Overdrive/Distortion frequency shaping - numerical recommendations

[lepra85]

Hi all,

My doubt today is about the frequencies that are important for an overdrive pedal and for a distortion too. In the Forum I found that there are some recommendations but they aren't specific numbers/ranges of frequencies, there are recommendations as "less bass" or "cut the highs".
From what I read in the forum, a common configuration  for the overdrives/distortion pedals is:
Input Buffer/bass attenuation/amplification/distortion/highs cut off/output buffer.
In some pedals these stages are clealy defined, and in another, some of them are mixed.
The 2 stages that in don't fully understand are bass attenuation and the highs cut off.
Doubts:
1)At what frequency is recommended to roll off the bass to avoid the "muddy distortion"? I saw roll off frecuency from 50 hz to almost 800hz in different pedal schematics.
2)At what frequency is recommended to cut off the highs for an overdrive and for a distortion? Is saw frecuencies from 700hz to almost 10khz?
3) I know that there is no only 1 answer correct. But maybe there are zones that can be specified in some way (eg. Until 1khz=subtle distortion, above 5khz= harsh, above 10khz=only noise). I really don't know.

Thanks,

[GibsonGM]

Good question, Lepra, and welcome.

Ok, there are some other factors we'd need to talk about.  WHERE in a circuit are we cutting the lows/highs?  What is the "Q" of the filter we're using - what kind of slope are we looking at? (how fast do freq's roll off...).  Are you cutting stuff pre - or post-distortion?

In GENERAL, cutting bass would be done around <200Hz.    200Hz - 1kHz would be your mids, and above that are highs.  600-800Hz are pretty sensitive areas for guitar.

What I suggest you do is find a cheap equalizer, or EQ pedal, and EXPERIMENT.    Depending on what you've done to your signal before you cut/boost, or you do after, you'll see that the results vary.    For one circuit, maybe you DO need to cut at 10kHz...but for another, leaving that alone might be nice!    Sometimes you want to bring mids up to push thru more, and sometimes to cut them for more "fat" and "heavy" tone....

You're seeing these ranges because of this - there isn't any hard and fast rule, only that 'too much bass can be boomy' when overdriven, and boosting the highs too much can be fizzy/icepick....most all of this is based on experience and personal preference!   

The reality seems to be that for guitar purposes, the bandwidth we're talking about gets trimmed back pretty well....we don't WANT high fidelity.    Mess around, get a sense of where YOUR idea of sweet spots are! Try the suggestions and find what works for you 

[lepra85]

GibsonGM,
Thanks for your prompt response. I am triying to design a similar overdrive to the AC booster from xotic, but trying to understand every component position and  each value.
I am totally agree with you regarding that the sounds must be experiencied.
I found that the TS9 has a single pole filter just before the clipping stage and before the tone stage (cut off @ 720hz aprox.).
Similar single pole RC were in the proco rat(cut off from 415 to 32k), MXR distorsión III (cut off @ 300) and Boss ds1(cut off @ 7.2khz)
With that information i thougth that for a overdrive it would be necesary to a put single pole filter arround 700hz after the tone control as strategy to avoid the harsh of the "high" frequencies.
It would be great if i could guess how is going to sound before assembly it.
I will aply your sugestión about the eq.
Theorically we hear from 20hz to 20khz. I will investigate at which frequencies the fizzy/icepick starts. Other thing o am interested in are the frequencies that most contribute to the noise.
Thanks again GibsonGM.

Ps. English is not my native language, sorry about the writing mistakes

[GibsonGM]

Your english is good!   

The differences in filtering that you see between MXR, Rat and DS1 are part of what makes them different than each other.   If you build one of them, and change the cap values, you will have something much different! 

As a general rule, try to cut some bass on the input, and then cut some highs after you clip, maybe at 10kHz - see what that sounds like!    The bass can make your tone "flabby", it can overdrive following stages and sound muddy.   The highs can be fizzy and yes, can have noise mixed in.   

You will learn where to cut highs so that you still have sparkle, but less noise and cold sounds       You can make some of this variable, also - you could make a variable HPF at the input, to see what more or less bass sounds like...or use another EQ pedal there!    Many recommend having TWO eq's, and using one at input, one at output.... 

[Transmogrifox]

These are my personal opinions:
250 Hz is the center of the "mud" frequency.  If you want your guitar to cut through a mix and blend well with a bass player then make a gentle scoop centered at 250 Hz.  The BMP style tone stack lends itself well to this kind of scoop.

Going into the distortion typically aim for 1-pole cut-off at >400 Hz for less muddy sounds.

To my ears the RAT is fuzz, and I generally hear what I would call fuzz when the bass frequencies below 400 Hz aren't significantly attenuated.

Intuitively this makes sense.  The high E string is  played at the 3rd fret (G) is 392 Hz fundamental, so any notes played from that string on up will sound about the same whether you have any low cut or not if you go much below 400 Hz.  This is in the range of playing chords or dual-string bends higher above, say, the 8th fret where the cut-off makes little difference. 

This is helpful to keep in mind so you know where your fundamentals are in the spectrum:


Basically the high E played at the 12th fret is ~700 Hz, so you can see the relationship between the TS-style overdrive and the notes you can play on a guitar.

Also keep in mind that notes on the small strings high on the fretboard are much closer to a pure tone, so the natural higher harmonics drop off rapidly above 700 Hz.

However, transients (pick attack, string slaps, plucking) contain much higher harmonics on the initial attack, and this dynamic behavior interacts with the filter in a way that is difficult to understand by frequency-domain analysis.  Our understanding of frequency response is based upon the assumption of LTI (Linear Time Invariant) systems.  Distortion pedals do not meet these requirements....well now I got off topic.  Back on topic...

The much "tighter" overdrive sound to my ears starts with 1-pole HPF around 600 Hz (I like the more body sound of the lower cut-off so I set my TS to 630 Hz).  As the 1 pole filter is decrease below 600 Hz I consider it to get muddy.

However, a 2-pole filter might be different (I haven't tried yet).  I would guess you might lower a 2-pole HPF down to 450 Hz for a similar effect.


On the high-end after distortion now you have harmonics reaching up above 8 kHz.  My opinion is a strong >2.5 kHz boost after distortion will produce "ice pick" sounds, but conversely you can't roll off at 2.5 kHz or you lose the "sparkle" which lives in the same frequency range as the "ice pick".  Between 2.5 kHz and 10 kHz it's more about shaping it.

The TS is a good case study on this.  It has a post-distortion 1-pole roll-off at 723 Hz, matching its boost.  On a clean setting its frequency response is relatively flat out to as high of frequencies you can hear on clean guitar.

Then the tone knob allows a boost at about 3.3 kHz when cranked all the way bright.  As you turn the tone pot both the amount of boost and the frequency move downward.

A lot of distortion circuits  I see aim to approximately correct pre-emphasis on whatever is going into the distortion.

Pre-emphasis->clip->De-emphasis->tone stack

...And tone stack is where you shape the final sound

[dschwartz]

This is such an interesting subject. I have been tweaking my last design tryig to get the filters right.
Filtering and voicing is the signature of a drive pedal..much more important han the clipping method..
For this design i filtered the pre clip bass at 400hz for low gain and 750hz on high gain..after clipping, the lpf cuts at 800hz.
I wanted it punchy, tight and very amp-like..
I got to these values comoaring it with the dirt channel of my tube amp....

[antonis]

Basically the high E played at the 12th fret is ~700 Hz

Bent to 13th fret...

[scaramoustache]

OCD and Hao Rust Driver (and propably many other) have a clever take on EQing before and after distortion. Both cut bass at around 1500hz before distortion and roll off treble way lower around 200-300hz, if I remember correctly. Bass cut and treble rolloff work against each other and so in overall the region between that 200hz and 1500hz is flat EQ-vice. At the same time bass cut and treble rolloff switch roles: frequencies below 200hz and above 1500hz are attenuated. And though overall EQ is flat between those frequencies, distortion gets funkily frequency-dependent.