FET distortion

[sbirkenstock]

Hi Everybody,

I am tinkering around with distortion boxes.
I found three basic ways to create distortion.
a) clipping diodes in the signal path (going to ground)
b) clipping diodes in the negative feedback loop (going to ground)
c) driving the input of a FET so hard, that the output clips

For a) and b) i found plenty of examples / schematics, but not for c)
Does anybody know a schematic for method c) or any product on the market using this method?
I found the fuzz face doing it and probably several other fuzz boxes are like that, but I´m looking more for a hot rod Marshall sound.
Or is there any downside to that method?

Best regards,

Stephan

[R.G.]

You might be interested in "Distortion 101" at geofex.com: http://geofex.com/effxfaq/distn101.htm There are more than three ways.

Amplifying devices can be driven to create distortion by either driving the input with signals the input devices cannot follow, or to output signals that are too big for the power supply limits of the device. There are other odd distortion mechanisms that are speed, offset, and time dependent according to the hifi tweako set, but they're not generally in play for distortion pedals, if they exist.

All amplifying devices will distort if you drive their inputs to make them try to put out a signal bigger than their output stage can swing. Some devices can be set up so their input devices can't follow the inputs even though the resulting output would be within the power supply limits of the device.

For the FET case, there are two main kinds of FETs, the Junction FET (JFET) and the MOSFET. They differ in that the MOSFET can't be driven to input distortion without destroying the device, and that the most common JFETs are depletion mode versus enhancement mode for the MOSFETs. The input of a JFET is a reverse biased diode, and it is possible to forward bias that diode to cause input clipping if you set up the output correctly. Takes some doing to make that happen.

But your question confuses me. The fuzz face does not use FETs at all. The two transistors in the fuzz face are bipolar junction transistors (BJT), which are fundamentally different animals than either kind of Field Effect Transistor. Which did you mean?

[GibsonGM]

Hi S.B.

You have some good questions...R.G. pointed you to a great resource for some more answers!!!  I'm only chiming in because it's raining here and I am bored    This is just an expansion of what R.G. just wrote.

C.   When new, it's a little confusing, so hang with it and learn as much as you can.   If you want to push FETs into clipping, you basically do the following, which also holds true for opamps and BJTs...any amplifying device, AKA "active devices"....tubes, MOSFETs......

You have an input stage, a GAIN stage, that amplifies your microscopic guitar signal from the 1/2 volt range up to a couple of volts....no clipping here, this is so that the signal is a lot louder than the NOISE (hiss....) that is part of active device circuits.  A guitar signal on its own can't clip anything, it's too small.

This now louder signal is fed to another gain stage...because it tries to amplify the now-several volt signal many times (20x, 100x....whatever it's set up for), it will run out of voltage to do this with.  Picture a 1 volt signal trying to be amplified by 20....in a 9V circuit....so, because we ran out of HEADROOM, the signal is clipped off.       This results in distortion, which looks like a square wave due to the (literal) clipping off of its top.    How this is manipulated is the heart of distortion, fuzz, OD....!!

Sound variations are made possible by the choice of device you clip (active like FETs (tube-like), BJTs, opamps....) or passive like diodes.  And how the signal is SHAPED before clipping is huge, too.   Mostly, bass is cut before clipping to avoid muddy sound and blatting (blocking distortion in some devices, too).    The sky is the limit on how this is done, and how much...so, the voicing is really up to the designer.

[Ben Lyman]

Does Joe Davisson's JFET Vulcan qualify? That thing is a BEAST! 
http://diystompboxes.com/analogalchemy/sch/jfetvulcan.html

[GGBB]

http://runoffgroove.com/peppermill.html

[robthequiet]

I believe I have seen Tim Escobedo's variations on FF with FETS and MOSFETS on paper, although tbh I haven't tried them and don't know what they sound like. But for the Marshall stack sound, I would look at the BSIAB or Thunderbird, or generally at the multi-stage FET family. As Ben mentioned, the Vulcan would be a great candidate. Following the other commenters, it generally takes multiple stages to get that sound, which you'll see by looking at the circuits.

[nocentelli]

The Wampler Plexidrive uses a single JFET input stage to drive two further JFET stages into distortion. The Wampler Velvet fuzz uses additional boost stages (either a BJT or the LM386 amplifier chip) in front of the Plexidrive circuit to drive the Marshall-esque distortion into a fuzz-into-a-Marshall sound.

[sbirkenstock]

Wow!
Thank you guys. That is a lot of very usefull information for me.
Of course the fuzz face is not FET, but BJT. Sorry for that (I come from tube amps, just started on transistors).
The examples are all very usefull.
The peppermill is using a MOSFET first and then a JFET.
I tried it the other way around so far.
Seems a lot more variations are used and possible with transistors than with tubes.

I actually do own the Wampler Plexidrive and I did find the schematics.
So far it is not my favorite distortion pedal, but that is likely not the FETs fault.

And I agree, one clipping stage is not enough, same true for tubes imo.

I got all the info I was looking for and more!
Thank you all very much,
Stephan

[TejfolvonDanone]

There are lot of variants of the fuzzface including FET: http://www.oocities.org/tpe123/folkurban/fuzz/fuzzyfaces.html

[jfrabat]

Why not try this one:

http://diy-fever.com/effects/the-ultimate-overdrive/

It's the ULTIMATE OVERDRIVE by Bancika.  You can choose whatever diode you like on each side of the wave, including FET.  So, with one pedal you can have multiple overdrive effects. I built one, and I am happy with it...  And trust me, if I can build it, anyone can!  And if you want even MORE flavor selection, use an OpAmp in a socket and you can then change that as well for an even more personalized sound!