Need help with understanding Mosrite Fuzzrite circuit.

[Meehah]

I love this fuzz and have built quite a few. I understand how the transistors are distorting the signal and the basics of the design of the circuit but would be very interested to learn what the individual capacitors and resistors are doing in the circuit and how their values were chosen. I've looked for more information but am really struggling to find anything. Here is a schematic: http://ilovefuzz.com/viewtopic.php?f=151&t=18290. Thanks in advance.

[antonis]

You better copy the diagram to a simulator program (like Spice, TINA etc) and tweak values observing changes in voltages..
(or else we would need a multi-page analysis/calculations thead..)

[Fancy Lime]

[...] would be very interested to learn what the individual capacitors and resistors are doing in the circuit and how their values were chosen.

Well, what the resistors and caps do exactly depends on the circuit topology but if you understand what a resistor, capacitor and transistor do in general, then that is fairly easy to figure out. A good exercise too. People here will be glad to help you with specific questions (like "what does C4 do?") but it's a it time consuming to explain all components. R.G.s "Technology of the ..." series is always a good start. Look at the fuzz face article (http://www.geofex.com/article_folders/fuzzface/fffram.htm) and see if you can transfer that info to the fuzz rite. It is quite a different beast but resistors are still resistors and caps are still caps.

As for the second part of the question: "how their values were chosen"? The reality of practical design is such that components are often chosen for price or availability, not necessity. There is quite a lot of wiggle room in terms of component values. You can get the same exact results from very different component value combinations if you combine them right. So if you find an old 60s fuzz with a bunch of 71nF capacitors in them it is most definitely not because that was exactly the value they needed but because they had a bucket of those lying around. Considering component tolerances, using a modern 68nF will sound exactly the same, within the general variability of individual units.

Andy

[Plexi]

What a great fuzz..the 2nd I've built in the early days.

[Hatredman]

What a great fuzz..the 2nd I've built in the early days.

Exactly How many fuzzes do you have, Plexi?

.

[thehallofshields]

I've also been curious about this topology. It definitely has a unique character.

[Fancy Lime]

The main magic in this type of topology lies in how the "fuzz" knob works, which the Mosrite Fuzzrite calls "depth". The shin-ei compaion FY-2 has the same basic topology and calls this knob "fuzz". The Orpheum seems to also call it "depth". What this knob does, is blending the relatively clean signal of the first transistor with the totally overdriven signal of the second transistor. Because both transistors are wired as common emitter amplifiers and this type of amplifier inverts the signal, the "depth" or "fuzz" knob mixes the two signals together with a 180° phase inversion between them. The resulting wave form at medium settings of the knob looks a bit strange and has very strong harmonics (especially the third) and a partially canceled-out fundamental. Hence the hollow and aggressive sound. At full "fuzz" it actually sounds nicer and fatter, because the fundamental is less canceled.

These things are super responsive to different types of transistors. I like 2N2222 transistors in this. Reverse biased transistors or piggy backed sound interesting too, especially in the Q2 position.

For more info:
Piggybacking:
http://www.diystompboxes.com/smfforum/index.php?topic=21801.0

Reverse biasing:
http://www.muzique.com/lab/reverse.htm

Hope that helps,
Andy

[Plexi]

What a great fuzz..the 2nd I've built in the early days.

Exactly How many fuzzes do you have, Plexi?

.

Without all I've built in breadboard?
Only 3 with enclosure, 9 without enclosure, and about 12 sold.

Answer to your question: not enough fuzz's. 

[kaycee]

Two questions occur to me. What would happen if the phase inversion was cancelled? And if the answer isn't silence, then how would one go about that?

[Fancy Lime]

Hi kaycee,

the answer is the opposite of silence. If you either invert the "clean" path as well or invert the "dirty" path back to the same phase as it had after the first transistor, you basically get a "normal" fuzz (whatever that means) with a clean blend. I tried different versions of that a few years back. Didn't sound bad but was not exactly what I was looking for at the time. To do that, you can simply add another stage that looks exactly like the two others either in the "clean" path or after the second BJT. However, you will want to reduce the gain on this stage and possibly the other stages as well. Else in the first variant the "fuzz" knob would simply mix two almost identical signals (so it would do nothing much) and in the second variant you would get unusably extreme fuzz (not really much more fuzzy but MUCH more noisy). You can reduce gain by reducing the feedback resistor (between collector and base) to something in the hundreds of kilo ohms range and adding an emitter resistor between emitter and ground (hundreds of ohms to a few kilo ohms). Be aware that these circuits are very sensitive to any changes because by standards of design for which the transistors are actually intended, fuzzes are a horrible misuse of the components. So to get it to sound good (subjectively) you'll have to play around with a lot of values, try to figure out which changes have which effect and slowly slowly inch towards your desired sound. Or just be lucky and nail it on the first try 

Have fun,
Andy

[kaycee]

Thanks Andy, I  understand what you have written, I can have a go at those ideas on the breadboard. Been having a play with the Fuzzrite of late and enjoying it. Just built the version with an added tone control, from Ghost Effects schematic of his Lunar Incantation, very versatile.

I currently have the Orpheum on the BB with the same tone control added, still fettling it and juggling transistors before committing to a build though.

Cheers for the info

[thehallofshields]

The resulting wave form at medium settings of the knob looks a bit strange and has very strong harmonics (especially the third) and a partially canceled-out fundamental. Hence the hollow and aggressive sound. At full "fuzz" it actually sounds nicer and fatter, because the fundamental is less canceled.

Great description. I someone knew all of this already, but these types of succinct explanations give clarity that everyone can understand.

[thehallofshields]

Wouldn't it also stand to reason that the value of the 'Fuzz' or 'Depth' pot is also contributing to the Negative Feedback that's limiting the Gain of Q2? Or does that 22k Resistor change things?

[Fancy Lime]

The 22k has some effect in reducing the feedback through the Pot back to the base of Q2 because it forms a voltage divider with the output impedance of Q2, which is roughly the value of the collector resistor. So the feedback only sees about 1/20th of the signal going through the pot. That may have been the thinking behind the curiously large collector resistors. Or not, 60s fuzz designers made many of their design choices based on part availability and some, by the looks of it, with the aid of creativity enhancing chemical substances in their bloodstream, so who knows what they were thinking half the time. Kidding aside, "when it sounds good, it IS good" is even more true for fuzzes than for other stuff, so I won't judge component value choices in a great sounding circuit.

Andy