Low-cut mid-cut hi-cut mid-pass passive filters in P-bass

Discussion in 'Pickups & Electronics [BG]' started by Andy GG, Jul 24, 2018.

  1. Andy GG

    Andy GG

    Jun 17, 2018
    Scotland, UK
    Hello good people.

    I have some ideas and would like to share it here to see if you could point me in the right direction.

    I plan to mod my P-style bass with 4 tone pots / switches and use them as following passive filters: low cut (HPF), mid cut (notch filter), mid pass (BPF) and high cut (LPF).
    They all will be used separately one at a time and fine tuned by dedicated pots. I want to be able to switch between them quickly. They must be passive RC. I'm far from even thinking about active circuits.

    Pots draft.

    I am an electronics beginner with a vision of what I'd like to achieve but not enough knowledge or experience to actually get there. Hence my questions to those who know. Hopefully topic would, at the same time, help others just like myself.

    Bass guitar.
    The bass is played with a pick and used for a rock/metal music. It features a p-style pickup with resistance measured at 11.3k and 500k pots.


    It's tuned for a bright, arrogant, industrial sound with a clang, more highs (7), a bit less lows (4-5) and little mids (3), used with wide variety of distortions. Strings are heavy gauge roundwound nickel on steel tuned all 2 steps down. Bass is only used for recording and solitary home practice.

    Anyway, the filters I want to use/make are as follows:

    1. Low-cut.
    This is to be used as a "solo" kind of switch for parts of a song when soloing in a guitar-like style.

    Freq_filters low-cut.png

    2. Mid-pass.
    This is to be used as a "special effect" tone in some parts of a song.

    Freq_filters mid-pass.png

    3. Hi-cut.
    This is to be used in calmer or darker songs to get rid of that arrogant harshness.

    Freq_filters hi-cut.png

    4. Mid-cut.
    This is to be used "normally" as a everyday tone shaper.

    Freq_filters mid-cut.png

    This sharp-cut shape I assume is the simplest to achieve. But the dipped shape would be more ideal:

    Freq_filters mid-cut2.png

    Now, below is how I think it should work in the circuit. My general logic is that if cap has cutoff point then it will pass signal above it and will cut signal below it. Pots aesthetics: signal goes into middle lug of the pot; when pot rotated left to the max - signal goes out the left lug (lug 1); when rotated right to the min - signal goes out the right lug (lug 3). I'm also placing vol pot at the end of the chain.

    1. Low-cut.
    0.047uF or 0.1uF cap connected along the signal path (in series). It will pass only the hi and mid frequencies. 500k pot.

    Filters_wiring low-cut.png

    Lug 2: signal input. Lug 1: clean output when pot turned to the left. Lug 3: filtered output when pot turned to the right.

    Question: can I just solder cap directly to lug 1 and 3?

    2. Mid-pass.
    This as I understand can be simply done by cutting some of the low end and then cutting some of the hi end of the same signal (subtractive operation). Meaning I use lower frequency hi-pass and higher frequency lo-pass filter at the same time.

    Filters_wiring mid-pass.png

    Lug 2: input. Lug 1: clean output when pot turned to the right. Lug 3: output signal heading towards both cutoff filters. The resistor on the orange route is suppose to prevent clean signal (lug 1) from entering the orange route and then the blue route which is grounded and would filter the signal no matter how you turn the knob making the circuit useless. Am I correct? 500k pot.

    4. Hi-cut.
    0.022uF or 0.047uF cap connected between signal and ground wires (in parallel) just as usual. 250k pot.

    Filters_wiring hi-cut.png

    4. Mid-cut.
    (For a sharp-cut shape) This I assumed would need a signal to be divided into 2 separate signals, then each of them treated accordingly: one with hi-cut filter and second with low-cut filter (both cutoff points around the same mid area) and then combined together again (additive operation). After a while I realised it could perhaps be done by using a double/stacked pot (?). 250k/500k pot.

    Filters_wiring mid-cut.png

    Pot A lug 2: input. Lug 1: clean output when pot turned to the left. Lug 3: filtered output when pot turned to the right. Pot B lug 2: input. Lug 1: empty. Lug 3: filtered output goes to ground when pot turned to the right.

    Question: how do I prevent clean signal which leaves pot A lug 1 from entering the orange route to the cap and as result being filtered? Do I put resistor as in mid-pass?

    Now logically caps could be of the same value, say 0.047uF with the cutoff point somewhere in the mids territory - low cut filter will cut below this point and hi cut will cut above it, am I right?

    For the dipped shape I've only heard rumours that it could be done using cap and inductor (band-stop filter) but the whys, hows and whats.. no clue. :-(

    Would those circuits work as expected?
    Any thoughts and suggestions appreciated.

    This is post 1 out of 3.
    Post 2 is about capacitors and 3 about switches.
    Please treat them separately whenever possible.
    Thanks for looking.
  2. Andy GG

    Andy GG

    Jun 17, 2018
    Scotland, UK

    Question 1.
    How exactly do I find values of capacitors that need to be used? For example I found this calculator here RC Filter Cutoff Frequency Calculator - Electronic Products
    All's fine but then in the description it says "This calculator assumes a low source impedance, which usually is small enough that it does not change the corner frequency"... but...

    Question 1B.
    ..But my source (pickup) has 11.3k resistance (impedance?) so does it mean I cannot use this kind of calculators? Or do I just put this 11.3k into the resistance field anyway and forget that description?

    Question 1C.
    I don't think I should include my tone pot in calculations here as when it is rolled to the max for a full cutoff effect then its resistance is nearly zero. Isn't it?
    Could someone please help with this? What kind of resistance those calculators mean?
    Please note that my vol pot is at the end of the chain.

    Question 2.
    I just found about PTB filters..
    For the bass-cut pot they use 0.0022uF cap, how can this be? Let's take our standard 0.047uF cap as an example, if you connect it in parallel (hi-cut) then it "CUT" as we know some of the mids and all highs SO THEREFORE: if you connect it in series (low-cut) it should PASS (as logically expected at least) the very same mids and all highs to the signal (while "cutting" lows and rest of the mids). Right? One capacitor working on one specific cutoff point both ways - passing or cutting the very same frequencies.
    This is how I imagine this:

    cap cutoff.png

    The drawing above seems more than logically correct, doesn't it? It certainly does to me.
    Why then in PTB tone mod the low-cut pot uses 0.0022uF cap when logically that job should be done by the 0.047uF cap - this cap passes lows and some mids with standard hi-cut pot, SO it should cut the same lows and same mids in a low-cut pot, shouldn't it?
    Intuitively the 473 cap should do the low-cut job better than the off-the-range 222 one.
    Continuing my reasoning, if we take that 0.0022uF cap and mark its cutoff point onto the above drawing then it would land far to the right from our example 0.047uF cap, and consequently such cap (0.0022) would cut not only lows as intended, but also all mids and half of the highs as well! Well here's how I imagine this:

    cap cutoff 2.png

    Where's the catch? Why such small cap can work as "low-cut" when logically it should really be a "low-mid-high-cut"?

    I may not understand the theory so could someone please show me a drawing or chart?
    There's one I found which I think is absolutely brilliant and highly explanatory:

    Cap 47nF cutoff.png

    I would be grateful if a good person could produce such a chart (similar to the above) with marked cutoff points and slopes for a series of caps covering the whole useful frequency spectrum, say for caps with values 47uF, 4.7uF, 0.47uF, 0.047uF (47nF), 4.7nF, 0.47nF, 0.047nF ...
    Something like this:

    caps chart.png

    Sure I do realise it depends on all sorts of variables but let's not complicate it more than necessary and let's assume a "typical scenario" with typical bass with typical p-style pickup (11.3k Ohm) using typical 500k pots and typical lenght of wires.
    Such a chart would help enormously not only me but the whole group out there in understanding the whys and hows while planning mods.

    Sure I could go and experiment, but first I would need to buy a bunch of caps. But buy what exactly? if I don't even know what range of values would be needed, and buying bunch of useless caps is not a good idea. Therefore kind asking for such a chart is a reasonable thing to ask I hope.

    To be honest my first reason for starting this topic was the urge to ask and desire of having such a chart. Even if it couldn't precisely tell a correct frequency in every scenario, it surely will show a very helpful pattern to assist in finding where about will you land when you use a particular capacitor. Or similarly what range of caps to use when you want to cut a particular frequency.

    If you're a pro in electronics you may have forgotten or not know that cap frequencies is one of the reasons keeping beginners and tone-shaping lovers and future mod-geniuses from improving their skill and exercising their ideas and contributing to the community.

    You may say "cap frequency question has been asked million times man!" and I believe so, pointing out that this is nothing but a proof that million hobbyists wanting to start the art of modding got stucked on this very topic: cap cutoff frequency.

    So pretty please, please generate such a chart so we could stick it for all to see.

    All comments and suggestions welcomed.
    Whenever possible please explain slowly and in plain English and don't assume that reader knows too much about electronics.

    Ideally use lots of images and diagrams.
    Last edited: Jul 24, 2018
  3. Andy GG

    Andy GG

    Jun 17, 2018
    Scotland, UK

    I started with positive attitude being sure that the switch I need is the most basic and simple one out there.
    What I had in mind was either one of these two:


    But what I found out is that only rotary switch may do what I want in a straightforward way. And the typical rotary switch is too big for my setup. Those toggle and slide switches seems to do weird things such as connecting lugs in a row then in column and then perhaps even across.. at least I see it that way :woot:

    Typical slide switch has too small slide handle for being practical on a guitar anyway. Does anybody know of a toggle switch that looks like those in a sketch above and is small enough to fit into not very large cavity?

    After some brainstorming I decided I will most likely use three ON-ON SPDT switches with 3 lugs - middle for input and side ones for both toggle positions in a following manner:


    Signal goes to lug2 of the first switch, ON(1) feeds the first filter and ON(3) feeds next switch into lug2, where ON(1) feeds the second filter and ON(3) feeds third switch into lug2, where ON(1) feeds the third filter and ON(3) feeds the fourth filter. When all 3 first filters are "off" (corresponding switches in position 3) signal feeds the last fourth filter which works as a default, everyday filter. When you switch any other filter ON(1) then the fourth filter gets switched off.
    Ha! How genius is that :)

    With a 4-way slide switch it would be tricky to hit and land on a specific position while playing at the same time. Many times you'd end up in the wrong place.
    But in my chain you'd only need to grab correct lever which is big and comfortably spaced from other two and just flick it up (or left). If you need to turn OFF any previously ONed switches you'd only need to slide your hand down across all of them to touch/flick them down to make sure they are all OFF from now on and then while your hand is down there already just flick the chosen switch up with your finger and you're done. All in about 0.02 sec.

    You could also use a speed trick while using filter 1 and will want filter 2 or 3 right after: you can turn the switch ON(1) for filter 2 or 3 ahead of time - it won't engage this filter until you turn the switch 1 OFF. Two birds with one stone.

    Well, now thinking it's a perfect chain for special effects! Imagine tapping or however else strumming with your left hand and at the same time switching rhythmically back and forth between low-cut and hi-cut filter! Boom-clang, boom boom-clang... haha!
    I already have a killswitch button installed here!

    This is the last one out of the 3 introductory posts.
    What are your thoughts good people?
    And sorry for the lenghty essays.
  4. Stumbo

    Stumbo Guest

    Feb 11, 2008
    Programmable EQ like Source Audio might work.
    Andy GG likes this.
  5. ofajen


    Apr 12, 2007
    92.4W 38.9N
    You may want to check out my thread:

    If P, then Q!

    Far less ambitious, I just added the same basic passive mid-cut on my P-bass that I was long accustomed to on my Gibson Ripper. It’s very musical and very versatile.

    Andy GG likes this.
  6. ofajen


    Apr 12, 2007
    92.4W 38.9N
    As you can see, the high cut and mid cut on my P bass run in parallel so they are available at the same time, which I find musically essential.

    You could even add an inductor based low cut with a third control, also in parallel and simultaneously available. That would look just like the standard high cut tone control wiring, but with the appropriate value of inductor in place of the capacitor.

    With low cut and high cut simultaneously available, I don’t see much of a musical use for also adding a mid pass, since you can accomplish a similar effect with the high and low filters, but YMMV.

  7. Andy GG

    Andy GG

    Jun 17, 2018
    Scotland, UK
    This pedal seems to do exactly what I'm after. But still, I want to take up on the challenge of constructing onboard passive filters. That's not only great exercise but something that I could be proud of :)

    I hear you, but my plan is to have 4 *separate* filters, having them available only *one at a time*; not using them together or mixing them. Almost like having 4 programs in your pedal so you can switch between them. That's actually very accurate comparison - I want 4 different sounds ready to switch between them throughout a song. And not just having 4 knobs to fiddle around for your perfect tone.

    That's why I'll use separate toggles for each filter with only one filter available at any given moment.
    I surely don't want to mix low-cut and hi-cut with mid-cut ;-)
    Last edited: Jul 25, 2018
  8. Killed_by_Death

    Killed_by_Death Snaggletooth Inactive

    You control whether they're on or not by the position of the knob.
    With G&L's PTB (Passive Treble Bass) controls when both knobs are fully clockwise there is no cut.
  9. ofajen


    Apr 12, 2007
    92.4W 38.9N
    A few thoughts:

    1) I run the Ripper much as you describe: two pickups with 4-position rotary selector switch. Mine has series, neck, bridge and series reversed polarity. Each setting has rather a different tone, quickly accessed, more so than the more common two volume or master/blend controls on two pickup basses.

    2) in practice, the eq adjustments can be quick since I know where to set them for each position. I usually don't scoop mids on the bridge setting, I turn the mids up to around 8 to 10 because that is the weight of the tone in that setting. OTOH, I usually scoop a lot of mids (down around 2) on neck setting to create this wonderful, deep and woody tone. Since I know this, it only takes a second.

    3) finally, I think you lose essential functionality if the modes are not able to be used together. It's great to be able to roll a bit of top end regardless of whether I'm scooping out low mids or not. Conversely I'd hate to be limited to not scoop any mids if I want to roll off some highs.

    Obviously, YMMV.

  10. Andy GG

    Andy GG

    Jun 17, 2018
    Scotland, UK
    Yes that's precisely the idea.

    You may be right. I can always add a piece of wire that will short two filters if I decide to use them together.
  11. ofajen


    Apr 12, 2007
    92.4W 38.9N
    I guess underlying my preference for the simultaneous and adjustable controls is the fact that musically I need to be able to adjust them for each pickup setting (and honestly for different music, too). I wouldn't presume to know, without trying it out, whether a particular low cut or notch would be musically suitable for me. It helps to be able to adjust the eq to find out.

    I kinda like the idea of getting the digital device to experiment with. Once you identify the settings you find useful, you could design specific passive circuitry to produce (approximately) that response and then set it up to use a rotary switch to select the various passive networks. This is kinda what the ToneStyler does, but using a larger space of possible networks.

  12. ofajen


    Apr 12, 2007
    92.4W 38.9N
    Here's one more thought:

    You could take it in stages:

    1) start with a single tone control and a rotary selector switch to put the various filter components in line.

    2) play each mode and adjust the tone control to your preference.

    3) measure the setting of the pot and replicate that setting with a simple resistor network that will work with that filter network.

    4) hardwire those resistor networks to the various filters so that the rotary switch automatically selects to preset tone you have tested out.

  13. Passinwind

    Passinwind I know nothing. Commercial User

    Dec 3, 2003
    Columbia River Gorge, WA.
    Owner/Designer &Toaster Tech Passinwind Electronics

    For starters, I'd recommend that you look in on the Pickup Makers Forum here: Pickup Makers

    Your pickup has a DC resistance of 11.3K Ohms, but AC performance is mostly what matters in this case. To calculate the impedance curve you also need to know its inductance and capacitance figures. The inductance is particularly mission critical. None of your posted filter configs are straight R-C ones, they are all actually R-L-C since the pickup is part of the circuit.

    I may be able to help a little with some LTSpice sims, but passive solutions are not really my thing at all and there are many others here who can undoubtedly do a better job. Best of luck, and have fun!
  14. Andy GG

    Andy GG

    Jun 17, 2018
    Scotland, UK
    Here's what I'll do.

    In a few days I should finish levelling frets of the bass in question so I can put new strings on.

    In first step I'll use my old 15 position equaliser and note the frequencies for each planned filter. This I'll compare with my Ashdown eq pedal which I actually use with the bass.

    Next I'll just start with the easiest filters (lpf and hpf). Hopefully till that time someone would have a look at my sketched circuits for the remaining filters.

    Thanks for your input and interest.
  15. Thanks for the pm, sorry I missed this interesting thread!

    I'm not really a passive bass kinda guy. I've always been underwhelmed by passive tone controls in general, but I still would encourage you to have a go at this. If nothing else you will learn a lot.

    As others have said, you are asking a lot of a passive circuit. If I was asked to design something like this, the first thing I would do is test the pickup for its inductance, resistance and stray capacitance (as others have also mentioned). Setting up a spice model with an inductor, resistor and a cap is by no means an accurate predictor of a pickup's tone of course, but it will get you started.

    I'd recommend using a high output, low impedance pickup, like a stingray pickup (wired in parallel).

    For LPF, check out my video on designing an adjustable LPF for my P bass. A cap straight across the pickup will give you the sharpest rolloff, but will need a little bit of taming with the odd resistor to get it to behave more like an active LP filter.

    For HPF, you can just run a cap in series with the pickup, but this will have a very gentle slope. And it will change, depending not just on the pickup's specs, but the load of the volume pot, the capacitance of the guitar lead, and the input z of your amp. (In fact all these filters will be like this.) Adding an inductor might help, but this will have its own load on the pickup.

    For band pass and band reject, I think you'll need an inductor. You may try a cheap audio transformer with all its windings in series, or if the inductor is across the pickup, an old guitar pickup might work. But again, you won't get a response like your sketches.

    Really, if you want a passive bandpass kinda tone, then I'd put a hot midrangey J pickup (like the Dimarzio model J) near the bridge, perhaps with a cap across it. For a scooped mid-cut kinda tone, I'd use a stingray pickup wired in parallel, near the neck.
    Last edited: Jul 26, 2018
    Andy GG, Stumbo and Passinwind like this.
  16. ofajen


    Apr 12, 2007
    92.4W 38.9N
    Should have mentioned, but it’s in my thread, Wilde Pickups (run by Bill Lawrence’s widow, Becky) sells a ready to go Q- filter component (inductor and capacitor in a potted unit) that you can use to make the mid cut filter. Same wiring as a standard HF tone control, you just put the Q- filter component in place of the tone cap.

    The mid cut filter is musically profound but more subtle and not as far-ranging as the active EQs that also emerged in the late 70s, so it kinda got lost in the shuffle when Gibson came out with some of Bill’s designs like the Ripper back then.

  17. Andy GG

    Andy GG

    Jun 17, 2018
    Scotland, UK
    One of my Eureka! moments.
    There's my mid-pass circuit:

    test mid pass.png

    When pot rotated full right signal goes through both caps from lug3, one cap cuts lows one cuts highs and signal goes to output from lug1.
    When pot rotated full left signal goes straight to output from lug1.

    Am I right?
  18. Andy GG

    Andy GG

    Jun 17, 2018
    Scotland, UK
    Frequencies update.

    I have decided for the following EQ settings:

    EQ hi-cut .png
    EQ low-cut.png
    EQ mid-pass.png
    EQ mid-cut.png

    Before changing my strings for Rotosound rb50 (heavy gauge nickel on steel rounds), I have "upgraded" my pickup with additional magnets.
    Underneath each half of my p-style pickup there's a few small 10x10x4 mm square neodymium magnets now arranged in following manner:


    Now the bass sings. It never sounded that good before!
    ELynx likes this.
  19. Do you have photos of it?
    Why this particular pattern? A magnet under each pole plus a magnet closer to the center per pair, amiright?
    Any noticeable sustain loss from increased magnetic field?
    And mostly, any records of before and after, from your description difference should be easily heard.
  20. Andy GG

    Andy GG

    Jun 17, 2018
    Scotland, UK

    I shall definitely prepare a comparison track as soon as I fix my broken computer. I'll take some photos then.
    The pattern was just the first idea that came to my mind having these magnets in hand. Four magnets across pickup so they cover position of poles and then two additional magnets below so those four wouldn't slip and move out of their place ; -)

    Due to huge volume gain i don't think i would notice loss of sustain even if there was any.
    Additionally those new strings make it nice and bright and they ring for hours.

    Once I'll be able to record anything I'll probably make new topic with the results.

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