Filter values...

bifurcation

bifurcation

Well-known member
Hey all,

I've been (largely unsuccessfully) trying to get my head around different tone stack circuits, and as an exercise I'm trying to mod a Foxx Tone Machine clone to adapt it for bass.
foxxschematic.gif

Looking at the input, and using a filter calculator, it looks like the 1M resistor and .1 uF capacitor trims off everything below 1.6 Hz, which is fine for guitar or bass.

From the little I understand, when I focus on the tone stack:
foxxschematic_tone_stack.gif

I think it works like:
foxxschematic_high.gif

Turn the knob to the left, it pulls signal through the upper capacitor and... uh... sends to ground through the 22K resistor?

Which makes it filter around 2,413 Hz?

Alternatively:
foxxschematic_low.gif

Turn the knob to the right, it pulls signal... past the lower capacitor and... uh... through the 4K7 resistor?

Which makes it filter around 678 Hz?

So, assuming any of that is correct, (which I'm doubting the more I write this out,) could I just pull out that 4K7 and replace it with a 120K to bring the low end response down to around 26 Hz? (the low A on a drop-tuned 5-string bass)
 
I would probably change the cap value on the lower part instead of the resistor, but I think you have the right idea.
 
Not quite. The tone network is a bridged T filter, similar to the Big Muff Pi tone stack. Treble goes around the top and bass goes around the bottom. When the wiper on the TONE pot is up, you get a mostly treble with some bass and a mid scoop. When the wiper on the TONE pot is down, you get mostly bass with a little treble. The two caps, two resistors, TONE pot and Q4's input impedance are all part of the filter. That 10uF cap is big enough that it does not affect the tone. The math gets a little messy, so I used Duncan's Tone Stack Calculator to simulate the TONE control (see pic). R2 isn't in the FoXX, so I made it so big that it acts like it isn't there. R3 represents the TONE pot and the 4.7K resistor below it. At the Bass end of the pot, I set TONE to 1 instead of zero to simulate the effect of the 4.7K resistor. R4 is the load from the 470K and 47K bias resistors and Q4. Blue trace is TONE all the way down, green trace is TONE in the middle and red trace is TONE at max. As you can see, there is a mid scoop for most of the TONE control's range. You can change the location and depth of the mid scoop by fiddling the two caps. Make 'em both bigger and the scoop moves to the left. Make 'em both smaller and the scoop moves to the right. Make the 3nF cap smaller (or the 50nF cap larger) and the notch gets deeper. Make the 3nF cap larger (or the 50nF cap smaller) and the notch flattens out or can even be made into a mid-range hump. Another way to look at it is R1 & C2 set the low-pass freq; C1, R3 and the load set the high-pass freq. There is some interaction between the parts. The tone stack may be ok as-is, depends on the desired sound. Something to consider when designing tone controls for dirt pedals: when the tone control is after the distortion section, it shapes the added harmonic content as much or more than it shapes the original instrument sound.

The parts that limit the low freq response are the input cap and the feedback network from Q2 to Q1. Looks like it rolls off around 30Hz. You can extend the response down another octave by doubling the two 100nF caps. That 1M resistor up front does not influence the low-freq response because it is on the upstream side of the capacitor. Here too, what you do to shape the low-freq response depends on the sound you're after. The only way to find out how it sounds is to build it and experiment.

Foxx tone.PNG
 
OMG, thanks! I feel much clearer on the why's and where's.

Chuck D. Bones said:
I used Duncan's Tone Stack Calculator to simulate the TONE control
Click to expand...

I love this app. :) I reproduced your settings and messing with C1 made it even clearer. I hadn't even thought about the fact that the clone I'm messing with has a "scoop" / "no scoop" switch on it. A little investigation shows that "scoop" uses a 1nF cap for C1 (instead of a 3nF) and "no scoop" uses a 33nF cap for C1. The DTSC shows a difference between a dramatic scoop and a slight mid hump. Nice.

I might replace that 1nF with a 3nF, not only to get it closer to the Foxx, but also because the current scoop sounds unusably deep to me.
 
Putting a "scoop" switch there is a common trick. If you use an SPDT with center off, you can get three different capacitor values. I've done that on the Ungula. Or use a DPDT center off and switch combinations of both caps, I did that on a custom Ultra-Muff I built on Vero.
 
Going to sit down and study this after I get back from vacation, any good reading you would recommend on the various tone stacks? And their differences.
 
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I have no recommendations on reading about how they work, others might. You can download user manuals for amps & pedals to read about what the tone controls do and how to use them. Other than that, you have to try out the various amps & pedals to find out what works for you.
 
Good stuff! With dirt pedals, I usually have no use for treble boost, only treble cut. I've used this one to good effect (pun intended) as an add-on to the ROG Double-D, it has a variable cutoff freq:

ROG Double-D.PNG

I also like this one, it has a fixed cutoff freq but variable attenuation:

FB tone.PNG

Both are simple and don't affect the bass & midrange levels.

The Fender/Marshall TMB tone stack shows up in the Covert (and others), it's useful for amplifier emulation.

The Friedman pedal clones (Thermionic, et. al.) have active filters in them, they are very effective.

Baxandall filters show up in a number of pedals, the EQD Talons is a good example.

It really comes down to what kind of tones you're after.
 
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