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

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.



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



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



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



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





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.



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.



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.



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.



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.

 

Capacitors.

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:



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:



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:



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:



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.

 

Switch.

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

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.

 

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 ;-)

 

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.

 

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.

 

One of my Eureka! moments.
There's my mid-pass circuit:



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?

 

Frequencies update.

I have decided for the following EQ settings:






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!

 

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.