Adding eq into the circuit of a dirt

[WolfCathedral]

Hey Guys, ive built a few fuzz/od pedals and while i like them I would like to be able to add a mid pot, bass pot, or treble pot. How does one incorporate that into the circuit? Is there a standard "section of the gain stage where this should be added to have the correct effect? Thanks!

[GibsonGM]

Hi Wolf, welcome.

Good question, the answer is "It varies".   

Tone stacks are notorious for having low input impedance...they 'suck tone'.   This is manifested by a loss of highs.  All passive tone controls have 'insertion loss', measured in dB, which means you will lose a certain amount of signal level by running thru them. 

So, just tacking a tone stack after a gain stage MAY do ok, but may not, also...depends on the values of the components the stack is made of, and the output impedance of the stage.     

To cure the above problems, a tone stack is usually (but not always) run from a BUFFER, which will stop the tone stack from interacting with the circuit blocks that come before it.  It has a high input impedance and low output impedance, allowing it to source current from the power supply for the stack.

Long story short - IF there is a standard place to put a tone stack, it would be after distortion (clipping) has occurred, and after a buffer...most tube amps follow this setup.   You can always TRY on in various locations, but it may sound like crap, to put it bluntly. 

If you only want to trim highs, or get rid of bass...THAT can be done nearly anywhere, altho many like to trim bass early (prevent a farty sound when clipping later) and trim highs later, after they've generated a lot of neat harmonics.   

Look at a bunch of schematics to see this - don't forget, passive networks of R's and C's are doing this in a fixed way thruout circuits, too! 

If I HAD to make a rule of thumb, I'd say 1 knob circuits, put them near the end of the circuit.  Tone stacks, after a buffer and also at the end, both types with maybe a gain makeup stage following before the output (see Big Muff Pi schem..). 

Very in-depth subject, wait a bit and there will be TONS of replies as this involves a FEW electronic principles both sound quality and impedance/level....

[WolfCathedral]

Wow thanks for all the info!! I will definitely take that advice. I've heard that it will suck tone so I've always wanted to figure out to get around that.

[GibsonGM]

Yeah - basically, when you are talking about 'tone sucking' (like lots of wahs do...), the impedance mis-match is really what's happening there.   Most tone stacks have that low input impedance (altho SOME designs are out there that can be used with some regular gain stages (mostly tube)...it's safe to assume a given tone stack is low impedance, and you can 'sim' it with software to see this).   

You want to have a high input Z (symbol for impedance), and low output Z in each stage to avoid loss of high frequencies.  Adding a buffer between a gain stage with low Zout and a tone stack with low Zin will permit the buffer to DRIVE the tone stack.

Keep in mind doing this stuff will change the 'nature' of whatever circuit you're working with, and it'll likely sound way different than what the original did (altho it might be awesome, too!)   

>>>  I should say that "Impedance" is a complex subject (but not crazy complex)...it is the combination of "AC resistance" and DC resistance.  An easy way to think of something with HIGH input impedance is to picture an amplifying thing, like a transistor, that doesn't need to "take" very much input current at all - just a little tiny amount of input will let it do its thing no problem.  Like a bike that's in a gear where you don't feel any resistance but it cruises right along with easy pedaling....

A LOW input impedance would pull in LOTS of current from what is driving it (your guitar, a radio, whatever), and the first thing that suffers when the input thing (guitar etc) can't deliver are the high frequencies.     Like rolling downhill very fast on a bike, you can't take your feet off the pedals, and pretend they can't freewheel, ha ha....the source (you) is LOADED DOWN.   

If that makes sense to ya, then you have enough to go play with stacks and things until you (maybe) decide to learn about reactance and impedance!     

[antonis]

I should say that "Impedance" is a complex subject (but not crazy complex)...

But compelx enough to drive you crazy.. 

until you (maybe) decide to learn about reactance and impedance! 

Better to learn about susceptance and admittance and then turn them upside down.. 

[blackieNYC]

I'm a big fan of pre-dirt EQing, which is complicated for similar reasons. A mid bump before distortion is like a wah before distortion of course. And there's something about having the dirt-created harmonics occur as a "natural" product of the waveform at its input, whether it is a bump, a notch, or a high end roll-off.
Trying this before a fuzz face type circuit may be problematic, but a full spectrum boost to pre-empt the tone stack loss, followed by the tone stack, followed by any other dirt of your choice is cool.
A big muff tone control positioned like this can be seen in the Azabache. (But change that 47k resistor to 4.7k. Otherwise the Azabache is bright.)

[robthequiet]

I recommend the articles at Jack Orman's Muzique website regarding the Stupidly Wonderful Tone Control that Mark Hammer presented on Ampage

There are some passive configurations presented that might be food for thought.

You may also like to explore Duncan's Tone Stack Calculator

Two things to consider just to expand on the ideas is that you can use a graphic equalizer in the signal chain to shape the sound without doing surgery on your pedals, and also consider that a guitar signal that is just amplified by a booster going into an amp will make the amp do the dirty work, so to speak, so that the amp's tone controls shape the tone. Carry on, then...

[Twhjelmgren28]

Hey Guys, ive built a few fuzz/od pedals and while i like them I would like to be able to add a mid pot, bass pot, or treble pot. How does one incorporate that into the circuit? Is there a standard "section of the gain stage where this should be added to have the correct effect? Thanks!

I don't have anything to add but wanted to say that was a solid question.  I'm super curious about this subject as well and am just starting to read/dig into filters and tone stacks.  There more I read / look at schematics, the more questions I have on the subject. 

Awesome question - I will be staying tuned.   

[wavley]

A good place to practice all of these great suggestions is to breadboard up some building blocks.  So put your fuzz circuit on the breadboard and then you can breadboard up some of the aforementioned tone controls and a buffer or two and maybe a gain stage.

Then you can patch the different building blocks into different parts of the circuit (just patch the ins and outs just like pedals on your board) for some real world experience on how they interact.  Of course there are always good practice rules of where things should go, but use your ears to tell you what's right.  The guy that does the Voice of Saturn stuff lives in town now and sometimes hangs out at my repair shop/studio and he was telling me about when he built the Voice of Saturn he ran it on a 9 volt battery and when he did a proper board and added decoupling to the power rail it didn't sound as good, a big part of the sound was the circuit pulling the rail all over the place.  I honestly would have never found that out because I always start with a solid power supply, sometimes bad things turn out to be good sounding. 

[nocentelli]

......many like to trim bass early (prevent a farty sound when clipping later) and trim highs later, after they've generated a lot of neat harmonics.   

Look at a bunch of schematics to see this....

Two good examples of schematics to look at are the Skreddy Lunar Module (fuzz) and Paul Cochrane's Timmy (OD). Both employ a bass control either directly before (Lunar Module) or as part of the clipping stage (Timmy), allowing you to trim the bass down when you turn up the gain to reduce the mushyness. The treble control is placed after clipping in both circuits so you can brighten up a lower gain sound, or dampen down the high end when you crank the gain.

[GGBB]

I'm a big fan of pre-dirt EQing, which is complicated for similar reasons. A mid bump before distortion is like a wah before distortion of course. And there's something about having the dirt-created harmonics occur as a "natural" product of the waveform at its input, whether it is a bump, a notch, or a high end roll-off.

I'll second that approach. Carefully filtering high-end out before distortion is a key part of the overall tone and IMO preferable to filtering after. But the two approaches sound quite different. A bit like how turning the tone down on your guitar sounds different than turning the treble down on your amp. Distortion harmonics are wonderful for mid frequencies but not so much for high frequencies - this is partly why many opamp based dirt circuits use a gain bypass filter cap in the feedback loop (which creates a pre-distortion low-pass filter) to "smooth" out the sound as well as prevent excessive noise or oscillation. Cutting the highs out after distortion affects those mid harmonics as well, but cutting highs before distortion allows you to preserve them.

[Twhjelmgren28]

I just wanted to ask a follow-up question or two related to this thread (rather than making a new thread - I didn't want to be redundant with threads). 

In screwing around with adding a variable high pass filter as a passive bass control on a simple JFET boost, I'm wondering if it's possible to wire it so the signal isn't completely lost when the HPF is turned all the way down.  Or would that require a tone stack coming after a buffer like Gibson GM stated?

It looks like the stupidly wonderful tone controls do this for LPFs (if I'm reading into that correctly) - is there an equivalent for HPFs?  For clarity's sake, I have the HPF setup as shown on the r-c filter calculator on AMZ.

Then I have a side question.  Does putting a buffer before a tone stack make it active or is it mainly there for impedance? 

Side note:  I found this to be a great article to help me begin to wrap my head around impedance - https://www.mrblackpedals.com/blogs/straight-jive/6629774-buffers-impedance-and-other-internet-lore

[GibsonGM]

Hi Tommy,

If I'm reading you right, looking at AMZ calc. HPF setup (the typical RC HPF).....What happens when the signal goes totally quiet is that your variable resistor is shorting the signal to ground via the wiper going to ground.

If you place a low value resistor between the pot and ground, you'll up up with the pot having "zero" resistance, but your fixed resistor won't be shorted, so you still have signal.   The compromise (there always is one...) is that the resistor will limit how much travel your control has - how much it can affect your signal.       

You would select the fixed resistor to give the level of signal you want at "zero".    Maybe that is 100 ohms, 220, 470 (?)...something you can play with til it does what you want.   Buffer not needed....


A buffer before a tone stack is basically there to 'separate' the circuit before the tone stack and the stack.....or the stack will draw too much current, and the highs will suffer, as will overall signal level.   It allows the stack to draw current thru IT, rather than the circuitry before...its input (opamp input, gate, base, etc) is not affected by the draw on its output (within reason).   

I believe that since that's the only work being done by the buffer, it's not really 'active', altho someone may correct me.   The buffer works across a range of frequencies (typically pretty broad - has a large bandwidth) where an active tone control would have a 'preference', would be more narrow spectrum....

[Twhjelmgren28]

That makes sense.  Thank you for the help!

[ElectricDruid]

I'd just like to add that there's a bit of a default assumption going on here that the "tone stack" that we'll all be using is one of the many variations of the passive tone stack from one of the famous amps - Fender/Marshall variants.

There are plenty of active tone control circuits out there that could be used instead. The Baxandall tone control is probably the most famous (either Bass/Treble or Bass/Mid/Treble variations) but you can have active lowpass rolloffs, shelving filters, mid boosts, notches, parametric peaks, etc etc. If you're willing to include an active component (an op-amp most likely) you can stop worrying about impedances so much, and if you were going to put a buffer in anyway, you've already got the active component.

HTH,
Tom

[Twhjelmgren28]

I'd just like to add that there's a bit of a default assumption going on here that the "tone stack" that we'll all be using is one of the many variations of the passive tone stack from one of the famous amps - Fender/Marshall variants.

There are plenty of active tone control circuits out there that could be used instead. The Baxandall tone control is probably the most famous (either Bass/Treble or Bass/Mid/Treble variations) but you can have active lowpass rolloffs, shelving filters, mid boosts, notches, parametric peaks, etc etc. If you're willing to include an active component (an op-amp most likely) you can stop worrying about impedances so much, and if you were going to put a buffer in anyway, you've already got the active component.

HTH,
Tom

That's great info.  Thanks!

I haven't downloaded the duncan tonestack software yet but will be doing so soon.  Thanks for all the help - and thanks for starting this thread WolfCathedral

[GibsonGM]

Yup, Tom points out a point to keep in mind.    FMV (fender marshall vox) tone stacks, baxandall, james (you'll see these in the Duncan TSC program!) require low input impedances (usually done with a buffer).

But there are some things that are (as you asked about) Active.     Here is an active HPF made with an opamp - notice how much it's like the passive RC one, just followed by a gain stage!  http://www.electronics-tutorials.ws/filter/filter_6.html

If the Rs and Cs are there alone, you may assume you need a buffer (or sometimes just a reg. gain stage) to drive them.   If you see opamps, transistors etc., you might assume they are an active circuit and those parts are 'taking care of' those impedance issues...

Also - as you've seen with SWTC or a basic RC HPF/LPF - you can usually just tack them in , the insertion loss isn't so great as to disrupt most circuits in a major way - no buffers etc., they are so simple!   You can model that with LT Spice and other programs - you see the amt. of signal needed to drive one RC filter is not that great.   The buffering becomes more important as the circuit (stack) becomes more complex.

[ElectricDruid]

+1 for the Duncan TSC program - it's great, and really handy.

You can do some cool stuff tweaking the values of these circuits. For example, I built a distortion pedal with a Bass/Treble Baxandall tone control on the output, but rather than have the two bands cross over somewhere in  the middle (which is typical) I deliberately left a "gap". This meant you could get an apparently "scooped mids" sound by putting both controls at max - it wasn't actually scooped, it was simply that the other parts of the spectrum were boosted. Similarly, you could get a midrange-boosted tone by cutting the bass and treble. Plus all the other variations, of course. You could do the same thing in more detail with a full three-way tone control of course, but I faked it and saved a knob.

Parametric or semi-parametric EQs are really good on dirt pedals too, but that does start to make the number of knobs add up! At the simplest, a sweepable mid control gives some versatility.

Tom

[Twhjelmgren28]

Thanks for all of the info!  There is so much I have to keep reading through.  I successfully put a hpf  filter in my jfet boost and it's perfect. I used a 10k in series with my pot.  I don't lose much volume that way but there is a good difference in bass.

It's also helpful to know that tone stacks aren't necessarily passive and you won't necessarily need a buffer with a passive tonestack (as in simpler circuits).

If the Rs and Cs are there alone, you may assume you need a buffer (or sometimes just a reg. gain stage) to drive them.   If you see opamps, transistors etc., you might assume they are an active circuit and those parts are 'taking care of' those impedance issues...

Also - as you've seen with SWTC or a basic RC HPF/LPF - you can usually just tack them in , the insertion loss isn't so great as to disrupt most circuits in a major way - no buffers etc., they are so simple!   You can model that with LT Spice and other programs - you see the amt. of signal needed to drive one RC filter is not that great.   The buffering becomes more important as the circuit (stack) becomes more complex.

So just to be clear, if there is just a rc filter it's most likely passive...these could be 'driven' by putting a gain stage after the filter -  Such as having a jfet or op amp follow the filter?

If there is an hpf/lpf coming directly off of an active component, you likely have an active filter and shouldn't need to worry so much about impedance.  How would this look with a jfet?  I can picture filters off an op amp, as shown in Gibson's link.... 

Or is it more the idea that an op amp / jfet is in itself part of a particular tonestack?







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[antonis]

A simple RC filter is always passive..

If it's impedance is high, it's loaded by anything follows - if it's low, it loads anything preceds..

Depending on every individual stage, it may need 2, 1 or none buffering..
(e.g. if it's driven by a Op-Amp and it drives a Fet, it doesn't need impedance "recovery"..)

You can easily realize it on designs of band-pass filters (a HPF followed by a LPF) where there is a recovery amp between them..