3-band tone control on custom OD pedal

[rhaggart1]

I'm currently in the development stages of my first overdrive/distortion type pedal, and I'm basically looking to cram a bunch of functionality into it so I've got a fun new toy!
I've spent the last couple of weeks slowly adding functionality, getting the components, testing, etc. and I'm almost ready to commit it to something more permanent than breadboard & get it into an enclosure. I now have a pretty basic circuit with the capability to switch between hard/soft, silicon/germanium, symmetric/asymmetric clipping, and I'm pretty happy with the sounds I'm getting out of it, considering this is my first 'real' pedal!

Though I'm just thinking: whilst I'm at it, I might as well add a 3-band (bass, mid, treble) tone control just to give myself even more room for shaping the sound.

I'm not *totally* sure how to go about it, however...

I know some schematics exist for standalone EQs, but I wonder how simple it would be to just tag one onto the end after my gain/clipping stages? In my searches I've come across this: https://guitarpcb.com/wp-content/uploads/2018/07/BD_Tone-TwEQ.pdf
However, I really can't say that I understand how this is working... I have a pretty good understanding of passive frequency filters, and some understanding of active LPF/HPF, but I can't see anything that looks too familiar in this circuit... (e.g. what are R4-R9 for? what is the first op-amps purpose?)

Or is it possible to get an entirely passive tone stack in here? Would this then require a buffer between volume & tone to prevent the two from interacting?

I've attached my (messy - sorry!) schematic so far - note this is a scruffy schematic intended only for reference and so component names & values aren't all accurate or up to date with the latest version.


Really what I'm hoping to get out of this post is an understanding of how I'd go about doing this. What to look out for, how this kind of thing works, etc.


Thanks very much!

[garcho]

Welcome to the forum!

I really can't say that I understand how this is working... I have a pretty good understanding of passive frequency filters, and some understanding of active LPF/HPF, but I can't see anything that looks too familiar in this circuit...

Don't be afraid to say you don't know something around here, you're in good company. That schematic is based on a very standard setup for guitar amplifier EQ from the golden days, and it's made its way into many pedals. It's basically three passive filters in parallel, with an inverting buffer before and inverting summing amplifier afterwards. The three passive tone controls do their RC thing and get summed at the virtual ground point of the inverting op amp.
By the way, when we talk about filters here, they are always "frequency filters", even the odd man out - all-pass filter i.e. phaser filter. If you're "filtering" amplitude, we just call it a volume control, unless it's part of an active amplifier, then we might call it "gain".

(e.g. what are R4-R9 for? what is the first op-amps purpose?)

R4-9 are the resistors in the resistor/capacitor passive filters for each band. I thought you knew about passive filters!
The first op amp is there to buffer the signal, so there's a steady current moving through all those filter sections, and to make sure the filters live in their own little bubble, not knowing what guitar pickup or pedal or other circuitry might be before them throwing off the math. It's also there as an inverting op amp so once the filtered signal is amplified by the inverting summing amplifier, it's back to its original phase. Phase is more of an issue designing serious stuff, it doesn't often matter all that much in guitar pedal world, but it's still good practice, and important to know.

One problem you might have squeezing an EQ into your circuit there is what's called a "recovery gain stage". EQ is as you said, a frequency filter. It filters frequencies out, meaning they are no longer audible, meaning the overall amplitude has been lowered. Once you carve away the offending frequencies, you might need to boost that carved signal back up. That's the reason for the second op amp in the EQ circuit you posted.
If you want to add that tone stack, you'll probably want to add that recovery stage. There are a couple other reasons why you would want to add it in as well, having to do with output impedance and other analog esoterica.

I would say the first thing to try is adding everything to the right of C2 (220nF capacitor) in the EQ schematic to your circuit, right after the op amp and right before the volume control. Luckily, you're in the breadboard stage so you can do whatever you want still! My favorite part of the process  

[Mark Hammer]

Gary's correct in that everything after the op-amp is entirely passive, such that any tone-shaping would involve selectively removing frequency content.  And since the diodes used for clipping already limit the maximum signal amplitude, you'd be subtracting from that.  If the pedal was always being used with gain and volume dimed, then a little bit of passive loss might be okay.  But people also want to aim for subtler overdrive sounds, so subtracting from a reduced output level now risks effect level being lower than bypass, or at least not much more than bypass.

There ARE ways to shape your tone, perhaps not quite as flexibly as might be achieved, without needing to resort to the addition of a gain-recovery stage (not that there is anything wrong with adding one).  For instance, C3 is shown as 100nf.  At maximum gain, it rolls off bass below 723hz.  If C3 was a larger value (e.g., 220nf or 470nf), it would retain more bass throughout the entire gain range.  And since much of the signal "lives in the basement", as I like to say, retaining more bass would effectively give you more drive at any gain setting.

Similarly, you don't seem to have any cap in the feedback loop, relying entirely on a post-output tone control.  Sticking a cap in parallel with R5, the feedback resistor, would roll off highs (e.g., none, 68pf and 150pf).  Note that what is produced via any clipping diodes is always a result of what you have fed them.  So shaving off highs would result in less fizz to trim back, after the clipping.

A pair of 3-position SPDT (on-off-on) toggles, one for highs (feedback cap selection) and one for lows (different effective C3 values) would provide a reasonable degree of tonal shaping.  No mids control, I'll grant you, but little "redesign" required.

[rhaggart1]

Note that my response here is prior to me doing any more detailed research into the topic - will return with more knowledge shortly... hopefully...

It's basically three passive filters in parallel, with an inverting buffer before and inverting summing amplifier afterwards. The three passive tone controls do their RC thing and get summed at the virtual ground point of the inverting op amp.

This is a beautiful piece of information that I hadn't been able to deduce myself! Thanks so much! I kind of recognised the RC filters in the feedback path but I wasn't confident that they were doing what I thought they were doing.
I recognise the LPF formed by R4/C4, and I assume the 'low' pot then just sets the level of this band of our signal.
I don't understand how R6/C3 are shaping the highs, however...

A pair of 3-position SPDT (on-off-on) toggles, one for highs (feedback cap selection) and one for lows (different effective C3 values) would provide a reasonable degree of tonal shaping.  No mids control, I'll grant you, but little "redesign" required.

Thanks for this, too. This is a nice and simple idea that I'll look in to implementing

[idy]

Is the GuitarPCB tone tweq not a baxandall and therefore active boost/cut? In my imperfect thinking these either filter the input, cutting a band or shelf, or they are filtering the feedback loop, "forcing" the opamp to produce more of that band/shelf. The pots "pan" between those two. No?

[antonis]

500k pot shouldn't drive a 250k one..

Tone should dramatically be altered by Volume pot setting..

[rhaggart1]

So I understand this part (the top branch of the Tone TwEQ (set V_B as 0V for now).
The feedback resistor (R2) is effectively the fixed 10k on the right + the right hand half of the pot. By moving the wiper we change the R2/R1 ratio, and so the gain changes. We apply this gain to the band of frequency that is passed by the RC LPF created by the left hand fixed resistor + the cap. All makes sense to me!

However, as I say, I don't think I can commfortably apply this logic to the bottom branch (highs) of the tone TwEQ in isolation... perhaps theres some interaction between the branches?

The pots "pan" between those two. No?

From my understanding, this part is right... With the wiper dead centre, the resistance on either side of the wiper is equal and so we have unity gain. Moving to either side of centre introduces a difference in the two halves, and so we either boost/cut depending on position.

500k pot shouldn't drive a 250k one..

My mistake! Thanks for the heads up

[POTL]

You need to figure out which equalizer you want, active or passive.
Passive will only cut frequencies.
Active will boost or cut them.
If you plan on using medium to high gain, I recommend avoiding Baxandall and James circuits, the low frequency response will be very wide and the more distortion, the worse the sound will sound.
High frequencies can also be too sharp if distorted.
However, with a clean sound and minimal overdrive, this will sound fine.
Equalizers with narrow bandwidth perform best.
The equalizer should be placed after all the distortion stages.

[niektb]

you already have a tone (treble) control, and if my memory serves me correct pot R6 can also serve as bass control (check f.e. Zendrive). why not add just a single mid control?

[idy]

This is coming around to the idea that although a tone stack is nice for mimicking an amps tone controls, and "eq:" type controls like baxandall nice for "high fi" treatment of your sound, in the context of a distortion box, it is often more fruitful to get rid of bass before or during the first clipping stage to avoid swamping everything with those great big waves ("tighten things up") and then reducing treble after to smooth off the jagged edges. Less rational-engineer thinking, more how-do-we-really-use-our-rigs thinking.

Right the above poster is: baxandall is nice for a "preamp" type box, not so much (too bassy, etc.) for the heavy drive sound.

At the other extreme a graphic eq works for a fuzz box... but so does a sweepable mid filter "wah" circuit.