Frequency response altering circuits

[booj]

These three popular pedals have a frequency modifying circuit (circled) which changes the gain at a particular frequency. At a frequency where the Xc equals the input feedback resistor, (I don't know the term, F3?) the change in gain would start to become noticeable as the freq. went above or below I'm assuming.
for the OCD, the first stage has a 2.2k / 68n combo. 68n has a reactance of 2.2k at about 1000Hz. Does that mean that above 1000Hz, the gain increases by double every time the frequency doubles?
On the second stage of the OCD, there is a 100n / 39k combo. 100n has a reactance of 39k at about 40 Hz. Does that mean there is a decrease in gain of 1/2 every time the frequency is halved?
On the Proco Rat, the 4.7uf cap would have a reactance of 560 ohms at about 65 Hz, and the 2.2uf would have a reactance of 47 ohms at about 1.6 KHz. So does that mean we're looking at a boost at frequencies above 1.6KHz and a cut on frequencies below 65Hz?
On the Voodoo Labs circuit, the 2.2uf would have a reactance of 47k at about 1.5 Hz? I guess that would be for full frequency response?
Anyway I've been looking at these circuits for and for some reason these little circuit bits just started popping out at me. Could someone please clarify what these resistor cap combos do?

[Steve Conner]

In the first stage of the OCD, it works the other way round to what you thought. The gain starts off 1 at low frequencies, then increases with frequency. It reaches its maximum value (determined by the Drive pot) a little over 1000Hz, and then stays constant for higher frequencies. The goal is presumably to remove mud from the tone.

I'll leave it to someone else to explain the other ones. There are only four possible frequency responses from passive RC filters: lowpass, highpass, low shelf and high shelf, and it's just a case of figuring out which one the circuit in question is producing. The example above was high shelf.

Actually if you want to get pedantic, there is a fifth one: notch. This requires a "T network" that is distinctive looking and pretty hard to hide in a schematic, though.

[Steve A.]

Actually if you want to get pedantic, there is a fifth one: notch. This requires a "T network" that is distinctive looking and pretty hard to hide in a schematic, though.

Would that be related to the bridged-T network that was used on some of simpler Gibson and Epiphone amps from the mid-60's? I thought that it makes a basic tweed-style amp sound more like a BF amp with the notched mids. Although I used discrete components for the network Gibson used little modules which encapsulated the circuitry.



Steve Ahola

[Steve Conner]

Yes, that's exactly the thing I'm talking about.

I didn't say Bridged-T because there is another network called the Twin-T that achieves the same thing, and maybe a few other topologies that I forgot about.

[booj]

Thankyou so much for answering, Steve. I figured the gain would increase. Then at 2khz the reactance would be 1.1k. OH.. ok. The gain would not double. And eventually the difference would be small compared to the 2.2k. Ok I get it. In other words, the gain is boosted a bit above 1 KHz, maybe giving an aural impression of a mid-boost?

[J M Fahey]

Picking Steve's torch and running a block , no more (thanks to overweight, lack of air and other ruinous physical conditions), let's see what we have.
To begin with, an old trick to produce usable SS distortion out of something as crude as a diode clipper, is to cut lows (= cut mud/farting) *before* clipping and to cut highs after clipping to lower buzz.
Mind you, a couple distortions, called "Fuzz", love the "bees in a can " sound and boost highs or cut bass.
One thing is for sure, they can't be ignored .
To each his own.
OCD: the gain of Op Amp 1 is =
1 + ([Drive+18K] / 2k2) so it can go from 9.2x (min) to 455x (max) .
*BUT* this is true at higher frequencies, where we may "forget" C2 but below F3 we must add the C2 impedance (reactance) to it.
We know that a Cap has higher impedance at lower frequencies, which in this case means loss of gain.
So, for example, at max gain our pedal has almost 500X gain (a LOT) .... at higher frequencies, *but* below 1000Hz it loses gain steeply.
At 100 Hz it has 1/10 the gain it had at 1000 Hz, go figure. (same as being 20dB down, a LOT).
This is not only done in pedals, but in amps (tube or ss) meant to distort.
That's why the typical Fender cathode combination went from 1K5/10uF (calculate F3) to Marshall's 2K7/.68uF (also calculate F3)
And Marshall also went from the .02/1M pot of Tweeds


to .0022/1M pot of Plexi's Lead channel.

You do your homework on the other pedals ... and on "famous" amps too
Good luck.

PS: and in a nutshell: there are not "magic" values, far from it; everything was made for some reason

If you understand this truth, you'll eventually be able to design on your own; otherwise it's difficult to progress beyond the stage of "tweaker".
Oh well.

[J M Fahey]

Picking Steve's torch and running a block , no more (thanks to overweight, lack of air and other ruinous physical conditions), let's see what we have.
To begin with, an old trick to produce usable SS distortion out of something as crude as a diode clipper, is to cut lows (= cut mud/farting) *before* clipping and to cut highs after clipping to lower buzz.
Mind you, a couple distortions, called "Fuzz", love the "bees in a can " sound and boost highs or cut bass.
One thing is for sure, they can't be ignored .
To each his own.
OCD: the gain of Op Amp 1 is =
1 + ([Drive+18K] / 2k2) so it can go from 9.2x (min) to 455x (max) .
*BUT* this is true at higher frequencies, where we may "forget" C2 but below F3 we must add the C2 impedance (reactance) to it.
We know that a Cap has higher impedance at lower frequencies, which in this case means loss of gain.
So, for example, at max gain our pedal has almost 500X gain (a LOT) .... at higher frequencies, *but* below 1000Hz it loses gain steeply.
At 100 Hz it has 1/10 the gain it had at 1000 Hz, go figure. (same as being 20dB down, a LOT).
This is not only done in pedals, but in amps (tube or ss) meant to distort.
That's why the typical Fender cathode combination went from 1K5/10uF (calculate F3) to Marshall's 2K7/.68uF (also calculate F3)
And Marshall also went from the .02/1M pot of Tweeds


to .0022/1M pot of Plexi's Lead channel.

You do your homework on the other pedals ... and on "famous" amps too
Good luck.

PS: and in a nutshell: there are not "magic" values, far from it; everything was made for some reason

If you understand this truth, you'll eventually be able to design on your own; otherwise it's difficult to progress beyond the stage of "tweaker".
Oh well.

[booj]

I think I understand the basic principles you are addressing, Juan. I've been using my own distortion pedals, both feedback and clipper types for years. I worked very hard to get even tone response and natural transparent sound. so...When I compare my boxes to others on the market I start to realize, yes I have achieved my original goal. they sound great. but when I compare them I also feel like I "like theirs better". Why? because theirs sound "brighter" "more lively" "able to achieve harmonics and controlled feedback sounds" It's little disturbing actually. My whole thing was to create an un-colored natural sounding gain machine. Now years later, I've got it and it's not what I want at all. I want controlled feedback and harmonics and a bright lively sound. My curiosity has turned to the devilishly decadent "midrange" frequencies. If you want to see my boxes you can see them at 51bobtube on youtube. Oh I've got the sound down, baby. I just don't like it. I want the magic. I'm currently working on a mid-boost knob.
Thanks for your response!

[J M Fahey]

Ok, I will.
As of getting your own sound, the way you have it "in your head", nobody said it would be easy
I've been at it for 43 years now .... have achieved a lot, but still tryin'
Oh well