About soft clipping and saturation + a design idea

[scaramoustache]

Hello fellow stompboxers! I've been following this forum for quite a while and thought I could contribute something on my behalf.

I've had a long grudge with soft clipping overdrives/distortions especially with the way they jump from set gain to clean blend when hitting the diodes threshold. What I'm after is soft clipping that doesn't do this jump but gradually lowers gain when signal level rises, aka "true" soft clipping that would work nicely with solid state amps and low gain.

I'd like to share a brute force attempt at this which I have on my breadboard at the moment made from components i had lying around. Basic idea is to use a full wave rectifier and jfet to vary gain. As signal level goes higher (or lower) than zero the jfet works as a variable resistor and lowers gain. The diodes are there to ensure that the jfet operates in its linear region (and might actually work better in hard clipping configuration).

The schematic has some fossils from previous attempts that were needed to make it work at the time. Nevertheless the circuit on my breadboard is working and sounding good. However I'm total amateur with electronics and I welcome any corrections or refinements to the design.

About the schematic: Its a little bit messy, limited software + inexperience with them =... you'll notice. Didn't have a symbol for jfet hence bjt in schematic, also didn't have space for both part numbering and values. I thought it'd be easier to make references if parts were numbered. Oh and r14 wasn't actually on my breadboard.

https://drive.google.com/file/d/0B8HrKgi5-lgmOFBtT3ZzT00yQjg/edit?usp=docslist_api

Teemu

[ggedamed]

Could you make a sound sample? I'm curious about the sound of the decay part of the envelope.

LATER EDIT: looks like you want to prevent hard clipping, so there would be no clipping artifacts, but still I would like to hear it.

[Rob Strand]

There was an article in ETI magazine around 1980 which used the following structure:

Input ->  Compressor -> Distorter -> Expander

Basically it reduces the signal variation presented to the distorter.

[scaramoustache]

Gabi,

I'm not aiming at removing clipping artificials. Quite contrary, I'm trying to introduce slight distortion even to lowest levels of signal and let highest peaks clip all the way.

Sound sample:

https://drive.google.com/file/d/0B8HrKgi5-lgmTjA5ZmY3OU5xVUk/edit?usp=docslist_api

clean ( Fender RocPro and Harley tele) - slight drive - max drive.

EDIT: sound sample in context with same setup:
https://drive.google.com/file/d/0B8HrKgi5-lgmdXRZdXhmX2dmMTg/edit?usp=docslist_api

[scaramoustache]

Hi Rob, thanks for input. I know some people use compressors before their overdrives to get smoother and more sustaining sound. Expander after that; can't think what would it do soundwise.

Bringing compressors up is actually quite accurate now that i think about it. What i've been trying to wire up is basically a compressor that works instantly without those attack and decay times. Thanks for pointing in that direction.

[scaramoustache]

Schematic posted above is rubbish. I cleaned up the clipping section since there was just too much useless stuff making things complicated. Instanly way more sustain and smooth decay. I did some testing in Spice to understand better what is going on and ended up simplifying even more. Can't do sound testing any more today but plots look the way they are supposed to.

Input:
https://drive.google.com/file/d/0B8HrKgi5-lgmczdpVGJnUjZya1k/edit?usp=docslist_api

Gain 50k:
https://drive.google.com/file/d/0B8HrKgi5-lgmVEpwOTFGR1RHalk/edit?usp=docslist_api

Gain 200k:
https://drive.google.com/file/d/0B8HrKgi5-lgma2hXU0JxSEZvYjg/edit?usp=docslist_api

[samhay]

Mixing distortion with explicit envelope detection can be great fun.
I did something similar recently - http://www.diystompboxes.com/smfforum/index.php?topic=110346.0
In this case, I tried taking the envelope both before and after the clipping - feed forward and feed back. The feed forward didn't work very well for me, but might be worth trying in your design - you could try taking the feed from the input buffer and then add some gain to your detector.

[scaramoustache]

Hi Sam,

I checked your DiSCO schematic and have to say I'm impressed. Great sustain and grit! I had toyd around softening clipping threshold by moving bias point but didn't come up with a solution that would do more than just sum the original signal with.. original signal (inverted or not). Adding compressor in the mix seems to work nicely,

I tried feedforward before, didn't work. Feedback on the other hand does and I'm now quite satisfied with the results.

Multitracked samples:

https://drive.google.com/file/d/0B8HrKgi5-lgmbnBaallTa2FMbzA/edit?usp=docslist_api

Should start working on eq, or amp miking..

[samhay]

Thanks.
It can be tricky to get feed-forward to work properly and may not be terribly useful in this capacity anyway.

Afraid I don't seem to be able to play the new clip.

[scaramoustache]

Don't know why the clip won't work for you. Tested the link on my wife's laptop, it should be ok.

As far as soft clipping goes I think I have cought the golden goose I've been after. At least to some extent. To make the whole thing sound good is about pre and post eqing, right? So on with that then.

Meanwhile, here's the schematic for the distortion part:

https://drive.google.com/file/d/0B8HrKgi5-lgmb0F6VVFaaXhDcFk/edit?usp=docslist_api

[scaramoustache]

Finally had time to put it together.  Works as overdrive/saturator and compressor and doesnt sound as boxy through solid state amp as some other dist/drive pedals i have. I guess one could squeeze clean blend and octave effect out of it too. Not much pre or post eq, just slight bass reduction and some treble roll off in addition to baxandall. Hope you like it!

https://drive.google.com/file/d/0B8HrKgi5-lgmb1BrZFJBUVZxd28/view?usp=docslist_api

[Digger1770]

http://www.muzique.com/lab/sat2.htm
This Jack Orman site has some great design ideas for clipping I have used the warp control for soft - hard clipping , and I love being able to label a control WARP CONTROL!