Passive filter pedal

[jgrab]

Hi folks.  New member here and hoping someone here could answer a few questions.

I've been thinking about a simple pedal that will filter my tone for parts of a song where I want a limited frequency response (e.g: Intro, verse, etc.) to contrast with the full frequency tone for say a chorus.

I just watched the latest That Pedal Show episode with Simon Jarrett of Kingsley: https://youtu.be/pmUq8uK_AL8

Around minute 37 things got really interesting for me. So, to the experts here:

1.  How would one build a very simple passive circuit to vary input impedance and swap coupling capacitors to filter a guitar sound on both the low and high end?

Could this just be two jacks with a typical guitar potentiometer to ground in between and a capacitor in series after?

2.  Would this be useless without a gain stage?

3.  How might this compare to a pedal with a typical guitar tone control and passive bass filter like Reverend or G&L guitars sometimes use?

Maybe I'm barking up the wrong tree and would be better off buying a small graphic EQ like the MXR M109S or that EHX %^&*ed wah pedal?

[Fancy Lime]

Hi jgrab,

welcome to the club! How complicated your request is depends on how flexible you want the thing to be. But the range goes from "super easy" to "still pretty damn easy". So, a few questions:

1) Do you want to be able to control this with an expression pedal (fairly expensive enclose and not easy to get) or just on/off with a footswitch (cheap and easy)?

2) Do you only want to take away bass and/or treble (this can be done passively but the result will always depend on the output impedance of your guitar and the input impedance of your amp*) or do you want to have the option to boost as well (much more flexible but needs an active circuit)?

3) Do you want a fixed bandwidth or more flexibility by tuning it with some knobs?

I personally would recommend building an active 2-band equalizer. Possibly with switchable corner frequencies or even semi-parametric. This is not difficult and I'm sure well be able to find you a design that is easy to build for a beginner.

Cheers,
Andy

Footnote:
*or more precisely: the output impedance of whatever precedes the thing (lets call it the Attenuator) and the input impedance of whatever cames after the Attenuator.

[jgrab]

Thanks for the feedback Andy!

My idea was for this to be on/off only and first in the chain to take advantage of all interaction with guitar electronics. The attached video demonstrates how extreme the effect could be with what seems like an incredibly simple circuit.  Any variability is simply to find what the right electrical values are for a particular guitar / sound, not something to be messed with while playing.

I've wired guitar pots and swapped a resistor on a wah pedal once but that's the extent of my electrical engineering prowess. So if someone sells something like this in a small form factor I'd consider buying it!

[Fancy Lime]

Me again,

I watched the video a bit further and I think I now know what you are getting at. What these guys do there only works the way it does because they feed it straight from the guitar and the passive "impedance mismatching circuit" has a treble booster after it. You can build this thing completely passively but it will only work the same way when the guitar goes straight into it (no other pedals or even a tuner with a buffer before the Attenuator) and if you know reasonably well what the input impedance of the next thing in the signal path is going to be. The latter condition is more easily met, since guitar amps and most pedals (exception: most fuzz pedals and some treble boosters) and guitar amps have a very high input impedance. The simplest case really only involves two jacks, two capacitors and two potentiometers. But it needs to be specifically tuned to your setup for best results.

To your second question: It would not be useless without a gain stage but much more useful with a gain stage, in my opinion. Building what they have in the video is very easy. Building something much better and more flexible is also easy. If you can figure out at which end to hold a soldering iron, we can teach you to build this.

3.  How might this compare to a pedal with a typical guitar tone control and passive bass filter like Reverend or G&L guitars sometimes use?

I don't quite understand the question. How it compare really depends on what you build. The basic working principle is the same, if that is what you mean. So any difference in sound can be purposefully designed by choosing the right component values.

Maybe I'm barking up the wrong tree and would be better off buying a small graphic EQ like the MXR M109S or that EHX %^&*ed wah pedal?

Building stuff yourself is fun, trust me. Also, for what you want (IIUC) a graphic EQ would be cracking nuts with a sledgehammer.

Andy

[Fancy Lime]

Thanks for the feedback Andy!

My idea was for this to be on/off only and first in the chain to take advantage of all interaction with guitar electronics. The attached video demonstrates how extreme the effect could be with what seems like an incredibly simple circuit.  Any variability is simply to find what the right electrical values are for a particular guitar / sound, not something to be messed with while playing.

I've wired guitar pots and swapped a resistor on a wah pedal once but that's the extent of my electrical engineering prowess. So if someone sells something like this in a small form factor I'd consider buying it!

OK, I see. Then I would probably suggest buying a treble booster kit and adding the passive circuitry up front. Problem there: there are tons of those things around and they sound quite differently and some work better with humbuckers, others with single coils. What kind of guitar and what kind of music do you play?

Andy

[jgrab]

Yeah maybe a treble booster kit or mod is a good way to go.

If I back up a step or two, I have some more fundamental electric questions that I thought this forum could help with.

Is input impedance to a circuit determined by a resistor to ground off the hot wire or is it more complicated than that?   What about the coupling capacitor that takes out lows - does that go in series on the hot wire?  On the tap to ground? (Isn't this is a low pass filter standard tone control?).  Before or after the tap?

Sorry for the super basic questions.

[Fancy Lime]

Is input impedance to a circuit determined by a resistor to ground off the hot wire or is it more complicated than that?

It depends. Yeah, I know, suuuuper helpful. Let me try and explain. The input impedance is determined by the *effective* resistance to ground. What this effective resistance is, depends on the design and the active device. For example, JFET or MOSFET transistors have a very high "gate resistance" at their input, which means that they themselves contribute very little to the input impedance. In these cases, the input impedance of the circuit is indeed mostly determined by the *bias network* of the transistors. For JFETs this bias network can be as simple as a single resistor to ground. For MOSFETs it generally takes at least one resistor to ground and one to the positive *supply rail* (usually +9V), and then the input impedance is calculated as these two resistors in parallel. The total resistance (R_tot) of paralleled resistors (R_1 and R_2) is calculated as: R_tot = 1 / ((1/R_1) + (1/ R_2)). So if both resistors were 2.2MΩ (Mega Ohm), you get 1.1MΩ (also written as "2M2 and 1M1", so the unit multiplier in place of the decimal point and leaving out the Ω; sometimes you also see "R" instead of Ω for Ohms). For BJT transistors such as are used in most treble booster designs, things are more complicated because these have a rather low internal resistance at their input, which is now not called "gate" anymore but "base". For BJTs, calculating the input impedance involves characteristics of the individual transistor and probably going a bit too far at this point. Important thing to note here is, that the highest possible input impedance you can get from a BJT (without further tricks that we shall put aside for now), is much lower (in the hundreds of kΩ range) than for JFETs and MOSFETs (MΩ range). So from all that you probably got that a JFET-based booster is the simplest and easiest to build, right? Well, not so fast. JFETs are quite inconsistent among different individuals of the same type and need careful biasing via the *drain resistor*. Nevertheless, I think for your purpose this sort of thing may be best. Here is a fantastic collection of classic circuits by the great Tim Escobedo:
http://www.diale.org/escobedo.html
Scroll down to find the "Duende JFET". This may be the easiest thing to build that suits your purpose.
Here is a stripboard (aka Veroboard) layout for the Duende JFET and its cousin, the JHS MiniBomb. The right board has no way of adjusting the bias, so I recommend the left one:
https://tagboardeffects.blogspot.com/2013/04/escobedo-duende-jfet-jhs-minibomb.html
As you can see, this is super simple, even without a kit.

What about the coupling capacitor that takes out lows - does that go in series on the hot wire?

Yes, that one goes in series with the signal path on the input, which I assume you mean by "hot wire". Just before the place where the resistor to ground is that defines the input impedance and biases the transistor (in case of a JFET device, that is). Together, the capacity and the input impedance (which is effectively a resistance that is frequency dependent but can for our purposes be regarded as a simple resistance) form a "first order high pass RC-filter". You can calculate the cutoff and look at the frequency response with this tool:
http://sim.okawa-denshi.jp/en/CRhikeisan.htm


The *series impedance* of your guitar pickups and cable forms a low pass filter together with the capacitor in the guitars tone control. You can pretty much copy you guitars toe control into the very beginning of your pedal and get the same result, but foot-switchable. Because guitar pickups are coils, they have a lot of inductance in addition to some resistance, so simple RC-calculations do not work here. If you know the inductance and resistance of your pickups, you can use this tool to calculate the cutoff as an RLC filter:
http://sim.okawa-denshi.jp/en/RLClowkeisan.htm


Hope that helps,
Andy

[andy-h-h]

Hi folks.  New member here and hoping someone here could answer a few questions.

I've been thinking about a simple pedal that will filter my tone for parts of a song where I want a limited frequency response (e.g: Intro, verse, etc.) to contrast with the full frequency tone for say a chorus.

What about a Fuzzrocious Feed Me?  Really simple to make, and would probably do what you need.  You could add something to play with impedance if you wanted to, but I'm not sure how necessary that really is.  Maybe not be a good idea to leave it entirely passive though, as you lose level with filters.  A little full range boost after the filters to make up gain would fix this.

[bartimaeus]

Just go for it. Solder it up and see if it meets your needs. If the loss of volume is bugging you, it wouldn't be hard to add a gain stage before the output of the pedal. I find it's often easier to build a project, see for myself why it doesn't work exactly how I'd like it, then add improvements.