Understanding the ZVex Fuzzolo

[turdadactyl]

So, I've been checking out this schematic:



I'm far from an expert on this stuff, but what I gather is, if you take off the MOSFET boost on the right, this is essentially an NPN Fuzz Face with the added PW pot.  How exactly is the PWM section working?  It seems nearly unchanged from the NPN Fuzz Face circuits out there like this one:

[anotherjim]

Maybe more of a clip symmetry adjust than pulse width? C2 can filter out audio fed back to Q1. .
Not sure about the protection zener for the MOSFET, it looks like it's borrowed from an input pre-amp scheme and not needed here?

[Transmogrifox]

PW pot changes the bias so it gets increasingly asymmetric as the pot value is increased toward 500k.  At the extreme end it basically biases the transistor in a saturated state so the notes get gated as the signal level fades.  You can hear this if you google the Fuzzolo and listen to demos.  ProGuitarShopDemos has a pretty good demo on youtube.

You are correct this is essentially a Si fuzz face with some tweaks (PW pot being the most noteworthy).

This is a freaking high gain beast!

[turdadactyl]

This is a freaking high gain beast!

Are you bored enough to go through the math?  Would love to follow along.

[Transmogrifox]

This is a freaking high gain beast!

Are you bored enough to go through the math?  Would love to follow along.

Maybe not in rigorous detail, but here's a rough run-down:
First stage gain can be approximated by (R5 | rpiq2)/req1
req1 ~ vt/ic = 25 mV/118uA = 212 ohms
rpiq2 = beta*vt/ic = 250*25mV/600uA = 10.4k
G1 = (51k | 10.4k)/212 = 41

Second stage gain (pass-band) is the same sort of thing:  (R8 | R9)/req2
req2 ~ 25 mV/600 uA = 42

G2 = 10k/42 = 238

Final stage has a small amount of negative feedback, but 1M compared to ~20k makes this almost zilch, so we can just look at the gain of Q3 stage:
[EDIT] I had neglected the 50k output pot in my original post [/EDIT]

G3 = (50k | 51k)/3.9k = 6.5

Because the MOSFET gain and small amount of negative feedback are not taken into account then let's just fudge it to gain of 6.

G3 = 6 (fudged)

Gain = G1*G2*G3 = 58.5k

Because of all the fudging stating 58.5k implies some level of computational precision that is not there.  Suffice it to say the gain is somewhere in the region of 60,000 in the "passive" setting, and about 1/2 that (30,000) in the "active" setting.

[EDIT] Spice simulation gives a gain of about 57,000.  My guesstimations come pretty close to simulation and I would suppose reality is in this general 60k ballpark[/EDIT]

Maybe you can also see that the selected transistor will have a significant effect on the overall gain so there could be some value in experimenting with different types of transistors (lower-gain trannies probably more useful).

This kind of gain is similar to a typical op amp open-loop gain.  You might be able to put a pot between output and input for feedback which would allow you to adjust the gain.  Doing it this way may result in a nice crunchy distortion.  It might also result in an unstable circuit

Note the somewhat non-conventional way of using "re" instead of gm directly.  It's just my way of thinking about this stuff, notice re = 1/gm, so it's equivalent. 

This way of thinking about it is useful when there is an additional resistor in the emitter of a circuit, then you can add RE and re to get an equivalent emitter resistor.  That is when this method of considering gm as equivalent emitter resistor "1/re" makes the small-signal gain calculation relatively straight-forward.  Also is a handy value for selecting bypass capacitors for a certain high-pass corner freq.

[Gus]

this might help

http://www.diystompboxes.com/smfforum/index.php?topic=112217.msg1037302#msg1037302

[turdadactyl]

So, here's an interesting little exercise:

[Transmogrifox]

So, here's an interesting little exercise

It appears Mr Vex found what he considers to be the perfect ice cream and has been at work to offer a nice variety of toppings. The pinch/PW knob is really at the heart of this species.

[turdadactyl]

It seems a lot of people like what he's scooping into their bowl.

[turdadactyl]

Maybe not in rigorous detail, but here's a rough run-down:
First stage gain can be approximated by (R5 | rpiq2)/req1
req1 ~ vt/ic = 25 mV/118uA = 212 ohms
rpiq2 = beta*vt/ic = 250*25mV/600uA = 10.4k
G1 = (51k | 10.4k)/212 = 41

Ah, crap.  Here I go down another engineering textbook rabbit hole.  I'll be back...who knows when?!

http://www.learningaboutelectronics.com/Articles/How-to-calculate-r-pi-of-a-transistor
http://www-inst.eecs.berkeley.edu/~ee105/fa14/lectures/Lecture12-Small%20Signal%20Model-BJT.pdf

[Plexi]

Great work turdadactyl!

I guess I'll build a Mastotron and add a switchable SHO at the end.

[amz-fx]

Some quick calculations seem to indicate that the collector resistor on Q2 would be better at 10k than 20k. I'll have to breadboard it to see if it works out.

A simple mod would be to substitute a 10k resistor in series with a 10k pot (as rheostat) for the Q2 collector resistor. This would allow bias change over a decent range.

Best regards, Jack