Crowhurst’s ”Fuzz”

[tca]

Hi,
I've been reading Teemu's book on SS guitar amplifiers and at page 254 there is a small circuit called Crowhurst's "Fuzz":



It is not a very common soft clipping circuit, I havent seen this type of soft clipping any where. Citing Teemu's

"The surprising feature of the circuit is the "attack": The amount of clipping is gradually decreased during sustained overdrive. In
practice, the amount of attack (or the rate of clipping) is controlled by the capacitor's charge, thus the circuit will react quite interactively to playing dynamics: Acquiring the heaviest clipping requires tranquil passages that allow the capacitor to discharge."

Any refs, examples?

Cheers

[Electron Tornado]

I wonder how high "fairly" is.

Put it a circuit and try fiddling with the R and C values.

[earthtonesaudio]

Put a 10:1 transformer on the left side and you have yourself a wall wart.

[electrosonic]

Found a link with slightly more content and commentary

http://audible.transient.net/archive/doc/curious-reversion.html

Andrew.

The book "Electronic Musical Instruments" by Norman Crowhurst (1971) which that is taken from is available at my local library - I put a hold on it so I should get it in a few days.

[artifus]

^ an entertaining read - thanks for posting!

[tca]

Found a link with slightly more content and commentary
http://audible.transient.net/archive/doc/curious-reversion.html

That is the same website were the image is found... very amusing!

Cheers.

[tca]

Here goes a simple sim. I guess it deserves some breadboarding! (left: high clipping (low R5), right: soft clipping (high R5))



Cheers.

P.S.

I wonder how high "fairly" is.

More than two diode voltage drops.

[kingswayguitar]

you couldn't help me could you. r5 is labeled 100m?? what is low r5, high r5. thanks

[tca]

you couldn't help me could you. r5 is labeled 100m?? what is low r5, high r5. thanks

No problem! low R5=.1R, high R5=3k

R5 is a 10k pot (look first image).

Cheers.

[kingswayguitar]

you couldn't help me could you. r5 is labeled 100m?? what is low r5, high r5. thanks

No problem! low R5=.1R, high R5=3k

R5 is a 10k pot (look first image).

Cheers.

cool

[Jazznoise]

It's basicly a flawed Full Wave Rectifer. One could do the same with a centre tapped transformer and simple put an RC around either diodes.

I've thought of doing similar to my Octavia to change the Rectification from Full>Partial>None

[samhay]

The idea - to have "the amount of clipping ... gradually decreased during sustained overdrive" is the rationale behind my 'Decompression Overdrive' (good to see that I was only 32 years late to that party) and I enjoyed the rant Andrew posted enough to give this circuit a try on the breadboard.

It has its limitations, but it 'does what it says on the tin'. Replacing the 10k resistor with a 10k or 100k pot is a must and it gives a nice 'attack' or 'decompression' effect. Only clipping above 2 diode drops is a bit of a problem and using 1N4148s, you have to hit it with a pretty hot signal. This is a circuit of the Ge era, and it works reasonably well with Ge diodes - I tried 1N34s.
To drag it into the 21st century, I put it in the feedback loop of an op-amp. Works better this way (in my opinion) and you can get away with Si diodes.
As-is the gain is pretty high (>100), so a parallel gain pot helps to make it a bit more civilised. It is pretty easy to clip the op-amp rails, so clipping LEDs are a good addition too.

I am still in two minds about it. It has its charms, but doesn't really do anything my 'Decompression Overdrive' can't do and I'm not sure if I will build it. Will let it sit for a few days and give it another try.

[tca]

^ Just found your 'Decompression Overdrive' entry, both entries differ in 18 days... I guess there is some cosmological coincidence in all this   I haven't breadboard my version yet, but I do like the idea of a passive clipping (just one opamp) but I'll have a go on putting the diodes on a feedback loop (sims firt). Any sound samples of the 'Decompression Overdrive'? Couldn't found any on your post.

Cheers.

[samhay]

Yeah - I thought it was a bit of a conincidence, but then I susspect I am not the first person to have thought of something like this since Crowhurst.

While Crowhursts' idea lends itself to a passive section, I think you will find it works much better this way with Ge diodes. I didn't try it this way, but its still on my breadboard and I will give it a go.

I have been meaning to post some sound samples, but have been having a few 'technical difficulties' (finger injury). This has mostly healed, so will try to do something this weekend. If the stars align, I should be able to do a comparision.

[R.G.]

Enclosing some DC circuit inside a full wave rectifier in a manner similar to that is an old trick for making an AC version of a fundamentally DC circuit. I've seen several versions.

As in all circuits, the details matter.

[Earthscum]

I've played around with this configuration a little bit. The last time I tried it I subbed in different diodes to see what they sounded like. I used LPB type clean boost in front and jfet buffer at the end. With 1n4148's, it was kind of overdriv-ish, not too bad, but it didn't really seem as "soft" as I expected. Schottky diodes were easier to get into the distortion range, and the "softness" started to show. GE's were almost too soft, almost like  a tonally modified clean signal. All in all, I wasn't impressed enough to commit anything. It seemed to lack any real character compared to opamp feedback clipping, but that is just my opinion. I originally found the circuit in a patent file somewhere, I believe.

Since I'm slowly making my way back into the effects building game, I may have to revisit this one, though. 

[samhay]

Haven't tried it with Schottkies, but I think Earthscum's comments are probably spot on. I did try with asymmetric clipping - 3 Si + 1 Ge and it sounded a little different again.

tca - I have put a quick clip of the decompression overdrive up on that thread. Will think about side-by-side a comparison with Crowhurst's "Fuzz" during the week.

[tca]

^I'll have a look, thanks.