any idea? I don't mind trying different cap/resistor values to find the best balance.

    Thanks in advance.
     
  2. FunkMetalBass

    FunkMetalBass

    Aug 5, 2005
    Phoenix, Arizona 85029
    Endorsing Artist: J.C. Basses
    Does your bass have separate outputs for the piezo? If so, you can just wire a small cap (maybe .001 uF) across one of the leads (so it's in parallel). If not, that's a toughy - the capacitor will react with the magnetic pup's signal in parallel.
     
  3. All you need is a simple LPF, so a capacitor across the output should do it.
    Or, why not just EQ it out?
     
  4. SebbyNC

    SebbyNC

    Mar 17, 2005
    Guadeloupe
    The magnetic pickups go through a Noll preamp (with its own EQ), then to the Ghost AcoustiPhonic buffer, but on the auxillary slot. The piezo pickup is plugged into the AcoustiPhonic pizo circuit. Both mag and piezo outputs are still separate when going out of the buffer, they just go each to either the ring or tip lug of a 9-lug jack connector.

    The problem with general EQ at the bass output is that it would also affect the magnetic signal, which is not desirable. I just need to lower the highs from the piezo if possible.

    Should FunkMetalBass's cap be put after the buffer circuit, or could it be be put between the piezo pickup and its buffer?
     
  5. FunkMetalBass

    FunkMetalBass

    Aug 5, 2005
    Phoenix, Arizona 85029
    Endorsing Artist: J.C. Basses
    The Piezo and the buffer, me thinks. I could be wrong.

    On a side note, how do you like that Noll? I've been looking at getting one, because it seems like the absolute best 4-band preamp out there (damn close to the Aguilar OBP-3's frequency centers).
     
  6. Thunderthumbs73

    Thunderthumbs73

    May 5, 2008
    The free version:
    Turn the high-end/treble down on any amp/preamp or deactivate the tweeter in any cab, if you can.

    Otherwise, the preceeding comments...
     
  7. Anything you do after the buffer is going to affect both the piezos and the magnetics, so you will want to do it before.
     
  8. SebbyNC

    SebbyNC

    Mar 17, 2005
    Guadeloupe
    As I said 2 posts before yours, I don't want to affect the tone of the magnetic pickups, only of the piezo, so the amp is not going to satisfy me. Thanks anyway.


    In fact my Noll is a 3-band with parametric mid frequency. I love its flexibility and I think it sounds great, I'm sure the 4-band is great too.

    Ok so I'll try that solution, if like FunkMetalBAss said the cap should be in parallel, it should be quite easy to implement. Do you both agree the .001 uF is a good starting point?
     
  9. Thunderthumbs73

    Thunderthumbs73

    May 5, 2008
    Sorry. Just read the OP, and responded. I didn't read the whole thread.
     
  10. MNAirHead

    MNAirHead Supporting Member

    Hi pass cut filter?
     
  11. staindbass

    staindbass

    Jun 9, 2008
    i think the same thing of those pickups. too extended and it makes it sound fake, like one of the guitars on an old 80's robert plant record. (sounds good on the guitar i must say though). so then i tried just adding a pinch of piezo to add the glasslike highs to the normal magnetic pickup, then i noticed since they are 8 inches apart the phase was so far out the piezo still messed with the low end of the magnetic pickup. in other words they were picking up start and stop of each vibration at slightly different times leading to some kind of cancellation. i ended up not using it. like the other guy said, a cap across the + and - of the output jack should work fine. i think somewhere between 3-6 khz roll off would do what your seeking. im sure there is a formula online somewhere where the output impedance will factor in the figurings to give you the right capacitor. johnny a. / staind
     
  12. FunkMetalBass

    FunkMetalBass

    Aug 5, 2005
    Phoenix, Arizona 85029
    Endorsing Artist: J.C. Basses
    AKA, Low Pass Filter? That's exactly what a cap in parallel does.
     
  13. FunkMetalBass

    FunkMetalBass

    Aug 5, 2005
    Phoenix, Arizona 85029
    Endorsing Artist: J.C. Basses
    If you do it on the output jack instead of before the buffer, you'll affect the magnetic pickup as well.

    The formula for a cutoff frequency is 1/(2*pi*R*C). Since, to my knowledge, piezos have very high impedance, you'll need a smaller capacitor to achieve the same effect as the typical magnetic pickup. If I'm wrong and they have very low impedance, you'll need a much larger capacitor. TS, I think .001 uF is a good start.
     
  14. SebbyNC

    SebbyNC

    Mar 17, 2005
    Guadeloupe
    Thanks guys. The 5 piezo saddles are all connected to a summing board, from which 2 wires go to the buffer: one carries the signal, the other one is ground. Sorry if it sounds dumb (I guess I am dumb in electronics), but is it correct to solder the cap across both signal and ground wires (one leg soldered to the signal lead, the other leg to the ground lead)?

    Regarding cap value, if I understand the formula, the lower the C value, the higher the cutoff frequency (i.e. more highs pass through)?

    Regarding the general sound of piezo on electric bass, I guess it's a matter of taste, you like it or you don't. I wouldn't say it sounds fake -what is "fake"? we could say magnetic pickups sound fake too, it depends on your point of reference. Piezo is different. When I hear Wooten play with piezo on, I like it as much as I do like his regular magnetic sound. You may not like Wooten's sound though, again it' a matter of taste. I like blending piezo and magnetic, I just can't do it as much as I would like to because of the high frequency content that my poor technique tends to emphasize... I guess ;) I'll try to improve technique, but in the meantime cutting some highs down will help me.
     
  15. FunkMetalBass

    FunkMetalBass

    Aug 5, 2005
    Phoenix, Arizona 85029
    Endorsing Artist: J.C. Basses
    You don't need to wire it across the ground wire as well.

    Yes, that's how the capacitor values work.
     
  16. SebbyNC

    SebbyNC

    Mar 17, 2005
    Guadeloupe
    So one cap leg is wired to the signal wire, but where should I wire the other leg? To the cavity ground?
     
  17. The other leg should be grounded.

    It doesn't matter where to, just as long as it's grounded.
     
  18. SebbyNC

    SebbyNC

    Mar 17, 2005
    Guadeloupe
    Thnaks guys for your ehlp, I'll report when I have time to get the cap and to wire it.
     
  19. bassbenj

    bassbenj

    Aug 11, 2009
    I presume the piezo saddles sum and then go to the buffer preamp and then the output of the buffer (it translates say 10 megohm down to a normal cable driving impedance of some Kohms) goes to a mixer circuit where it and the magnetic output are mixed together presumably with controls to determine how much of exah you mix in. Probably there is also EQ that operates on the MIXED signal as well. (as does your amp EQ). Is that right?

    If that is the case, then the place to filter would be to place a cap on the OUTPUT of the buffer. If that works, you are home free. But if it doesn't (takes way too big a cap for example) then you'll need to insert a resistor in series ahead of the cap. Just placing a cap on the output uses the driving impedance of the buffer as the series resistor which may or may not work.

    I'm guessing the series R should be around 10k. This is the R in the frequency formula and C is the capacitor value. The formula should get you in the ballpark (choose say 2,000 Hertz or so for a frequency) And from then just try cap values until it sounds good.

    If the Buffer is part of the mixer then you'll have to filter the sound at the Piezo ahead of the buffer. This is VERY tricky because the piezo impedance needs to be VERY high...like 10 megohms. If it isn't, then the bass notes won't come through. So this means that your filter would need a VERY large resistor and tiny cap. And keeping the resistor large (10 meg) makes the whole thing more prone to hum if it's not completely shielded. Like I said it's a tricky business.

    Let's just hope you can do this post-buffer!

    Good luck.
     
  20. SebbyNC

    SebbyNC

    Mar 17, 2005
    Guadeloupe
    Bassbenj, in fact we can consider the magnetic and piezo to be separate signal chains up to the output jack. There's no EQ nor mixing circuit for both. As I started with a magnetic only bass, I didn't want to complicate things and just added the piezo. Thus, the EQ is only for the magnetic pickups, and instead of a mixing board or balance pot, I have 2 separate Vol pots. The buffer circuit is only buffering the piezo and providing voltage to the mag preamp.
    I don't know if it's clear, it's a quite complicated design, so here's a diagram which I hope will make it clearer:

    final-diagram.

     

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