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a different kind of low pass filter

Discussion in 'Pickups & Electronics [BG]' started by bdplaid, Apr 13, 2015.

  1. bdplaid

    bdplaid Supporting Member

    Aug 31, 2007
    I'm looking for a different kind of onboard bass guitar LPF... The regular tone circuits operate by steadily decreasing (or also increasing, in the case of active) the slope of the filter at a fixed knee point, thereby cutting/boosting high end. I'm looking a tone circuit, preferably passive, that has fixed slope, but that slides a fixed cut (downward) slope of 12 or 18dB/octave upward and downward in the frequency range.

    Anyone know of such a thing?
  2. bdplaid

    bdplaid Supporting Member

    Aug 31, 2007
    thanks, but that's HIGH pass, not LOW pass. thanks anyhoo.
  3. That's not going to work out well. You would be better off with an active filter.
    blindrabbit, LoveThatBass and bdplaid like this.
  4. Antisyzygy


    Dec 8, 2014
    I think you're describing a "Shelf" filter with some sort of frequency cut-off selector. High-Shelf or Low-Shelf filters are different than High/Low pass filters. You may have to wire it up yourself. My intuition (which is wrong sometimes, I was in two circuits classes 8 years ago) tells me you might be able to do that with some sort of varitone that swaps out capacitors.

    I recall reading a thread about the G&L PTB system where they used a spreadsheet to see how different wiring schemes would affect the frequency response. I don't know if it will work for you but it's worth a shot.

    Bass-cut and the G&L PTB system | GuitarNutz 2
    Last edited: Apr 13, 2015
    petrus61 and bdplaid like this.
  5. bdplaid

    bdplaid Supporting Member

    Aug 31, 2007
    A shelf filter would be fine, as long as the knee is moveable and can go pretty high - like 7K Hz +, so I can get it out of the way. My experience with shelving filters, eg EMG and pretty much every active filter on the market except for a para-mid, is that way - the frequency knee is preset, and the amount of shelving, therefore the slope of the curve, varies. I'm looking to preset the slope/amount of cut and move the frequency.

    It occurred to me that I could probably get something similar to this with something like a 5-way switch and various caps, but not exactly what i'm after.

  6. Ok, so you want discrete steps between frequencies, rather than a continuous sweep throughout a range? That's easier to accomplish.
  7. bdplaid

    bdplaid Supporting Member

    Aug 31, 2007
    my goal is to have a sweep that would pretty much progressively clamp the high end output. Think of it as starting as having no effect - then sweeping downward to cut lower and lower frequencies, probably ending up around 300-500 Hz or lower; not sure about that. Not being a circuit designer, my assumption is that this would have to be active; but I'm not looking for any kind of boost in the circuit.

    If it has to be specific steps, i think that is probably possible by just buying something like a ToneStyler, but I'd rather have a sweep. I'm not a fan of the tonestyler.

    FWIW, I cobbled together a passive tone control that used a 3-way switch that selected different caps. It wasn't especially useful. I imaging the graduated step approach to be similarly unuseful.

    The more I think about it, something like an infinitely sweepable mid-Q control in cut mode would work if it was shelving mode cutting all frequencies above a set point, and the sweep went really high.

  8. Antisyzygy


    Dec 8, 2014
    They make rotary switches that have like 20 steps, that's more of what I was thinking. They also make variable capacitors but I don't know if there are any that are suitable for this application.
  9. Those are used in old fashioned radios. They tend to be very large, and are only capable of covering a low range of capacitance. (picoFarads)
    n1as and crentest like this.
  10. sprag


    Sep 15, 2011
    Melb Australia
    You are after a two knob Lpf circuit. 1 knob for volume 1 for frequency cutoff.

    Try diy stomp boxes for of the effects forum here. You may find circuits with more flexibility, just replace those pots with a resistor to keep them at a fixed value. It'll want to be passive or 9 volt if you stick a battery in your bass... I have no idea if you can make it fit into a bass cavity or not. I'm gonna guess that you can
    bdplaid likes this.
  11. Hmm, passively the rotary switch is probably the way to go. But if you need more than 12 steps, you can buy stepped attenuators that you can load yourself. You could maybe adapt one with caps instead or resistors. And I'm not sure you'll fit it in the bass. Plus the passive way will give you a very gentle slope...

    Or for active preamps, I believe this is exactly what the wal and alembic type preamps do. They are based on a circuit called a "state variable filter".
    It gives you a resonant LPF that you can sweep around all you like. I'm sure there'll be someone out there making a copy of one of these preamps. Try ebay...

    Edit: google or ebay search on "boogieman".
    Last edited: Apr 14, 2015
    Groove Doctor and bdplaid like this.
  12. bdplaid

    bdplaid Supporting Member

    Aug 31, 2007
    thanks for the info. that old variable cap looks too cool to hide inside a bass... maybe attach to it... :)

    I was looking more at the EMG Vari-mid and if that thing only swept higher, it would work.

    as for wal and alembic circuits, I think best bass gear sells something similar, I'll have a look. But i had an alembic some years back and don't recall it working anything like this.

  13. petrus61

    petrus61 Supporting Member

    I almost wonder if this could be accomplished thinking almost in terms of a wah pedal? I'm familiar with varitone circuits but not very familiar with wah pedals. Doesn't a wah basically function like a varitone of sorts? I'm not thinking so much the frequencies you're seeing in a wah, but more of the sweeping action and basic idea.
  14. Perhaps a modified speaker cross over?
  15. It's not a LPF, it's a BPF. (or a band-reject filter, if you are cutting of course.)

    A wah is also a BPF. But with an even higher resonance.

    If you need a LPF with a fairly decent slope, and a sweepable frequency, I believe teh state variable filter is the best choice.
    Last edited: Apr 14, 2015
  16. Martin Beer

    Martin Beer

    Dec 4, 2004
    It strikes me that the "Super Tone Control" circuit from the old Craig Anderton Electronic Projects for Musicians book could do this job. It was a 12db/octave state variable filter with Lowpass, Bandpass and Highpass outputs, with variable cutoff frequency and resonance. For your application you would leave off the Bandpass and Highpass level controls and leave the resonance disconnected. It uses two batteries though, so it might be better as an outboard thing.
  17. friendlybass


    Jul 19, 2012
    I'm no filter wizard, but it almost sounds like you're looking for like a moog ladder filter kind of thing. Am I way off base ir no?
  18. Azure Skies

    Azure Skies Commercial User

    Mar 21, 2012
    Businessman Importer/Exporter Broughton Audio
    Iron Ether's xerograph will do just this. Granted it's not passive, but it is selectable in slope and frequency. You don't want passive, anyway...to get that kind of slope you are looking at RLC circuits, and will quickly become lossy/noisy/cumbersome. Moog makes low pass filters as well.
  19. A wah is a variable notch filter.
    It takes a bite out of a small section of frequencys
    at various points.
    It might be of some use in part of it's range.
    Perhaaps that range could be lowered to be more usful.

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