Building Muff Fuzz question(s)

[scott_v]

I have refered to this topic: http://www.diystompboxes.com/smfforum/index.php?topic=77456.0

and I'm still learning alot and need some input... I want to take another or additional approach to building this circuit.  On GGG, on the build instructions page it says R2 can be switched out from 10K to 22k-50k to increase fuzz.  Is it possible to instead put say a 50k pot in place of fixed R2 so i can adjust this amount of additional fuzz? (or just a 25k pot?)  I'm looking at doing this as an option instead of goosing the gain on Q2 that Mark Hammer notes in the above thread.  I'm not sure what to expect by doing both other than having way to much uncontrollable nosie and fuzz. 

Also still thinking about adding the SWTC to control the fuzz tone, i guess my goal is to make a more controllable 3-knob muff fuzz thats not exactly a big muff and has abit more of a fuzz face tone.

Thanks for any help!

[foxfire]

yeah, you could use a pot...you may want to try using a 10K resistor in series with a 50K pot or a 24K/25K resistor with a 25K pot? the SWTC is a great tone control...

[scott_v]

yeah i just sorta realized that after starting this topic... i presume without the series resistor you can remove all the resistance or fuzz that characterizes the circuit, i think i'll start by trying a 10k resistor w/ a 10k or 25k pot...

i also found a post by Mark Hammer stating that "...bypassing the 2k7 resistance with a cap and something less than 2k7 will increase overall gain and fuzziness.  Use of smaller-value caps would apply the increased gain to mids and highs, but not bass.  Use of a larger-value cap (e.g., 22uf-47uf) would apply gain across the entire spectrum."
wondering what this sounds like too...

[foxfire]

it sounds good...

[scott_v]

ok stupid question or maybe not... does it matter where the pot is placed? should it be between the resistor (R2) and collector of Q2?

or the resistor (R2) and R1 (from the +9V) and C1 (47uF)?

and i'm trying to decipher what Mark's statement versus what he came up with in the thread in my first post...

could i replace 2k7 (R5) with a smaller resistor value and a cap (22uf -47uf) in series between ground and the emitter of Q2 instead of what is shown in the first schematic above?

[Mark Hammer]

If you look at the typical Fuzz Face or derivative, you'll see that the gain/fuzz is set by a pot between ground and the emitter of the 2nd transistor - precisely where R5 is.  Most frequently, you will see something like a 1k pot with the outside lugs tied to the emitter and ground, and the wiper tied to an electrolytic cap of some value.  When the cap is tied directly to the emitter, AC (audio content) has what is essentially a zero-ohm path to ground.  Which portion of the audio spectrum will depend on the value of that cap, which is why some folks will prefer, say, a 22uf cap in their Fuzz Face, while others might like a 33uf or even 47uf.  The larger the cap value, the lower down that "gain advantage" begins.

By connecting the cap tot he wiper, the overall emitter-ground resistance for the unadjustable portion of the signal remains constant, while you vary the resistance between the cap and emitter with one leg of the pot.  If yo look at Jack Orman's MosFET booster, you can see that the pot doesn't need to be part of the emitter-ground path.  You can simply stick a variable resistance between the emitter and cap and leave a fixed resistor in parallel between emitter and ground.  Same thing, functionally.

The Muff Fuzz sets a fixed rather than adjustable gain.  A simple way to make it adjustable, following the Fuzz Face model is to replace R5 with a 680 ohm  resistor and 2k pot.  One outside lug of the pot goes to the emitter, and the other goes to the 680R resistor which is then tied to ground.  Voila.  No matter where you set the pot, there is a fixed 2k7 resistance to ground.  Now, stick an electrolytic cap between the wiper of that pot and ground.  You now have adjustable gain.

[scott_v]

thanks for the input... unfortunately i don't have a beardboard at the moment to try all of these options.  I would guess a 10uF or 22uF wiper to ground would be a good place to start on the adjustable gain. and leave R2 fixed at whatever desired value (10k-50k). I guess this is what i could do or the "emitter cap to ground" mod not sure if the results of doing both together would be pleasing.

[Gus]

If you change R2 other stuff changes.

First it is a Si FF type circuit like Mark posted

overall gain is the 100K feedback/the guitar mess(RLC) with the openloop gain of Q1  X 10K/2.7K(2nd stages gain)

I would leave R2 at 10K for better drive to the volume pot and anything you add after the Fuzz

I would use a 2K gain pot and a 2K or 5K bias set up like like in the following circuit if you want to stay close to the stock biasing of the transistors.
http://www.diystompboxes.com/pedals/gusFuzzFace.gif

[Mark Hammer]

Yup, that's it.

[scott_v]

great! thanks for the help guys... this now gives me a more defined direction to go with this circuit.

I may start out with just the basic stock muff fuzz and work from there and see where i want to go with it by either adding an adjustable "more gain/fuzz" pot (by doing the "emitter cap to ground mod" addition to the circuit) or just making the existing "stock fuzz" adjustable (changing R5 from fixed to variable). and then adding the SWTC...

just one minor question of detail, if i were to use a 2.2K or 3.3k for R5 (instead of 2.7k) would it be noticably different or should i try to get as close to 2.7k (where i would use a 2.2k & a 470R in series)?  i'm just trying to use up some spare components i have laying around... before ordering anything new.

[Mark Hammer]

Either 2k2 or 3k3 are fine.  Once you add a bypass electrolytic cap to ground - and especially if that cap value is big enough to cover the guitar's full range - it likely won't matter since the cap starts to become the determining factor of what separates the emitter from ground.

As a sidenote, on a Muff Fuzz build I made with the variable gain control, I used two different cap values to get two different sorts of gain responses.  To do this, connect the wiper of a 10k linear pot to the emitter, and connect two different caps from the outside lugs to ground (observing the same polarity).  For example 3u3 and 47uf.  Set to the midpoint, the stock fixed resistor (2k7) obviously provides a much more efficient path to ground than the resistance of the pot leg on either side of the wiper (5k + 5k at the middle).  Rotate the pot to one side, and that resistance increases for one of the caps, but decreases for the other.  So, make the pot leg leading to the 3u3 cap smaller and you'll get increasing gain for the mids and treble, but NOT the bass.  In effect, a treble booster of sorts.  Rotate it the other way so that the 47uf cap gets tied to the emitter by a progressively smaller resistance, and  you apply more gain to the entire signal, resulting in more clipping across the board.

It's a nice simple way of getting different kinds of sounds from a 2-knob box (Gain + Output).

[scott_v]

Thanks Mark that sounds like a great easy option to increase gain across the eq spectrum. i'll probably give that a shot.  i have a 10k pot though its linear and not log like you mention so i think the perceived eq change while turning the pot will not sound pleasing, in another thread you mentioned using a 5k pot for doing the same thing, probably doesn't matter much...  i do have 100k log pot and could put a resistor across the outside lugs to drop the resistance. though its not as accurate then?

[Mark Hammer]

Reread the post.  You're fine.