Circuit analysis help: Experimental fuzz!

[tatou]

I've been breadboarding an experimental fuzz, combining some ideas from the fuzz face, muff fuzz, bazz fuss, and even the big muff.

Here is the current status of what I've hacked together (my "question areas" are boxed in red):


Quick overview:
* It's essentially a fuzz face topology (with tweaked values) running into a modified AMZ presence tone control
* The fuzz is controlled by an input pot rather than the normal fuzz control (e.g., R5 as a pot)... I found this to sound better & be more reliable
* A switch toggles between a single LED in the Q2 feedback loop vs. silicon hard clipping
* The amplifier output comes directly from the Q2 collector, unlike a typical fuzz face

It's sounding great, but I still have a few questions about how parts of the circuit work, and hope some of you can help me understand my own frankenstein a bit better!!

D1: WHY DOES ONLY ONE DIODE DO ANYTHING?
At first, I wired up two LEDs in a symmetrical typical clipping fashion, as in the big muff, etc. But I noticed only one LED lit up. With further experimentation I realized only one diode going from collector to base on Q2 appeared to do anything at all. The other direction simply has no perceivable effect. This is how things are wired in the bazz fuss, but I suspect you can actually still use symmetrical clipping in a single-transistor circuit like that (e.g., I've seen it in the muffer or 5th gear overdrive). But I'm wondering if there's something particular going on with the coupling of transistors in the FF topology (rather than having two distinct "stages"? Could it be that the feedback loop via R3 is somehow affecting the voltage swing in such a way that only one diode ever matters? If so, is this not a clipping diode in the conventional sense? It really does sound different... and the hard-clipping diodes to ground work in both directions. What gives?

Regardless, that single LED sounds great. It's a harsh, sputtering bumblebee-type fuzz, whereas toggling to the hard clipping network has a creamier distortion feel (and no diodes at all is a beefier, more open and uncompressed fuzz, and of course more output). The tone controls help balance out some of the tonal differences between those three "clipping" options.

R5: THIS SEEMS TO BE THE ONLY WAY TO GET A GOOD SOUND OUT OF THIS CIRCUIT
There's no equivalent to this resistor in any other schematic I've seen... but if you jumper it and go straight from the Q2 emitter to ground via R8 and C2, the pedal sounds... starved. Like the battery is nearly dead or something. The fact that R5 is even there is a happy accident, a breadboarding vestige from experiments getting a fuzz control to sound good in this part of the circuit. If any of you have any thoughts, please let me know!

D2-D5: SUBLE MULTIPLE CLIPPING DIODE QUESTION
I drew things here on the schematic as I accidentally breadboarded it up... with the anodes of D2/D5 touching the cathodes of D3/D4. In this way, it seems like some current can skip D3/D5 without going all the way to ground, but then I couldn't hear any difference when I "fixed" it (D4->D5 and D3->D2 directly, with no point touching in between). The only other place I've seen this is in some (but not all) Timmy schematics floating around. Does it matter to have this connection point between diode pairs in opposite directions? Why or why not?

C10: HOW DO YOU REALLY CUT HISS?
Adding this cap to ground seems to help cut a little bit of high-end hiss/noise out of the circuit (although probably not even enough to bother with the parts count)... However, it doesn't appear to me to be a proper low-pass filter. Any thoughts on why this works or what I should be doing instead are welcome.

The rest of the circuit I feel like I understand pretty well. It's a little low on output... if you dial the fuzz & tone knobs back and crank the volume to max, the effect is on par with unity bypass volume. With everything cranked there's quite a bit more headroom. Based on my research, a fuzz face adds 18db of gain and a BMP-style tone stack attenuates 7-15db of that, and with diodes on that cuts into things a bit more.

Any other thoughts you have about this circuit and how to improve it are welcome. So far it's proven to be a fun, versatile little beast!

Thanks!
(And apologies for cross-posting to other boards)

[gordo]

Hmmm, 2020 is shaping up to be fun year!

[thesmokingman]

where's the resistor that pairs with c10 to form a filter? ... also kinda the wrong forum

[midwayfair]

D1: WHY DOES ONLY ONE DIODE DO ANYTHING?
At first, I wired up two LEDs in a symmetrical typical clipping fashion, as in the big muff, etc. But I noticed only one LED lit up. With further experimentation I realized only one diode going from collector to base on Q2 appeared to do anything at all. The other direction simply has no perceivable effect. This is how things are wired in the bazz fuss, but I suspect you can actually still use symmetrical clipping in a single-transistor circuit like that (e.g., I've seen it in the muffer or 5th gear overdrive). But I'm wondering if there's something particular going on with the coupling of transistors in the FF topology (rather than having two distinct "stages"? Could it be that the feedback loop via R3 is somehow affecting the voltage swing in such a way that only one diode ever matters? If so, is this not a clipping diode in the conventional sense? It really does sound different... and the hard-clipping diodes to ground work in both directions. What gives?

The 5th gear schematic is actually wrong. Take a very close look at the muffer (or the big muff). There's a very, very important distinction between your circuit's diode feedback and the Big Muff's. The fact that something's only clipping half your waveform is a big clue. I am currently not going to give more than a hint though because I think you have the capacity to figure it out for yourself.

R5: THIS SEEMS TO BE THE ONLY WAY TO GET A GOOD SOUND OUT OF THIS CIRCUIT
There's no equivalent to this resistor in any other schematic I've seen... but if you jumper it and go straight from the Q2 emitter to ground via R8 and C2, the pedal sounds... starved. Like the battery is nearly dead or something. The fact that R5 is even there is a happy accident, a breadboarding vestige from experiments getting a fuzz control to sound good in this part of the circuit. If any of you have any thoughts, please let me know!

What are your voltages? I would guess that the collector of Q2 is not near the 4.5V it's "supposed" to be. As an experiment, move the 330R to the south side of the 1K, so that there's 1330R to ground still, but the 47uF is connected directly to the negative feedback resistor (R3). You can also move R3 to the south side of the 330R. Basically, you want to figure out if it's

D2-D5: SUBLE MULTIPLE CLIPPING DIODE QUESTION
I drew things here on the schematic as I accidentally breadboarded it up... with the anodes of D2/D5 touching the cathodes of D3/D4. In this way, it seems like some current can skip D3/D5 without going all the way to ground, but then I couldn't hear any difference when I "fixed" it (D4->D5 and D3->D2 directly, with no point touching in between). The only other place I've seen this is in some (but not all) Timmy schematics floating around. Does it matter to have this connection point between diode pairs in opposite directions? Why or why not?

Pretend you're an AC waveform. Half of you goes through, say, D2. You look at D5 and say, "can't go that way, gotta go through D3." This is no different than if you couldn't see D5 at all.

You can exploit this by giving your clipping switch a third option.

C10: HOW DO YOU REALLY CUT HISS?
Adding this cap to ground seems to help cut a little bit of high-end hiss/noise out of the circuit (although probably not even enough to bother with the parts count)... However, it doesn't appear to me to be a proper low-pass filter. Any thoughts on why this works or what I should be doing instead are welcome.

This is a perfectly fine low-pass filter. If it weren't, it wouldn't work. The output impedance of Q2 is your resistance. It's also why the diodes clip, actually.

[tatou]

Thanks all! There's also been a good discussion on DIYSB.

Apologies thesmokingman, you're right. Perhaps this wasn't the most appropriate forum for dev help. In the past, I've had more luck getting my questions answered here than at DIYSB or FSB, but in the past I've mostly been modding MBP projects (and I did use the hipster schematic as my FF reference for this experiment!).

Mods, feel free to disable this thread if it's too off-topic.

The 5th gear schematic is actually wrong. Take a very close look at the muffer (or the big muff). There's a very, very important distinction between your circuit's diode feedback and the Big Muff's. The fact that something's only clipping half your waveform is a big clue. I am currently not going to give more than a hint though because I think you have the capacity to figure it out for yourself.

midwayfair - If you're hinting at the capacitor in the muffer/BMP schematics (?), I did try wiring things up that way but it didn't matter. Voltage still only seemed to clip in one direction. However, someone on DIYSB reminded me that transistors can be wired as diodes... the 2n5088/89 forward voltage was around v_f = .75, way below the LED's (v_f = 1.6). When I tried a Si (v_f = .62) or Ge v_f = .45) diode going from the Q2 base to collector, I did get clipping. So I assume that's what's going on... unless I'm missing something?

Still planning to experiment with different ways to bias Q2 though...

[midwayfair]

Apologies thesmokingman, you're right. Perhaps this wasn't the most appropriate forum for dev help. In the past, I've had more luck getting my questions answered here than at DIYSB or FSB, but in the past I've mostly been modding MBP projects (and I did use the hipster schematic as my FF reference for this experiment!).

I'm pretty sure he meant that there's a subforum for member's projects.
http://www.madbeanpedals.com/forum/index.php?board=17.0

There's absolutely no reason people can't post their own projects here for advice. The only restriction is not to see boards for the same pedals that Brian himself sells.

It's true that there's already a diode in the transistor. (IIRC they actually act like zener diodes.) You probably still want the capacitor there if you're going to have the diodes go both ways. Otherwise they can mess up your biasing.