Giant Hogweed - Fuzz / Octave UP / Octave Down

[drolo]

This is a bit of a Frankenstein consisting of the Octave UP circuit borrowed from the FOXX Tone Machine, the Octave down from Joe Davisson's Shocktave. Both are fed into a Roger Mayer Mongoose. The name came from listening to a lot of early Genesis stuff while breadboarding this...

The artwork was made with a nice drawing from my lovely wife :-)

I stumbled upon the Mongoose by coincidence and decided giving it a try. Turns out it's a GREAT distortion. It has some similarities with a RAT but has a gain stage before the LM308 which gives it a lot more oomph. It has great sustain and can cover quite a lot of distortion territory. Somewhere between a Fuzz, a distortion and even overdrive on milder settings.

Here is a demo:

And the schematic:



The original Mongoose has the LM308 directly DC coupled to the output of the transistor gain stage that provides a bias of 5.5V. Since I had to separate the transistor gain stage from the LM308 to insert the mixing stage in between, I experimented with different bias settings. Turns out that this slight off center bias gave the distortion its particular character. It makes the op amp clip asymmetrically. With the usual 4.5V half supply bias it sounded a lot more behaved, not quite the same character. I found that I could create lots of different sounds by just  changing the bias voltage so I added a "Stability" pot that shifts the bias point for the LM308. CCW it literally starves the op amp and allows to dial in some gritty, gated and full CW you have 4.5V.

I'm not much of a parts snob usually (especially when it comes to op amps) but in this case the LM308 really does seem to make a difference (as has been said for the RAT). I tried several options and none was really doing the same thing. The worst was a 5534... ugh ... The closest I could find was a TL070.

For the tone control, I tried lots of different incarnations of the BMP tone control but decided to go with the WSTC in the end. Simple and effective.

I changed the gain control a bit. Reduced the gain pot from 10k to 5k. On the original circuit, when turning down the gain, the cutoff frequency also changed and gave a muffled sound. To avoid this I added a fixed gain path (a 4k7 resistor and a 100n) cap that adds some clarity on cleaner settings and is not noticeable on heavier settings.
The input signal is first buffered by the first op amp stage and split to the input of the FOXX and the Mongoose input transistor stage.

The output of the 2 octaves and the Mongoose transistor stage are then mixed together with an inverting op amp stage that goes to the LM308.

The octave on/off switch disconnects the 2 octaves from the mixing stage.

Not much to say about the 2 octavers. I did not change anything on the stock FOXX circuit except for using Shotkys instead of Germanium diodes.

The Shocktave takes its signal from the collector of the second FOXX transistor. The signal there is already boosted and filtered (some treble is removed) and goes into a SHO-like MOSFET booster (replacing the original gain stage) and another low pass filter before hitting the Shocktave.

All in all I'm pretty happy with how it turned out. I just wish I could have done the signal routing and mixing a bit simpler and also would have preferred not to separate the Mongoose's input gain stage from the LM308. There are plenty of sounds in there. Along with a good solid fuzz/distortion, it can do jackwhiteblackkeys stuff, neilyoungbrokenfendertweed stuff, even synth-like stuff.

[Mark Hammer]

Very nice work.  Some smart borrowing, and stitching together.  I'm itching to hear it through a phaser or flanger.

I'm always admiring of folks who can work on something like this, which obviously takes a lot of time, and keep it all under their hat until they have the bugs worked out, the schematic drawn, and the Youtube editted and posted.

Kudos.

[TGP39]

This is magnificent David. I would love to build this. Really great execution. Steve.

[drolo]

Very nice work.  Some smart borrowing, and stitching together.  I'm itching to hear it through a phaser or flanger.

I'm always admiring of folks who can work on something like this, which obviously takes a lot of time, and keep it all under their hat until they have the bugs worked out, the schematic drawn, and the Youtube editted and posted.

Kudos.

Hehe, that must come from my having being involved in art projects. One almost gets the tendency to value the presentation over the actual content ... :-D
Also I have the blind urge to edit any photo in Photoshop even when it's not necessary ...

Having a child really makes you consider your time differently. I used to work day and night on projects. Now I work a minute here, an hour there, but I take my time and structure my work better.

The last clip in the video is the Giant Hogweed going through a Chorus, not quite a Flanger or Phaser but it does react nice to all kinds of effects I tried, auto-wah, flanger, phaser. It does what you imagine it would do :-)

[bool]

Brilliant!

As for the presentation, on the "product photo" I see a slightly overpronounced DOF on the stompbox knobs, and on a personal note, a slightly more contrasty and undesaturated edit would perhaps look even better.

One thing I would suggest to measure and confirm if you indeed have the + and - on the C7 right (I mean the actual voltage on your actual build, in real-life).

[drolo]

One thing I would suggest to measure and confirm if you indeed have the + and - on the C7 right (I mean the actual voltage on your actual build, in real-life).

Hmm, they are oriented the same way as in the actual FTM schematic (and it's working right) so I guess it's ok.

[samhay]

Sounds cool and nicely presented! What's the story with the empty 3-pin socket on the PCB?
Didn't realise you were a fingerstyle player David - nice one.

[deadastronaut]

sounds excellent david...

mass sustain...very cool.

[digi2t]

Man, David, that seriously kicks ass! I'm very impressed by it's range. BRAVO!!

I've put it on my list of "veros I must do", but will you be having any more boards made? Maybe OSH Park?

[drolo]

Thanks guys :-)

What's the story with the empty 3-pin socket on the PCB?
Didn't realise you were a fingerstyle player David - nice one.

I left some space for an optional cap for filtering out high frequencies in case of oscillations. In this case it was not needed.
I started using fingerpicking a few years ago, a bit inspired by Daniel Lanois, I admit :-)
I don't think I'm particular good at it but I can't seem to go back to using pleks. Feels weird not to be touching the strings ...

I've put it on my list of "veros I must do", but will you be having any more boards made? Maybe OSH Park?

I did order a couple more PCB's than I needed actually. I would gladly sell one to you if you want (and if you don't mind fixing a little bug on the PCB, a forgotten via hole ...)
Just PM me :-)

[mac]

8-Up and down in a box, very cool!!!  

The name came from listening to a lot of early Genesis stuff while breadboarding this...

That song always reminds me of the old movie "Invasion of the body snatchers"  

mac

[Mark Hammer]

The last clip in the video is the Giant Hogweed going through a Chorus, not quite a Flanger or Phaser but it does react nice to all kinds of effects I tried, auto-wah, flanger, phaser. It does what you imagine it would do :-)

Then the difference between our respective degrees of patience is once again underscored.    I listened to about 2/3 of the clip, and decided that I had a pretty thorough sense of what it did and could do, and cheated myself out of some fun, apparently.

[drolo]

These demos seem to go on and on indeed ... :-)

I have a quesition about a certain quirk in the circuit.

That Stability pot shifts the bias voltage around for the LM308.

I had put the cap labeled C27 in because I was having U2 oscillate when dialing down the Stability knob, especially with op amps other than the LM308.
Now on the finished build I started to notice that the bias voltage, when stability turned ccw, would gradually shift back to its original point after a while. Taking out the cap stops it from doing that, so it works as I meant it to and fortunately there are no more oscillations now that I have it on a proper PCB. The oscillation must have come from the rats nest on the breadboard ...

Still I can't really understand why the voltage would change with the cap connected. I guess it must be charging up somehow, but it's strange ....

Also when I was breadboarding, I had noticed that when removing the bias at the input of U2, the voltage would not change immediately but only after a while (maybe half a minute or more)

Does the opamp do that? Can it store the voltage somehow??

[drolo]

8-Up and down in a box, very cool!!!  

The name came from listening to a lot of early Genesis stuff while breadboarding this...

That song always reminds me of the old movie "Invasion of the body snatchers"  

mac

It has that kind of vibe :-)
I find the idea of writing a song about that plant, the way they did it, very amusing.
Also Hackett's guitar work on this is amazing.

[garcho]

bravo drolo! can't wait to get settled in to my new place and start building again. dreaming of wires...