Electronic Audio Experiments w/ Obstructures - OxEAE Fuzz (gutshots, traced) [traced]

[whbeers]

Really nice (and _loud_) cascaded TL072 op-amp fuzz w/ what I've found to be an abnormally versatile blended octave stage. It's a ton of fun to play and by far the most flexible boost/overdrive/fuzz pedal I own. While it's remarkable in how loud it'll get, it's also not hard at all to dial in a wide range of more subtle effects.

The pedal uses four slider pots to control overall volume, post-octave gain, "texture" of the octave effect, and "weight" of the tone control on the front end. The pedal is constructed inside a heavy/thick aluminum enclosure using two PCBs connected by three 2.54mm pin headers.
- a host board with buffered (OPA2156) relay-based (TQ2-5V) bypass switching w/ RGB indicator LED, 24V power supply, 4x 100k slider pots (all linear, I think), and effect IO (un-switched mono TS)
- the effect daughterboard with all audio processing

The effect board is two layers, the host board has four with what seems like a variety of voltages of buried power fills (not fully traced).

Product page: https://www.electronicaudioexperiments. ... 0xeae-fuzz
Manual w/ high level technical description of the circuit: https://static1.squarespace.com/static/ ... +Rev+B.pdf

Notes:
* I'm reasonably confident in the effect trace (double-checked a few times) but still very much learning when it comes to the circuit design side of things, so would love if someone else could build on this trace with a block-by-block explanation of the circuit.
* At various points in the tracing I switched the left/right pin numbering of the slider pots. Given how many times I've gotten them backwards producing my own PCBs, I can 100% guarantee they're wrong, and they'd still be wrong if I swapped them back.
* I only traced enough of the host board to understand the control lines (reflected in the effect trace).
* I ran into some barriers tracing the host board, so am not including the partial trace of that board (barriers: unidentifiable components, buried traces, and general laziness/limited interest compared to the effect).
* As my first EAE pedal (of what will undoubtedly now turn into many), this has turned me into a huge fan of John Snyder's work. In digging into the company, I now realize I have several others that he's had a hand in (e.g. a few Caroline pedals), and unsurprisingly they're also some of my favorites.
* if you appreciated or learned something from this trace, go buy EAE/Snyder's pedals - they're awesome.

Schematic:

Effect board:

Host board:

(more pictures available if folks are interested)

[whbeers]

Just noticed that while it should match the original circuit, the tone stage at the front could be laid out much less confusingly in the schematic. I'll update the original post with a revised one a bit later on.

(done. critique welcome, especially if coupled with an explanation!)

[stradivari310]

Well that was fast lol. Glad you like it. Hopefully this doesn't end up on one of those PCB scalper sites any time soon. I got bills to pay... including the die for that custom enclosure. Sigh.

Anyway, the trace looks reasonably accurate to me. The remaining caps can be easily discerned using an LCR meter.

General block diagram is input buffer > MFB filter (Weight control) > split to octave > mixer w/ gain > high gain stages > output.

The multiple feedback (MFB) filter is a variable high pass with a tiny bump of resonance. Pre-gain bass makes a huge difference with fuzz circuits. The octave circuit is a precision rectifier straight out of various application guides. AFAIK other fuzzes haven't used this topology, but it's a lot of fun.

Not much else to say about it really.

[crxst]

Very excited to see this here!

My initial thoughts about the traced schematic:
- R13 1k between U1A/U1B only functions as a RC filter with CB7 1u as presented. Simultaneously the signal impedance at U1B input (MFB) is determined entirely by U1A output impedance as there is no resistor present at U1B -IN. I wonder if thats correct. Seems pretty wild.
- The whopping 24V operating voltage is interesting. It helps to diminish the effect of diode voltage drops in the rectifier part?
- R8 1k and R11 1k in series with U4 V+ and V- pins are present for tonal purposes (headroom lowering)? I've never seen the V- pin lifted from GND this way.

All the best to EAE! The striking aesthetic of this pedal is not showcased here. I encourage everyone to check that out if it you havent already seen it.

[stradivari310]

Very excited to see this here!

My initial thoughts about the traced schematic:
- R13 1k between U1A/U1B only functions as a RC filter with CB7 1u as presented. Simultaneously the signal impedance at U1B input (MFB) is determined entirely by U1A output impedance as there is no resistor present at U1B -IN. I wonder if thats correct. Seems pretty wild.
- The whopping 24V operating voltage is interesting. It helps to diminish the effect of diode voltage drops in the rectifier part?
- R8 1k and R11 1k in series with U4 V+ and V- pins are present for tonal purposes (headroom lowering)? I've never seen the V- pin lifted from GND this way.

All the best to EAE! The striking aesthetic of this pedal is not showcased here. I encourage everyone to check that out if it you havent already seen it.

That filter is indeed drawn correctly. It's a multiple feedback high pass filter with a pretty aggressive cutoff.

The 24V supply is to make it extremely loud, and because we use it on the boost in this series (which is also very loud). I generally prefer how most circuits sound at higher voltages and it's become a calling card of sorts.

The series resistance on U4's power pins was to intentionally destabilize it in a symmetrical fashion. Works pretty well for what we were going for.

Thanks!

[Jarno]

including the die for that custom enclosure.

That enclosure is very very cool, I like also how you've made that little inlay plate so you can use it for various products, easier to engrave/machine than a complete enclosure too.
I like the "brutalist" style
Here's the stage to play that pedal on:

[Ichabod_Crane]

I emulated that schematic, it seems respond well to frequency. (Of course I assumed that unknown small caps value, from 47pF to 100pF. Those cut treble).