Let's talk Fuzz Factory

[DrAlx]

I just built my first Fuzz Factory.   To try and improve the bass response, I put extra switches to give the option of adding 47 uF caps in parallel to the input and output caps.  I did this based on what I'd read in forums, without really thinking about things properly for myself first.  This is what I found...

The output cap change from 10 uF up to (10uF + 47 uF) made absolutely no audible difference to the sound.
A bit of theory explains why.  The RC high-pass filter in the output stage has a 3dB cutoff frequency of 1/(2 * pi * R * C) Hz.  Putting R = 220k and C = 10 uF,  gives 0.07 Hz.   That's already way below any audible frequency, so boosting the output cap is a pointless exercise.

Boosting the input cap from 10 uF up to (10uF + 47 uF) was a different story.  That didn't boost anything, but pretty much killed the signal completely..  The FF has a low input impedance by default.  Boosting the input cap lowers the input impedance even further.
In the end I noticeably  improved the bass response using a different cap mod...
Instead of putting a cap in parallel with the input cap, I left the input cap alone and added a lower value cap (0.47 uF) across the base of Q1 and ground (with the cap's negative terminal to ground).
You can think of this cap as leaking high frequencies to ground more than the lows.
The end result is a much warmer fatter sound.  If the sound is too warm or muddy for your taste, then try a lower value than 0.47 uF  (e.g. 0.1 uF).

[kodiakklub]

OMG i did the same thing. (amazing pedal by the way, i see why he made a name for himself with this one). i boosted the input and output caps like you did, no really audible difference at all. guess i will have a make another....WAHOOO! thanks for the explanation doc. you are a way smarter man than I.

[DrAlx]

Actually, after playing a little more I've decided that the 0.47 uF cap made everything too muddy for me.  Playing the high notes on the top E string were just too quiet.  So I've just taken out that 0.47 uF cap, and followed the advice further up this thread ... i.e. boosting the 0.1 uF capacitor between Q1 and Q2 instead.
I have a switch that puts a 2.2 uF capacitor in parallel with it.  If you do this mod, make sure the negative side of this cap is on Q1's collector, and the positive side is on Q2's base.  I have seen advice before saying the exact opposite thing!!  If you are not sure, measure the voltage  between those points of the circuit.  On my build, I found Q2's base at higher voltage than Q1's collector, so that's where the positive side of the cap went.

The result is a really FAT sound.  One side effect is that the self oscillations happen at lower frequency (so you get lots of clicks).  
If you do this mod, either make the cap switchable, or put a variable resistor in series with it so you can fade the big cap in and out,as opposed to switching it on and off like me.

I might post some soundcloud samples later.

[kodiakklub]

that 0.1uf cap is not polarized so it shouldnt matter which way round you put it in there, correct? i will try putting the switchable cap in that position instead. thanks again.

[DrAlx]

If you've used a non-electrolytic cap for the 0.1 uF (as I have on mine) then it doesn't matter which way you have put it in.
However the 2.2 uF cap that I put across it was electrolytic and has to be put in the right way.
You can ignore what I said about the guitar controls not changing the pitch.  They do, it's just that the really low frequency oscillations are just a series of clicks, and it's hard to notice them speed up or slow down when they are far apart.

[DrAlx]

My samples of the effect of adding 2.2 uF across the 0.1 uF cap.

http://soundcloud.com/alex-lawrow/fuzz-factory-modification

[kodiakklub]

sample is amazing. gonna redraw my FF now and etch a new version. maybe ill turn the old one into a 2 or 3 knob vanilla version!!

[Jaicen_solo]

Fat Fuzz Factory??? 

[petey twofinger]

i ordered a set of those germs from the guy in the czec republic ... 15 bucks , on ebay , .... i had been drinking a bit , and was feeling discouraged .

[DrAlx]

I'm glad the samples were useful. Just to confirm about the orientation of the extra cap.
I've done a simulation of the Fuzz Factory hooked up to my guitar using the Circuit Lab website.
What a great site.  You can even model the self oscillation, and the effect of the guitar controls on it's frequency!!

Anyway, a DC analysis shows that Q2's base will always be at least a few volts higher than Q1's collector (for all STAB values).
So the positive side of the extra cap definitely goes to Q2.  I didn't just fluke things on my build
Here's a screen shot.  The voltages shown on the left are for STAB set at maximum stability.  

[Plexi]

I worked on this simple looking, but complicated pedal.
The transistors hfe are very important.

I tried with silicon simple PNP, and the feedback where in almost every knob settings.
Tried with germaniums 2SB54, around 100/120 hfe, and they tamed a bit.

I need to find out how to bias correctly to use silicon ones.

Is that possible?

[POTL]

Hi, in most projects, fuzzies with germanium require negative power and positive ground
Here, the land is negative, and the food is positive, although PNP transistors
Usually this is achieved by requiring a voltage converter MAX1044 ICL7660s, etc.
Can you explain why this circuit works from ordinary polarity
How does the pedal feed (standard connector with a minus in the center)?

[bassjunkie]

Just modded Q1 in a Fat Fuzz Factory with a 2N1308 I had laying around from a hoof build. Works as described. Also made an "industrial fuzz" from guitarPCB using 2N404 transistors in Q2 and Q3 with an OC140 in Q1. Both sound basically the same (when the FFF is set to normal FF C2 setting)

I do like this mod very much because it sounds like a harder edged fuzz face. Something to be aware of when a Germ NPN is subbed in at Q1, the oscillation sensitivity changes and the knob positions that create the oscillation change. It still oscillates - just differently and in a more limited range.

Also the pedal got less noisey.

For some people - this might be the one mod that makes the fuzz factory circuit more applicable in their rigs. 

As always - YMMV