[sdiy] FETs for distortion

[James Patchell]

MOSFETs are a whole other pit of snakes.  I have in the past used them to 
build several power amplifiers.  They are not easy to use in linear 
circuits.  And they really vary all over the map as far as their parameters 
go.  And again...temperature stability is a huge problem...which is one of 
the reasons you just don't see them used much in linear circuits (IC's are 
a different story...).

         As far as clipping...depending on circuit topology...yes the 
clipping is probably different.  Both MOSFETs and JFETs have a much lower 
voltage gain than a BJT.

         My opinion, however...is if you want distortion, you should 
probably think more in terms of actually building a processor (analog or 
digital) where you can actually set the transfer function and thus get 
repeatable results...granted...it will me much more complicated than a 
single device that you run a signal through...

         I have always wanted to use a pair of multipliers to generate the 
square and cube of the input signal and then have a mixer where you can 
take the raw, square and cube and mix them....giving you y = Ax + Bx^2 + 
Cx^3....well, maybe someday...

         Anyway...good luck on your experiments.

At 02:00 PM 12/31/2005 +0000, Seb Francis wrote:
>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
>>
>>***************************************************************
>>
>>
>>
>

         -Jim
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http://www.oldcrows.net/~patchell

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