[sdiy] FETs for distortion

[Pat Kammerer]

Here is a tutorial about boosts based around mosfets, heavy on the theory;
http://www.geofex.com/Article_Folders/mosboost/mosboost.htm

Here is a project of a mosfet boost.
http://www.muzique.com/schem/mosfet.htm

I think these ideas would be good as a gain stage in a distortion which uses
several stages, maybe another stage with a BJT or FET.

I really like the one on this page called Muffmaster, though it is based on
a an NPN BJT.  I would recommend it for an input stage with maybe other
clipping stages.
http://www.muzique.com/schem/projects.htm

----- Original Message -----
From: Seb Francis <seb at burnit.co.uk>
To: James Patchell <patchell at cox.net>
Cc: Synth DIY <synth-diy at dropmix.xs4all.nl>
Sent: Saturday, December 31, 2005 8:00 AM
Subject: Re: [sdiy] FETs for distortion


> Hi Jim,
>
> Thanks for your reply.
>
> I have heard it said that FETs give a softer clipping than BJTs, hence
> wanting to try them.  Since I will breadboard my ideas first, I will be
> able to play around with biasing and driving levels to get a sound I
> like, then use the same actual FETs when I build.  The temperature drift
> is perhaps more tricky, so I will have to read the datasheet and see if
> I can minimise the effects of temperature (or add some compensation).
>
> So far (from the various schematics I have seen) I'm going to buy some
> J201 and 2N5457.  Now you have said they may vary a lot, I will buy a
> few so I can pick and choose.
>
> I think these two are JFETs.  I wonder if I would get a different sound
> using MOSFETs?  I haven't seen them used in any schematics, and I've
> heard they are damaged by static very easily, but someone on this list
> said recently that they give a nice soft clipping (I'm not sure if this
> comment also applied to JFETs, or was specific about MOSFETs).
>
> Seb
>
>
>
> James Patchell wrote:
>
> > One thing you should note about fets is that they have a tendency to
> > "vary" a lot from unit to unit.  Depending on the circuit topology,
> > you can pick a set of fets that will work perfectly, and if you then
> > replace them with randomly selected parts...it will not work the way
> > you expect.  Plus, fet parameters drift with temperature in ways that
> > you will definitely notice.  By comparison, BJT's are extremely well
> > behaved.  I am not suggesting you use BJT's, just letting you know
> > that you are going to be tackling.  Most people are pretty used to
> > using BJTs.
> >
> >
> >
> > At 02:10 AM 12/31/2005 +0000, Seb Francis wrote:
> >
> >> Hi,
> >>
> >> I'm just playing around making a few distortion circuits (to be used
> >> for electronic drums + synths, rather than guitars).
> >>
> >> I want to try out some distortions with MOSFETs and/or JFETs and I
> >> was wondering if anyone had any recommendations of some part numbers
> >> to buy.  Preferably they need to be available in UK from Rapid or
> >> Farnell, but if there's something particularly good I don't mind
> >> hunting around.
> >>
> >> I guess I'm asking whether one FET is going to be pretty much the
> >> same sound as another (just a case of biasing and driving
> >> appropriately), or are there some favourites for this type of app?
> >> Or are there in fact some particular specifications/attributes I
> >> should look for when choosing a FET?
> >>
> >> I hope this is not too vague a question ;)
> >>
> >> Seb
> >>
> >>
> >
> >         -Jim
> > ***************************************************************
> > http://www.oldcrows.net/~patchell
> >
> > ***************************************************************
> >
> >
> >
> >
>
>