Thunderchunk

[PBE6]

Just like to share an original distortion circuit with you that I'm going to call the "Thunderchunk":


(Please see the revised diagram further down this thread for input impedance fix!!)

At its heart it's a diodes-to-ground clipper, but it's got a beefy sound and some fun bells and whistles that make it unique:

• pre-gain tone shaping with bass cut (72 Hz roll-off, 340 Hz shelf) to reduce fizz
• tuned saturation control that lets you move between Dist+ and Tubescreamer-type sounds
• up to 26 dB of post-gain bass boost centered at 130 Hz for plenty of beefy chunk
• up to 20 dB of post-gain presence boost centered around 3.3 kHz to cut through the mix
• firm overall low-pass filtering to cut out the hash

With the bass and presence knobs up full, the pedal has a nice tight bass and plenty of sizzle. Rolling off the bass and presence gives you a cool band-passed mid-forward tone. The saturation control can be adjusted to taste to open up the sound, and the curtailed pot response ensures you never get too clean. Enjoy!

Thanks to all the posters who have contributed great ideas for me to knick, most notably RG Keen (saturation control), Mark Hammer (pre-gain bass cut and general tone shaping) and the folks at Runoffgroove (bass boost and presence boost circuits as found on their Thor/Thunderbird pedals). Cheers!

[Frank_NH]

Very cool.     Looks like you could use a TL074 for all of the op amps.  Any sound clips?

[Derringer]

Hey! I recognize those filters! 

Looks cool man. Does it get scooped sounding with the bass and treble both up?

+1 for clips.

[PBE6]

Just made up a little sound clip.
https://soundcloud.com/pbe6/thunderchunk

Here is the signal chain:

Strat-style guitar with EMG pickups (bridge pickup) --> pedal --> FMR Really Nice Preamp --> Logic Pro X (amp modeller - transparent preamp (set flat), vintage 4x12 cabinet)

Here are the settings:

1. Clean
2. Gain FULL, Sat OFF, Bass FULL, Pres FULL
3. Gain FULL, Sat FULL, Bass FULL, Pres FULL
4. Gain FULL, Sat OFF, Bass OFF, Pres OFF
5. Gain FULL, Sat FULL, Bass OFF, Pres OFF
6. Gain HALF, Sat HALF, Bass HALF, Pres HALF
7. Gain HALF, Sat HALF, Bass OFF, Pres FULL
8. Gain HALF, Sat HALF, Bass FULL, Pres OFF
9. Gain FULL, Sat OFF, Bass FULL, Pres FULL (again!)

[bluebunny]

Looks very interesting.  One for the backlog, I think.  Any chance of a link to a bigger schematic for us myopic folks?   

[antonis]

Any specific reason for R2's "low" value ..??


edit: Haven't seen about 72Hz roll-off  

(shouldn't be better with a 220k and 10nF or 470k and 4n7..??)

[PBE6]

@bluebunny hopefully this one is sized a bit better

[bluebunny]

Hmmm...  The word "identical" comes to mind.      Lemme just squint a bit... 

[PBE6]

@antonis I left the power scheme off of the schematic, but it's buffered like so:



The DC current through the 22k resistor doesn't have any where to go so I don't think the value is too critical, but I could be wrong. I happen to have a lot of 100nF caps lying around so I used one of them and then chose the resistor accordingly to get my preferred roll-off.

[PBE6]

Sorry bluebunny! I tried to make it 4 times the size, guess it didn't work. Does the second one zoom better at least?

[samhay]

^The DC current through the 22k resistor doesn't have any where to go so I don't think the value is too critical, but I could be wrong.

This sets the input impedance of the circuit. A value of 1M is quite commonly chosen for similar designs.

[Kipper4]

Sounds good mate.
Lots of tone shaping options.
Well done

[antonis]

The DC current through the 22k resistor doesn't have any where to go so I don't think the value is too critical, but I could be wrong. I happen to have a lot of 100nF caps lying around so I used one of them and then chose the resistor accordingly to get my preferred roll-off.

I was talking about input impedance so I proposed a larger resistor value...


edit: Sam is faster than me, anyway...

[PBE6]

Ahh yes, forgot about that. Then yes, a smaller cap and a larger resistor would be a better choice. I probably didn't notice anything because of the active pickups.

Actually, could the buffered power supply have anything to do with it? Usually Vref coming from a voltage divider tied to a battery is considered "ground" for AC purposes, but can the AC signal swim "upstream" back through the opamp buffer?

[antonis]

I tried to make it 4 times the size, guess it didn't work. Does the second one zoom better at least?

Better now..

[antonis]

can the AC signal swim "upstream" back through the opamp buffer?

If it doesn't then you can't create a high pass filter...

[PBE6]

Good point! And thanks for enlarging the diagram, not so easy to do on the iPhone.

[antonis]

And thanks for enlarging the diagram

You're wellcome...

(ONLY if you promise for a verified vero layout...)

[Frank_NH]

Great demo!  It's interesting how much the sound changes with the saturation control.  Definitely want to build this on vero with a TL074, as I have several of those chips just waiting to get used... .

[Frank_NH]

I have a question about the power section illustrated above.  I haven't seen many designs with an op amp buffered reference voltage.  What are the advantages of this approach, in general (stability, noise reduction)?