Variable Input Cap for Fuzz Question

[PhiloB]

If I am going to add a variable input cap to a silicon fuzz face...I would connect input to lug 2, high value cap to lug 3 and low value cap to lug 1?  I would connect the other ends of the caps together? 
If I understand the above correctly, then my question is:
What value pot would you use and why that value?


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[selfdestroyer]

Take a look at this 4-Knob fuzz face

http://music.codydeschenes.com/wp-content/uploads/2013/10/Fuzz-Face-4-Knob-Ataraxia-Engineering-Transfer.pdf

I have built 4 or 5 of them over the past year and they are great. GOt some cool mods on it and you can see the input "fat" knob in the schematic.

Cody

[PhiloB]

Thanks Cody.  That's what I'm looking for. 
Why is input connected to lugs 2 &3 and not just 2?
Also why 100k? 


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[selfdestroyer]

Thanks Cody.  That's what I'm looking for. 
Why is input connected to lugs 2 &3 and not just 2?
Also why 100k? 


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Unfortunately I can not answer those questions.. I built this one by the BOM and the fuzz face is not something I know well. Maybe someone else will chime in. Ataraxia-Engineering is a dude from DIYSB, maybe I can find some threads about it.

[PhiloB]

Thanks Cody.  I'll do what's listed for sure, just trying to better understand what in doing so:)


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[selfdestroyer]

Thanks Cody.  I'll do what's listed for sure, just trying to better understand what in doing so:)


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I am also curious what the answers are.

I did find this

from here http://www.diystompboxes.com/smfforum/index.php?topic=60710.0
Cody

[PhiloB]

That looks more like what was in my head.  But now the question 'why the 33k resistor before the caps'!? 


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[PhiloB]

Just did the mod in the diagram above.  Works great.  I used a 10nf and 2u2.  Used it on a silicon fuzz face circuit.  Goes from overwhelming bass to pretty thin.  May adjust the cap values but then again neither is too extreme.
Thanks Cody.
Hope someone chimes in with some answers to:
1. Why in the first linked schem does the layout connect the 2 lugs.
2. Why 100k?  Would other values work?


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[TGP39]

Doesn't the 33k resistor combined with each cap create a passive RC LOW PASS filter? So the cutoff frequency can be calculated as 1/2*pi*R*C. This is why you get very different sounds when you switch the caps into the circuit.  I hope I'm understanding your question correctly PhiloB. If not, my apologies.

Steve.

[PhiloB]

When I did it I left out the 33k Resistor and it seemed to function fine.  I guess that resistor changes the frequency cutoff.


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[GrindCustoms]

I've used variable cap setup lots of time, in many circuits and always using the same receipe.

Using a low value pot, 500K and under affect the transition between the capacitance and will also change the frequency response of the whole setup.

The schem shown is close to what i use other that i have lugs 2 and 1 tied, signal inputs on Lug2 and outputs at the junction of the 2 capacitors.

B1M pot is what have worked best, maybe if you use 2 caps that have a very large difference in capacitance you might need to use a Log or Reverse taper depending of how you want the orientation and sweep of the pot to behave.

Rej

[TGP39]

Hope someone chimes in with some answers to:
1. Why in the first linked schem does the layout connect the 2 lugs.
2. Why 100k?  Would other values work?

1.  In Cody's 4 knob fuzz link, the input goes directly to his Z knob (input capacitor blend). Lugs 1 and 2 are tied because we only need the variable resistance between the wiper and lug 3 for our frequency cutoff equation.  As you turn knob Z clockwise the resistance increases from minimal to 50k. This resistance along with the capacitance created by 2 input caps (C1 and C2) gives us our input frequency cutoff (via RC filter). Fc=1/(2*pi*R*C). As you can tell by the equation, the higher the resistance the lower our frequency cut off becomes. 

2.  We could use other values for our "Z" knob....that would just change our total resistance range for the above equation and therefore change our frequency cutoff range.

I am still learning, but I  believe the above is correct. Please correct me if I'm wrong. I hope this helps. 

I also found this link very helpful. http://www.electronics-tutorials.ws/filter/filter_3.html

Steve.

[Cortexturizer]

There's a lengthy explanation over at DIYSB by R.G. Keen on this subject. I didn't understand it back then, I'd have to go revisit it. From what I can recall there was complicated mathematics involved to do get it right, but R.G. had concluded that doing it like we intuitively are doing it (like Cody's drawing shows) is good enough for stompboxes.

I've used the principle a couple of times, and would also strongly recommend the 500kC pot for this. The thing what Rej said about a huge difference in capacitors works much better than let's say 100n and 1uF. Make that 47pF and 1uF and then you'll see the obvious difference. There some residual capacitance or something (I am writing from memory, and am probably way in over my head with this) so if you do the 100n/1uF combo you will never go as low as 100n. By making that one significantly lower, you will reach lower capacitance that you're wishing for (but following the same principle, never as low as 47pf).

I tried this in a bunch of fuzz face and tone bender circuits and it works very well. Obviously the amount of dirt you get with circuits like that is dependable on the bass content so sometimes I would get a fuzz face with not enough sustain and so on...but it's a good alternative to pregain controls on fuzz faces.

My 2cents on this. Hope I helped, and haven't told much jiberish

[PhiloB]

This has been a great help help!  Thanks guys.  Time to try out some different values for pots and caps.  May even break out my calculator!


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[midwayfair]

I wouldn't use the panner version with the caps on the outside lugs for a Fuzz Face. Use the version with just lugs 2 and 3 (variable resistor in series with a capacitor). The panner version will put constant resistance between the guitar and the circuit at every position except the far right and far left. It might work, but it is not going to work like a fuzz face except at the extremes of the dial. The input impedance of a Fuzz Face is only about 50K, so at the center of the pot, you are cutting the signal by HALF.

The way the variable resistor version works is that the pot forms a voltage divider with the input impedance of the circuit for all frequencies below the value of the capacitor. It's essentially a shelf filter being added to the high-pass filter that the input caps create. In other words, it adds capacitance as you decrease the resistance between the input and the capacitor attached to the pot. It ends up being in parallel with the smaller capacitor. The amount of resistance in series with the pot determines the amount of signal reduction for the larger capacitor. So you need to know the input impedance of the circuit (we have that for the fuzz face -- about 50K) to determine the proper size of pot. A 100K pot will pull those frequencies down to about 1/3 the volume of the frequencies let through with the smaller capacitor (100/50). A 500K pot would pull them down much further. I used a 500K on the Rangemaster I built with an input cap control and there is very good separation between the extremes of the dial.

You also need to know the input impedance so you can find the right value capacitor to be the "fixed" side. For a bass control I usually stick to about 200Hz. The 10nF is pretty convenient, though, and it puts the frequency at 318Hz. The actual cutoff will drop some because you'll have some amount of the lower frequencies bleeding through at all times.

The 33k is in series with that tone control because whoever designed it was putting it in a circuit that could handle the extra impedance. It's clearly not from a Fuzz Face.