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3 band Passive EQ

Discussion in 'Pickups & Electronics [BG]' started by Mykk, Aug 6, 2012.

  1. Hey Guys,

    I'm sure this has been discussed before. I've done a few searches and didn't turn up quite what I was looking for.

    Has anyone here successfully made/wired a 3 band passive EQ consisting of a Low Pass filter, Band Pass Filter & High Pass filter?

    I understand that in a traditional sense this wouldn't be an EQ, but rather would just remove certain frequencies from a passive circuit.

    Could you please share your wiring schematic. After my 'Dimebucker' in an Epi Les Paul swap, I have them wired in series with a 200k resistor inline straight to the jack. Leaving me with four vacant holes in my LP.

    A Volume, Low, Mid, High could be an interesting set up. I just need the basics, I can experiment with different caps to dial in the specific sounds for this particular bass.

    Also, anyone have a source for no-load linear pots?

    Cheers ~Mykk
  2. Band pass filter would be achieved with the LPF and the HPF. The G&L electronics come to mind.

    But there are schematics of passive mid section on the internet I belive because there are passive eq (ex: Pultec, Manely Massive Passive, Universal Audio - the cut section only, for boost they go active)
  3. eddododo

    eddododo Supporting Member

    Apr 7, 2010
    give me just 1 second to come back and post, but I just built an array of passive devices recently.. The hpf was/is tricky though
  4. FunkMetalBass


    Aug 5, 2005
    Phoenix, Arizona 85029
    Endorsing Artist: J.C. Basses
    A LC HPF is a pretty simple circuit. What aspects of it were tricky?
  5. Very simple. For the LPF, wire a standard tone control. For the HPF, put a capacitor in series with the signal, and a pot parallel to the capacitor to vary the impedance across it. For the bandstop, wire a standard tone control with an inductor in series with the capacitor.
  6. Sounds like a plan, worth a shot. How would this look on paper? I can mentally picture the LPF & HPF section, I'm not family with a bandstop
  7. The bandstop is just a standard tone control, plus an inductor to block the highs from passing to ground. If the frequency is high, the capacitor will start to pass it to ground, but the inductor will block it. If the frequency is low, the inductor will start to pass it, but the capacitor will block it. Frequencies in the middle will be passed by both the inductor and capacitor, and thus, their impedance will drop, being diverted to ground.
  8. SGD Lutherie

    SGD Lutherie Banned Commercial User

    Aug 21, 2008
    Bloomfield, NJ
    Owner, SGD Music Products
    A passive bandpass filter would probably rob a lot of output. A better way is bass/treble cut controls, as on a G&L, and then a single frequency mid cut like on the Gibson Ripper.

    Turning the lows and highs down will simulate a mid boost. ;)
  9. rumblinbass


    Aug 22, 2003
    Wimberley, TX
    not sure this is entirely accurate, but I could be wrong. Passive eq's or tone control are cut only. They are followed by gain stages to make-up the attenuation.
    in other words...
    when the EQ is set flat there is a specific amount of attenuation. The following amplifier has gain to bring the signal back up to input level. Cut attenuates further and Boost attenuates less.

    I may have misunderstood what you were saying though. Sounds like you were saying that an active stage is switched in or applied only when Boosted.
  10. eddododo

    eddododo Supporting Member

    Apr 7, 2010
    for the passive HPF i found it frustrating to find a frequency rolloff i liked, even with the pot in the circuit. Then it was even more problematic when using different impedance instruments (upright etc).
    I got a trimmer capacitor to sort of set (and so i could experiment ) a rolloff i liked without having to try 80 cap/pot variations.
    though i am pretty new and tend to jerry-rig stuff

    the hpf i made :
    View attachment highpass.bmp
    NOTE: its wired so that it 'cuts bass' as you turn the pot clockwise

    I also wired a classic tone control through a 5-way switch with 3 different value caps on it.

    View attachment deep.bmp

    it REALLY does. So does the Highpass, even. I kind of wanted it for that reason, so it works out, and you can set it to where the rolloff is at a pretty low freq, as a 'mud cut' or whatever, but it certainly has limitations.
    unless you have a little clean signal booster pedal after..?
  11. If I gather correctly, the general census agrees it would be better to just have a LPF & HPF? And that a HPF will reduce output some.
  12. eddododo

    eddododo Supporting Member

    Apr 7, 2010
    I'd say thats a decent summary..

    definitely try out those trimmer capacitors... they're convenient.
  13. I will, sounds more convenient than picking up a bunch of different values and than soldering in different ones to find what works.

    Although, they seem a bit rare.... and the values don't scope over traditional guitar cap ranges. It seems these would need to get paired up with a traditional caps to bring it into a usable range.
  14. exactly,

    for ex:
    On the Massive Passive you have a cut/boost switch per band, wich sets the filters from passive cut to active boost.

    On my La-610 is done exactly as you said above. the 2 selfs when cutting are passive and when boosting are active.
  15. father of fires

    father of fires Commercial User

    Nov 29, 2006
    Chief of Medicine at Damnation Audio
    You can run a passive bass through an amp style tone stack. You'll lose some volume but higher output basses should be fine.
  16. That might be an option.

    This bass is using two Duncan SH13's in series, I have a 300k resistor in series to bring it down to a usable output level and it still has more output than any of my other basses.

    edit: if this will work in practice as well as it sounds in theory. I think I found my circuit, based on this website:


  17. rumblinbass


    Aug 22, 2003
    Wimberley, TX
    I can see how that would work. I little too complicated for an onboard bass pre, IMO. I can certainly ask the man who designed the Massive Passive since I work with him.
  18. father of fires

    father of fires Commercial User

    Nov 29, 2006
    Chief of Medicine at Damnation Audio
    The "in" and "out" look shifted but that should work. The Out is from the center lug of the treble pot. The bass/treble are boost only and the mids are cut only. So be aware that there is no bass or treble cut but if you want less treble just boost some bass and if you want more mids just cut some bass and treble.

    You can add a volume pot at the output too.

    Good luck.
  19. SGD Lutherie

    SGD Lutherie Banned Commercial User

    Aug 21, 2008
    Bloomfield, NJ
    Owner, SGD Music Products
    It will "work" but you are going to lose a lot of output. Passive tone stacks always have a gain recovery amp after them to make up for the insertion loss. With passive circuits you almost always lose something. They can't add anything, unless it's a small resonant boost, or they are using a step up transformer like Villex does.
  20. father of fires

    father of fires Commercial User

    Nov 29, 2006
    Chief of Medicine at Damnation Audio
    Agreed but if the bass has a high enough output it'll work.

    My main bass is a G&L SB-1 which is atleast 20dB louder than my backup bass (Squier P) so I personally can stand to lose some volume.

    OP - I suggest you experiment with this before you commit yourself. You can wire your pickups straight to the jack and then get a breadboard and experiment with tone stacks and when you have something you like then you should put it in your bass. That's how I'd do it.

    Good luck!

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